•draS^ 


METEOROLOGY 


THE  MACMILLAN  COMPANY 

NEW  YORK    •    BOSTON  •    CHICAGO 
DALLAS  •    SAN  FRANCISCO 

MACMILLAN  &  CO.,  LIMITED 

LONDON   •    BOMBAY   •    CALCUTTA 
MELBOURNE 

THE  MACMILLAN  CO.  OF  CANADA,  LTD. 

TORONTO 


METEOROLOGY 

A  TEXT-BOOK  ON  THE  WEATHER,  THE  CAUSES  OP  ITS 
CHANGES,  AND  WEATHER  FORECASTING 

FOE  THE  STUDENT  AND  GENEEAL  EEADEE 


BY 

WILLIS    ISBISTER   MILHAM,    PH.D. 

FIELD    MEMORIAL    PROFESSOR    OF    ASTRONOMY 
IN   WILLIAMS    COLLEGE 


Neto  gorfc 

THE  ,MACMILLAN   COMPANY 
1912 

All  rights  reserved 


COPYRIGHT,  1912, 
BY  THE  MACMILLAN  COMPANY. 


Set  up  and  electrotyped.     Published  March,  1912. 


NortoooD 

J.  8.  Cashing  Co.  —  Berwick  &  Smith  Co. 
Norwood,  Mass.,  U.S.A. 


PREFACE 

THIS  book  owes  its  existence  to  a  course  on  meteorology  which  has 
been  given  by  the  author  in  Williams  College  for  the  last  eight  years. 
This  course  is  a  Junior  -and  Senior  elective  course  with  three  exercises 
a  week  during  a  half  year.  A  syllabus,  covering  both  the  text-book  used 
and  the  added  material,  was  prepared  for  the  course.  This  was  at  first 
mimeographed,  then  revised  and  printed.  Later  it  was  again  revised  and 
reprinted.  This  book  follows  the  order  of  topics  in  this  last  syllabus 
very  closely,  and  is  thus  essentially  a  resume  of  the  material  which  has 
been  gathered  for  the  course. 

This  book  is  essentially  a  text-book.  For  this  reason,  the  marginal 
comments  at  the  sides  of  the  pages,  the  questions,  topics  for  investiga- 
tion, and  practical  exercises  have  been  added.  A  syllabus  of  each  chapter 
has  been  placed  at  its  beginning,  and  the  book  has  been  divided  into 
numbered  sections,  each  treating  a  definite  topic.  The  book  is  also 
intended  for  the  general  reader  of  scientific  tastes,  and  it  is  hoped  that 
these  earmarks  of  a  text-book  will  not  be  found  objectionable  by  him. 
It  can  hardly  be  called  an  elementary  treatise,  but  it  starts  at  the  begin- 
ning and  no  previous  knowledge  of  meteorology  itself  is  anywhere  assumed. 
It  is  assumed,  however,  that  the  reader  is  familiar  with  the  great  general 
facts  of  science.  References  have  been  added  at  the  end  of  each  chapter. 
These  include  pamphlets  and  articles  in  the  periodical  literature  as  well 
as  books.  These  are  the  first  things  which  a  student  would  naturally 
look  up  in  order  to  gain  further  information.  In  appendix  IX  an  attempt 
has  been  made  to  summarize  the  literature  of  meteorology.  Here  the 
books  are  arranged  in  alphabetical  order  without  regard  to  age  or  value. 
Both  the  metric  and  English  system  of  units  and  the  Fahrenheit  and 
centigrade  thermometer  scales  have  been  used  in  the  book.  It  seems 
unnecessary  in  quoting  facts  and  data  from  many  sources  to  change  every- 
thing to  conform  to  one  set  of  units.  In  appendices  I,  II,  and  III,  the 
English  and  metric  systems  of  units  and  conversion  tables  have  been 
added.  The  facts  of  meteorology  have  now  become  so  general  and 
accepted  that  there  can  be  but  little  that  is  new  in  such  a  book.  The 
originality  must  lie  in  the  arrangement,  use,  and  perhaps  interpretation 

v 

236401 


vi  PREFACE 

of  these  facts.  Whenever  a  distinctive  idea  has  been  introduced  by  some 
investigator,  credit  is  always  given  in  the  text  when  this  idea  is  quoted. 
Such  credit  has  never  been  intentionally  omitted  by  the  author. 

Although  this  book  has  assumed  a  considerable  size,  it  can  lay  no  claim 
to  compTeteness.  No  single  volume  can  be  a  treatise  containing  all  known 
facts  to  date  and  all  the  explanations  which  have  been  offered.  Four 
aspects  or  applications  of  meteorology  have  been  entirely  omitted.  These 
are: 

(1)  Mathematical  Meteorology. 

(2)  Meteorology  applied  to  living  things ;  including  phenology  and  the 
influence  of  climate  on  man. 

(3)  Meteorology  and  medicine ;  including  climate  and  disease. 

(4)  A  History  of  Meteorology ;  including  a  biography  of  the  men  who 
have  contributed  much  to  its  development. 

To  each  of  these  subjects  a  long  chapter  could  be  devoted,  and  they  could 
easily  be  expanded  so  as  to  become  large  books.  Such  topics  as  "  Meteor- 
ological Apparatus,"  "  The  Daily,  Annual,  and  Irregular  Variation  in  the 
Various  Meteorological  Elements,"  "The  Isothermal  Layer,"  etc.,  could 
be  easily  treated  at  such  length  as  to  become  a  large  book.  In  fact,  each 
chapter  in  this  book  could  be  expanded  into  a  sizable  volume.  This  book, 
then,  makes  no  attempt  at  completeness,  but  it  does  attempt  to  give  a 
fairly  full  presentation  of  the  present  state  of  the  science,  and  also  to 
point  the  way  for  further  acquisition  of  information  on  the  part  of  him 
who  desires  it. 

In  the  preparation  of  this  book,  the  author  is  particularly  indebted  to 
the  United  States  Weather  Bureau  and  to  Professor  Willis  L.  Moore,  its 
chief.  Every  opportunity  was  given  to  make  use  of  what  is  probably  the 
largest  meteorological  library  in  the  world;  tables  of  data  to  illustrate 
various  points  were  supplied ;  and  free  permission  to  reproduce  and  quote 
much  that  has  appeared  in  government  publications  was  given.  The 
author  is  under  great  obligations  to  many  persons  who  have  helped  him 
in  various  ways:  to  Professor  William  J.  Humphreys,  Professor  of 
Meteorological  Physics,  United  States  Weather  Bureau,  who  has  read 
the  entire  manuscript  and  made  many  helpful  and  valuable  suggestions ; 
to  Edward  H.  Bowie  of  the  forecast  division,  Preston  C.  Day  and  Mait- 
land  C.  Bennett  of  the  climatological  division,  C.  Fitzhugh  Talman  in 
charge  of  the  library,  and  Cleveland  Abbe,  Jr.,  of  the  library  division,  for 
assistance  and  suggestions  during  the  final  revision  of  this  book  while 
in  Washington ;  to  Professor  Cleveland  Abbe,  who  has  read  a  portion 
of  the  manuscript  and  whose  kindly  interest  is  an  inspiration  to  any  one 
who  is  teaching  meteorology  or  doing  research  in  that  subject ;  to  George 


PREFACE  vii 

T.  Todd.  local  forecaster  in  charge  of  the  Albany  station,  and  Herbert  E. 
Vail,  the  first  assistant.  These  last  gentlemen  have  read  the  entire 
manuscript,  made  many  suggestions,  and  guarded  it  against  minor  mis- 
takes in  connection  with  the  routine  work  of  the  United  States  Weather 
Bureau.  They  have  also  responded  with  unfailing  cheerfulness  and 
promptness  to  the  many  calls  for  data  in  connection  with  the  Albany 
station. 

W.  I.  M. 

WILLIAMS  COLLEGE, 

WILLIAMS-TOWN,  MASS.,  July,  1911. 


CONTENTS 
PART  I 

CHAPTER  PAGB 

I.  INTRODUCTION  —  THE  ATMOSPHERE     ........  1 

II.  THE  HEATING  AND  COOLING  OF  THE  ATMOSPHERE   .....  28 

III.  THE  OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE           ...  60 

IV.  THE  PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE    ....  112 

A.  The  Observation  and  Distribution  of  Pressure 114 

B.  The  Observation  and  Distribution  of  the  Winds        .         .        .         .136 

C.  The  Convectional  Theory  and  its  Comparison  with  the  Observed 

Facts .156 

D.  A  General  Classification  of  the  Winds 164 

V.     THE  MOISTURE  IN  THE  ATMOSPHERE          .......     189 

^.   The  Water  Vapor  of  the  Atmosphere 191 

X    Dew,  Frost,  Fog 210 

"$!,    Clouds 218 

D.   Precipitation .         .         .239 

VI.     THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE         .         .         .         .  264 

A.  Tropical  Cyclones .  266 

B.  Extratropical  Cyclones  and  Anticyclones 283 

C.  Thundershowers 320 

D.  Tornadoes .335 

E.  Waterspouts  and  Whirlwinds 342 

F.  Cyclonic  and  Local  Winds 343 

VII.     WEATHER  BUREAUS  AND  THEIR  WORK      .......     353 

VIII.     WEATHER  PREDICTIONS       ..........     378 

PAET   II 

IX.  CLIMATE 426 

X.  FLOODS  AND  RIVER  STAGES 442 

XL  ATMOSPHERIC  ELECTRICITY 454 

XII.  ATMOSPHERIC  OPTICS 483 

XIII.  ATMOSPHERIC  ACOUSTICS    .  .         .  496 


ILLUSTRATIONS 

FIGUBE 

1.  Composition  of  the  Atmosphere  at  Various  Heights 10 

2.  Aitken's  Dust  Counter 11 

3.  The  Cause  of  Twilight 19 

4.  Typical  Undisturbed  Daily  Variation  in  Temperature 23 

5.  Daily  Variation  disturbed  by  a  Thundershower       .         .         .        .         .         .23 

^  6.  Energy  received  by  Perpendicular  and  Oblique  Incidence        ....       32 

^7.  The  Revolution  of  the  Earth  around  the  Sun .33 

8.  The  Presentation  of  the  Earth  to  the  Sun  on  June  21  and  December  21          .       33 

9.  Variation  in  the  Insolation  with  Latitude  at  Five  Different  Dates  ...      34 

10.  Annual  Variation  in  the  Insolation  received  at  Three  Different  Latitudes      .       35 

11.  Variation  in  the  Insolation  with  Latitude  and  Time 36 

12.  Showing  the  Contrast  in  the  Thickness  of  Air  passed  through  by  Vertical  and 

Oblique  Rays * 38 

13.  Insolation  on  Mont  Ventoux 40 

14.  Insolation  at  Montpellier  ...........       40 

15.  Diagram  Illustrating  Convection .45 

16.  Diagram  Illustrating  Mirage 46 

17.  Balloon  Equipped  for  Meteorological  Observations  ....       facing      48 

18.  Kite  Equipped  for  Meteorological  Observations        ....       facing      49 

19.  Graph  Illustrating  Vertical  Temperature  Gradient  .         .        .        .        .         .50 

20.  Temperature  Gradients 51 

21.  The  Original  Thermometer  of  Sanctorius 6T" 

22.  Thermometers  of  Various  Forms      .        .         .         .        .    x    .        .        .        .64 

23.  A  Thermometer  in  the  Making         .........      64 

24.  The  Error  of  Parallax  in  Reading  a  Thermometer 65 

25.  The  Thermometer  Shelter  of  the  U.  S.  Weather  Bureau         .        .        .        .67 

26.  The  French  Thermometer  Shelter .        .       68 

27.  The  English  Thermometer  Shelter 68 

28.  The  Russian  Thermometer  Shelter %  .        .69 

29.  The  Sling  Thermometer 69 

30.  Assmann's  Ventilated  Thermometer 70 

31.  The  Draper  Thermograph 71 

32.  The  Richard  Freres  Thermograph .       72 

33.  Weather  Bureau  Maximum  and  Minimum  Thermometers       ....       73 

34.  Six  Maximum  and  Minimum  Thermometer     .......       74 

35.  Black  Bulb  Thermometer 75 

36.  Graphical  Representation  of  Station  Normals  of  Temperature  at  Albany,  N.  Y.      80 

xi 


xii  ILLUSTRATIONS 


37.  Thermo-isopleths  at  Berlin 81 

38.  Thermograph  Record  showing  Typical  Daily  Variation  of  Temperature  at 

Albany,  N.Y.,  Oct.  14-18,  1908 84 

39.  Diagram  illustrating  the  Energy  received  and  given  off  by  the  Earth  during 

a  Day 85 

40.  Annual  Variation  in  Temperature  at  Five  Different  Places     .         .  .86 

41.  Isanomalous  Temperature  Lines  for  January .98 

42.  Isanomalous  Temperature  Lines  for  July 99 

43.  Isothermal  Lines  for  Spain  and  Portugal  for  January  and  July       ...  99 

44.  Annual  Range  of  Temperature .  100 

45.  Highest  Temperatures  ever  observed  in  the  United  States       .  102 

46.  Lowest  Temperatures  ever  observed  in  the  United  States        ....  103 

47.  The  Variability  of  Temperature  for  January  in  the  United  States  .        .        .  104 

48.  Torricelli's  Experiment .         .        .  115 

49.  Standard  Barometer         .        .         .        .        . 116 

50.  Cross-section  of  the  Cistern  of  a  Barometer ..117 

51.  The  Meniscus 118 

52.  An  Aneroid  Barometer 120 

53.  The  Internal  Construction  of  an  Aneroid  Barometer 120 

54.  The  Richard  Freres  Barograph 121 

55.  The  Mouth-barometer 122 

56.  Barograph  Record  Showing  the  Diurnal  Variation  in  Pressure        .         .         .  125 
67.  The  Diurnal  Variation  in  Pressure  at  Sitka,  New  York,  St.  Louis,  San  Fran- 
cisco, New  Orleans,  and  Mexico  City 126 

58.  Diurnal  Barometric  Wave  at  10  A. M 127 

59.  Diurnal  Barometric  Wave  at  4  P.M.          .         .        .         .         .        .         .        .  127 

60.  The  Distribution  of  Pressure  in  a  Vertical  Section  along  a  Meridian       .        .  134 

61.  Areas  of  High  and  Low  Pressure      .        .        .         .        .        .        .        .        .  135 

62.  The  Electrically  Recording  Wind  Vane  of  the  Weather  Bureau      .        .        .139 

63.  A  Simple  Deflection  Anemometer 141 

64.  A  Simple  Pressure  Anemometer 142 

65.  Lind's  Pressure  Anemometer   ..........  142 

66.  Robinson's  Cup  Anemometer  ......         ....  143 

67.  Pocket  Anemometer 144 

68.  Wind  Rose  for  January,  1909,  at  Syracuse,  N.Y .147 

69.  Wind  Rose  for  January.  1909,  at  Albany,  N.Y 148 

70.  The  Daily  Variation  in  Wind  Velocity  at  New  York,  St.  Louis,  and  San 

Francisco  for  January  and  July        ........  149 

71.  Diurnal  Variation  in  Wind  Direction  and  Velocity  at  the  Top  of  the  Eiffel 

Tower  during  June,  July,  and  August 151 

72.  The  Annual  Variation  in  Wind  Velocity  at  Philadelphia,  Chicago,  Phoenix, 

and  San  Francisco 152 

73.  Diagram  illustrating  Convection  in  a  Long  Tank  of  Fluid       .        .         .         .156 

74.  The  Meridional  Section  before  Convection  Started 157 

76.  The  Meridional  Section  after  Convection  was  Permanently  Established          .  158 

76.  A  Meridional  View  of  the  Air  Circulation       ...                                  .  158 


ILLUSTRATIONS  xiii 

FIGURE                                                                                            .  PAGE 

77.  The  Relation  of  Wind  Direction  to  Pressure  Gradients          .         .        .         .159 

78.  Air  Motion  about  "  Highs  "  and  «'  Lows  "  on  a  Non-rotating  Earth      .        .  160 

79.  The  Meridional  Section  of  the  Isobaric  Surfaces  in  a  Convectional  Circula- 

tion on  a  Rotating  Earth  ..........  162 

80.  Air  Motion  about  "  Highs"  and  "  Lows"  on  a  Rotating  Earth    .         .         .  163 

81.  Surface  Distribution  of  the  Planetary  Winds         ......  166 

82.  The  Air  Currents  in  the  Outer  Layer  of  the  Atmosphere       ....  166 

83.  The  Air  Circulation  in  the  Intermediate  Layer      ......  167 

84.  The  North-south  Component  of  the  Circulation  of  the  Atmosphere       .         .  167 

85.  The  Sub-equatorial  and  Sub-tropical  Wind  Belts 171 

86.  Pressure  and  Wind  Distribution  over  India  during  January          .        .         .  175 

87.  Pressure  and  Wind  Distribution  over  India  during  July         ....  175 

88.  Pressure  and  Wind  Direction  in  Spain  and  Portugal  during  January    .         .  176 

89.  Pressure  and  Wind  Direction  in  Spain  and  Portugal  during  July  .         .         .  176 

90.  Wind  Direction  in  Australia  during  January  and  July 176 

91.  The  Effect  of  Land  and  Sea  Breezes  on  the  Prevailing  West  Wind  on  the 

Shores  of  Long  Island .  178 

92.  Cross-section  of  a  Valley  showing  the  Isobaric  Surfaces        ....  180 

93.  Piche  Evaporimeter 193 

94.  The  Hair  Hygrometer 199 

95.  The  Psychrometer  .         . 201 

96.  The  Whirled  Psychrometer .         .         .201 

97.  The  Recording  Hygrometer 203 

98.  The  Annual  Variation  in  Absolute  Humidity  at  Washington,  San  Francisco, 

New  Orleans,  St.  Louis,  and  Bismarck    .         .         .         .        .         .         .  206 

99.  Relative  Humidity  at  St.  Louis,  Mo.,  Sept.  16-19,  1908          .        .         .         .207 

100.  The  Annual  Variation  in  Relative  Humidity  at  New  Orleans,  Washington, 

Bismarck,  and  Phoenix    ..........  208 

101.  A  Nephoscope 226 

102.  The  Burnt  Paper  Sunshine  Recorder 227 

103.  The  Photographic  Sunshine  Recorder 228 

104.  The  U.  S.  Weather  Bureau  Sunshine  Recorder 228 

105.  Snowflakes facing  241 

106.  The  Effects  of  an  Ice  Storm  in  New  England         ....       facing  242 

107.  The  U.  S.  Weather  Bureau  Rain  Gauge          . 244 

108.  The  Annual  Variation  in  the  Amount  of  Precipitation  at  Twelve  Stations 

in  the  United  States ,     .  .        .        .249 

109.  The  Annual  Variation  in  the  Amount  of  Precipitation  at  Eight  Foreign 

Stations 250 

110.  The  Distribution  of  the  Meteorological  Elements  about  a  Tropical  Cyclone  .  268 

111.  Temperature  and  Moisture  Changes  in  a  Tropical  Cyclone     ....  269 

112.  The  Path  of  the  Hurricane  of  Sept.  1-12,  1900 272 

113.  Locating  the  Center  of  a  Tropical  Cyclone     .         .         .        .         .         .        .  273 

114.  The  Five  Regions  of  Occurrence  of  Tropical  Cyclones  .                 .        .        .  274 

115.  The  Direction  of  Rotation,  Dangerous  Half,  and  Path  of  Tropical  Cyclones  275 

116.  The  Hurricanes  of  the  West  Indies  during  September,  from  1878  to  1900      .  276 


xiv  ILLUSTRATIONS 

FIGITKB  PAQB 

117.  The  Application  of  Ferrel's  Law  to  the  Air  coming  towards  a  Tropical 

Cyclone 280 

118.  The  Distribution  of  the  Meteorological  Elements  about  an  Extratropical 

Cyclone 284 

119.  Distribution  of  the  Meteorological  Elements  about  a  Low  with  a  Typical 

Wind  Shift  Line 288 

120.  The  Structure  of  an  Extratropical  Cyclone  at  Various  Levels        .         .         .     289 

121.  The  Distribution  of  the  Meteorological  Elements  about  a  Typical  Anticyclone 

or  High 295 

122.  The  Motion  of  the  Cirrus  Clouds  about  an  Area  of  High  Pressure         .         .     297 

123.  The  Tracks  of  a  Number  of  Lows  in  the  Northern  Hemisphere     .         .         .     300 

124.  Storm  Tracks  for  Europe        .         .         .        .         .         .        .         .         .         .301 

125.  The  Bigelow  System  of  Storm  Tracks  across  the  United  States      ...        .     302 

126.  The  Russell  System  of  Tracks  across  the  United  States         .        .        .        .302 

127.  The  Van  Cleef  System  of  Storm  Tracks  across  the  United  States .        .        .     303 

128.  The  Van  Cleef  System  of  Tracks  for  Highs  across  the  United  States     .         .     306 

129.  Diagram  Illustrating  the  Determination  of  the  Wind  Direction  and  Velocity 

at  Any  Point  near  a  Passing  Low    .        .         .         .        .         .         .        .317 

130.  The  Changes  in  the  Meteorological  Elements  during  a  Hot  Summer  Day 


with  a  Typical  Thundershower  in  the  Afternoon     . 

. 

323 

131. 

The  Cross-section  of  a  Typical  Thundershower      .... 

. 

324 

132. 

The  Typical  Form  of  a  Thundershower          

. 

328 

133. 

The  Typical  Path  of  a  Thundershower  ...... 

. 

329 

134. 

329 

135. 

Two  Views  of  the  Same  Tornado  at  Goddard,  Kansas  . 

facing 

337 

136. 

A  Tornado  at  Oklahoma  City,  May  12,  1896  

facing 

337 

137. 

Damage  Caused  by  a  Tornado  at  Rochester,  Minn.,  Aug.  21,  1883 

facing 

337 

138. 

Wreckage  of  Anchor  Hall,  St.  Louis,  May  27,  1896 

facing 

337 

139. 

The  Distribution  of  All  Recorded  Tornadoes  from  1794  to  1881     . 

, 

339 

140. 

The  Cottage  City  Waterspout,  Aug.  19,  1896,  1:02  P.M. 

facing 

343 

141. 

Diagram  Illustrating  the  Formation  of  the  Foehn  Wind 

. 

346 

142. 

The  Climatological  Districts  and  Forecast  Sections  of  the  Weather 

Bureau  . 

357 

143. 

The  U.  S.  Weather  Bureau  Station  at  Washington,  D.C. 

Frontispiece 

144. 

International  Storm  Warnings        ....... 

. 

374 

145. 

Diagram  Illustrating  the  Area  for  which  to  Predict  a  Cold  Wave 

. 

402 

146. 

The  Price  Current  Meter         

. 

445 

147. 

Two  Simple  Electroscopes      ........ 

. 

458 

148. 

The  Equipotential  Surfaces  over  an  Irregular  Surface    . 

459 

149. 

A  Lightning  Flash  .......... 

facing 

467 

150. 

An  Oak  Struck  by  Lightning  

facing 

473 

151. 

A  Black  Walnut  Struck  by  Lightning    ...... 

facing 

473 

152. 

A  Church  Spire  Struck  by  Lightning      

474 

153. 

The  Effect  of  Refraction         ........ 

. 

484 

154. 

Halos  and  Related  Phenomena       ....... 

. 

488 

155. 

The  Formation  of  the  Rainbow      

. 

489 

156. 

Cloud  Shadows  ;  "  the  Sun  drawing  Water  "          . 

facing 

490 

157. 

Selective  Scattering  bv  a  Turbid  Medium 

491 

CHARTS 


CHART 

I.  Isothermal  Lines  for  the  Year. 

II.  Ocean  Currents. 

III.  Isothermal  Lines  for  the  Year  for  the  United  States. 

IV.  Isothermal  Lines  for  January. 
V.  Isothermal  Lines  for  July. 

VI.  Isothermal  Lines  for  July  for  the  United  States. 

VII.  Isothermal  Lines  for  January  for  the  United  States. 

VIII.  Isotherms  for  the  North  Polar  Regions  for  January. 

IX.  Isotherms  for  the  North  Polar  Regions  for  July. 

X.  Isobaric  Lines  of  the  World  for  the  Year. 

XI.  Isobaric  Lines  of  the  World  for  January. 

XII.  Isobaric  Lines  of  the  World  for  July. 

XIII.  Air  Circulation  of  the  Atlantic  Ocean  for  January  and  February. 

XIV.  Air  Circulation  of  the  Atlantic  Ocean  for  July  and  August. 
XV.  Normal  Relative  Humidity  of  the  United  States  for  January. 

XVI.  Normal  Relative  Humidity  of  the  United  States  for  July. 

XVII.  Normal  Date  of  the  First  Killing  Frost  of  the  Autumn. 

XVIII.  Normal  Date  of  the  Last  Killing  Frost  of  the  Spring. 

XIX.  Sunshine  of  the  United  States  for  January. 

XX.  Sunshine  of  the  United  States  for  July. 

XXI.  Cloudiness  of  the  World  for  the  Year. 

XXII.  Normal  Annual  Precipitation  for  the  World. 

XXIII.  Normal  Annual  Precipitation  for  the  United  States. 

XXIV.  Normal  Precipitation  for  the  United  States  for  January,  February,  and 

March. 

XXV.     Normal  Precipitation  for  the  United  States  for  July,  August,  and  September. 
XXVI.     Normal  Annual  Snowfall  for  the  United  States. 
XXVII.     Weather  Map,  8  A.M.,  Sept.  8,  1900,  showing  Galveston  Hurricane. 
XXVIII.     Weather  Map,  8  A.M.,  Dec.   30,   1907,   showing  a  Typical  Extratropical 

Cyclone  or  Low. 
XXIX.     Weather  Map,  8  A.M.,  March  3,  1904,  showing  a  Typical  Extratropical 

Cyclone  or  Low  with  a  Pronounced  Wind  Shift  Line. 
XXX.     Weather  Map,  8  A.M.,  April  23,  1906,  showing  a  Typical  Anticyclone  or 

High. 

XXXI-XXXIV.  Four  Weather  Maps  Illustrating  Distribution  of  the  Meteorological 
Elements,  Tracks  followed,  and  Velocity  of  Motion  for  Highs  and  Lows 
(Jan.  30,  31,  Feb.  1,  2,  1908). 

1  The  fifty  charts  are  placed  together  at  the  end  of  the  book. 
XV 


XVI 


CHARTS 


CHART 

XXXV.     Weather  Map,  8  A.M.,  Sept.  2,    1904,  showing  the   Presence    of   Many 

Thundershowers  in  the  Southern  Quadrants  of  the  Low. 
XXXVI.     Weather  Map,  3  P.M.,  March  11,  1884,  showing  a  Low  which  gave  rise  to 

Tornadoes. 
XXXVII.     Weather  Map,  8  A.M.,  April  25,  1906,  showing  a  Low  which  gave  rise  to  a 

Tornado. 

XXXVIII-XL.     Three  Weather  Maps  to  illustrate  Forecasting  (Dec.  8,  9,  10,  1907). 
XLI-XLII.     Two  Weather  Maps  to  illustrate  the  Founding  of  a  Low  from  a  V-shaped 

Depression  (Dec.  27,  28,  1904). 
XLIII-XLVIII.     Six  Weather  Maps  illustrating  Forecasting  by  Similarity  (Jan.  9, 10, 

14,  15,  27,  28,  1908). 

XLIX.     Weather  Map  illustrating  Cold  Wave  Prediction  (Jan.  6,  1909). 
L.     Weather  Map  of  the  Northern  Hemisphere  for  Jan.  28,  1910. 


METEOROLOGY 


METEOROLOGY 


CHAPTER  I 

INTRODUCTION  — THE  ATMOSPHERE 
INTRODUCTION 

The  science  of  meteorology,  i. 

Outline  history  of  meteorology,  2,  3. 

Utility,  4-6. 

Relation  of  meteorology  to  the  other  natural  sciences,  7. 

THE  ATMOSPHERE 

The  atmosphere  and  its  properties,  8. 
Composition  of  the  atmosphere,  9-14. 
Offices  and  activities  of  the  atmosphere,  15,  16. 
Atmospheres  of  other  heavenly  bodies,  17. 
Evolution  of  the  atmosphere,  18,  19. 
Future  of  the  atmosphere,  20. 

THE  PRESSURE  AND  HEIGHT  OP  THE  ATMOSPHERE 

Gravity  and  its  effects,  21. 

Geosphere,  hydrosphere,  atmosphere,  22,  23. 

Height  of  the  atmosphere,  24. 

THE  METEOROLOGICAL  ELEMENTS 

The  meteorological  elements,  25. 

Weather  and  climate,  26. 

Periodic  and  irregular  variation  and  normal  value,  27,  28. 

Graphical  representation,  29. 

THE  THREE  METHODS  OF  INVESTIGATION,  30. 
THE  PLAN  OF  THE  BOOK,  31. 

INTRODUCTION 

i.   The  science   of  meteorology.  —  The  natural  sciences   deal  with 
the  phenomena  of  the  world  of  nature  about  us.     As  examples   of 
these  varied  phenomena   one  might  mention  the  fall  of  a  Definition 
stone,  the  rusting  of  iron,  the  growth  of  a  plant,  the  change   of  a  natural 
in  the  phases  of  the  moon,  the  formation  of  a  cloud,  or  the   £ 
erosion  of  a  valley  by  a  stream.     These  are  all  occurrences  in  or  phe- 
nomena of  this  world  of  nature,  and  it  is  thus  the  province  of  some  one 
of  the  natural  sciences  to  treat  fully  each  one  of  these  phenomena. 

1 


2  METEOROLOGY 

Since  these  phenomena  are  so  numerous,  complex,  and  varied,  they 

are  divided  among  several  natural  sciences.      Physics  and  chemistry 

are   the   two   fundamental   natural  "sciences    because   they 

Enumera- 
tion of  the  treat  matter  and  energy,  the  two  components  of  the  mate- 
natural  rjaj  worid^  in  the  abstract.  They  are  also  fundamental  be- 
cause so  many  of  their  facts,  laws,  principles,  and  methods 
are  usexLjn  the  other  sciences.  Biology  with  its  many  subdivisions 
includes  all  those  phenomena  where  life  is  involved.  The  remaining 
phenomena,  those  of  inanimate  matter,  are  divided  up  between  astron- 
omy, meteorology,  and  geology ;  astronomy  treating  the  heavenly  bodies 
beyond  the  earth,  meteorology  the  earth's  atmosphere  or  envelope  of 
gas,  and  geology  the  earth  itself. 

(^Meteorology  is  thus  one  of  the  natural  sciences,  since  it  has  a  group  of 
phenomena  to  investigate.  It  treats  of  the  condition  of  the  atmosphere, 
Definition  of  ^s  changes  of  condition,  and  the  causes  of  these  changes, 
meteorol-  Its  duty  is  to  arrange  the  facts  in  an  orderly  way  so  that 
the  relation  of  cause  and  effect  can  be  traced  and  generaliza- 
tions can  be  formed.  Meteorology  is  often  defined  briefly  as  the  study 
of  atmospheric  phenomena.  Since  so  many  of  the  laws  and  principles 
of  physics  find  their  application  in  the  atmosphere  on  a  stupendous 
scale,  meteorology  is  also  often  defined  as  the  physics  of  the  atmosphere. 

2.  Outline  history  of  meteorology.  --  The  annual  change  from 
summer  to  winter  and  back  again  from  winter  to  summer,  with  all  the 
Meteorol-  attendant  changes  in  vegetation,  the  daily  change  from  the 
ogy  an  old  heat  of  the  day  to  the  cool  of  the  night,  the  falling  of  rain 
and  snow,  the  coming  of  a  thunder  shower,  all  such  things 
must  have  profoundly  occupied  the  mind  of  man  from  remote  ages. 
The  antiquity  of  many  weather  proverbs  and  of  much  of  our  weather 
lore  also  shows  that  meteorological  observations  and  generalizations 
were  among  the  first  acts  of  an  intelligent  race.2 

The  word  meteorology  comes  from  the  Greek.  Socrates  is  spoken 
On  'n  of  the  °^  m  one  °^  P^o's  dialogues  as  "  a  wise  man,  both  a  thinker 
word  mete-  on  supra-terrestrial  things  and  an  investigator  of  all  things 
oroiogy.  beneath  the  earth."3  The  word  for  supra-terrestrial  fur- 

1  The  abbreviation  used  by  the  U.S.  Weather  Bureau  for  meteorological  is  met'l, 
and  this  form  will  be  used  in  this  book  whenever  the  word  is  abbreviated. 

J  In  the  British  Museum  in  London  there  are  Babylonian  clay  tablets  dating  from 
about  4000  B.C.  which  contain  weather  proverbs.  One,  for  example,  reads :  "When  a  ring 
surrounds  the  sun,  then  will  rain  fall."  See  HELLMANN,  "The  Dawn  of  Meteorology," 
Quarterly  Journal  of  the  Royal  Met.  Society,  October,  1908,  vol.  XXXIV,  No.  148,  p.  227. 

3  Sw/cpdtTTjs,  <ro<t>&s   &VTIP,  "rdre  /ueT^wpa  <J>pojTi<7T7js,  ical  T&.  virb  7775 
Apologia  Socratis,  cap.  II. 


INTRODUCTION  3 

nishes  the  root  for  the  word  meteorology.1  About  fifty  years  later 
(350  B.C.)  Aristotle  wrote  the  first  treatise  2  on  meteorology,  consist- 
ing of  four  books  or  parts,  containing  a  large  amount  of  information  of 
very  mixed  value.  Not  only  were  the  things  considered  to-day  under 
the  term  meteorology  treated  in  this  book,  but  the  appearance  of  the 
stars^  comets,  meteors,  earthquakes,  northern  lights,  the  composition  of 
matter,  and  other  things  as  well. 

3.  Four  periods  may  be  recognized  in  the  history  of  meteorology. 
The  first  lasts  from  antiquity  until  about  1600  A.D.     The  observations 
were  crude,  nearly  all  made  without  the  help  of  instruments,  The  four 
and  they  were  often  inaccurate  and  much  influenced  by  periods  in 
superstition  and  imagination.    'The  explanations  were  often  ^e  history 
fantastic  and  supernatural.     ^*  ogy  and 

The  coming  of  the  second  period  was  brought  about  by  theiTch&T~ 
,,       .  ,.          e  .  :,.  ..  „,       acteristics. 

the  invention  of  instruments  for  making  observations.     The- 

two  most  important  instruments  in  meteorology  were  invented  at  about 
this  time,  the  thermometer  in  1590  by  Galileo  and  Sanctorius  of  Padua, 
and  the  barometer  in  1643  by  Torricelli  ;  and  in  1653  Ferdinand  II, 
Grand  Duke  of  Tuscany,  established  stations  throughout  northern  Italy 
for  making  careful  observations  of  meteorological  phenomena.  The 
chief  characteristics  of  this  period  are  a  larger  number  of  observations 
and  a  great  increase  in  accuracy. 

The  third  period  begins  a  little  before  1800  and  runs  until  about 
1850.  Its  great  characteristic  is  the  attempt  to  give  logical  explana- 
tions for  the  various  phenomena  which  Were  now  being  observed  with 
ever  increasing  accuracy. 

The  fourth  period  is  the  modern  period,  and  it  began  about  1850. 
'  >r,tly  after  this  the  various  governments  began  establishing  weather 
bureaus,  interest  in  meteorology  increased  markedly,  and  great  advances 
began  to  be  made.  The  three  great  characteristics  of  this  period  are  : 
ceasehss  activity  in  gaining  information,  the  utmost  accuracy  in  ob- 
serving all  possil  e  phenomena,  and  the  rigid  testing  of  all  explanations 
and  hypotheses. 

4.  Utility-  •      The   utility  of  meteorology  may  be  noted   in  two  en- 
tirely different  directions.     First,  there  is  the  financial  saving  caused 
by  the  thflely  forecast  :-->g  of  tho^e  weather  conditions  which  Two  Hnes  of 

do  damage  to  commerce,  agriculture,  products  in  transit,  usefulness. 

. 

~  ^upra-tenv-      .:.  \67ot  =  description  or  treatise.     Thus  meteorology  was 


...  rial  thing 

•2  T><"  °-  v;is  TO.  u,(.~f'  ••••    *   - 


4  METEOROLOGY 

and  businesses  of  many  sorts;  and,  secondly,  there  is  the  educational 

advantage  in  the  study  of  meteorology.     The  storms  on  the  coast  and 

also  those  on  the  lakes  and  rivers  do  immense  damage  to  shipping  and 

commerce  and  often  cause  an  appalling  loss  of  life.     The  late 

saving  by       f rosts  of  spring  and  the  early  frosts  of  autumn  work  havoc  with 

the  farm  produce  which  may  be  exposed.     Sudden  changes 

in  temperature  from  hot  to  cold  or  from  cold  to  hot 


cause  the  complete  loss  of  produce  or  merchandise  which  is  being  trans-  j 
ported  by  boat  or  train  from  one  point  to  another.  Traffic  on  railroad 
and  street  car  lines  may  be  delayed  or  even  stopped  entirely  by  a  heavy, 
unexpected  fall  of  snow.  The  timely  warning  of  the  approach  of  these 
damage-causing  weather  conditions  allows  every  precaution  to  be  taken 
and  has  resulted  in  a  tremendous  financial  saving.  The  total  cost 
in  maintaining  the  Weather  Bureau  now  amounts  to  about  a  million 
and  a  half  dollars,  while  the  most  conservative  estimates  place  the  saving 
to  this  country  brought  about  by  timely  forecasts  at  many  times  the  cost 
of  maintaining  the  bureau. 

Mark  W.  Harrington,  a  former  chief  of  the  United  States  Weather 
Bureau,  says  in  the  preface  of  his  book,  About  the  Weather  :l  "  The 
Harrington's  more  than  twelve  hundred  thousand  dollars  expended  every 
opinion.  year  by  the  Government  may  perhaps  be  considered  an  ex- 
orbitant price  to  pay  for  learning  what  weather  we  are  likely  to  have  for 
the  coming  twenty-four  hours,  but  the  truth  is  that  no  public  investment 
is  so  immediately  and  so  immensely  profitable  as  that  applied  to  the  main- 
tenance of  the  Weather  Bureau. 

"  Not  only  are  cyclones  in  the  West,  but  the  floods  of  streams  and 
rivers,  especially  those  of  the  Mississippi  Valley,  are  foretold,  '  and*  in- 
calculable saving  of  life  and  property/  as  Mr.  J.  E.  Prindle  says  in  ? 
port  on  the  subject,  'results  from  their  warnings.     Before  the  daysof  the 
bureau/  he  proceeds, '  the  West  India  hurricanes  came  unannounced,  and 
sometimes  two  thousand  lives  were  lost  in  a  single  storm, 
warnings  of  the  Weather  Bureau  three  such  storms  have  parsed  in 
succession  without  the  loss  of  a  single  life,  and  the  propen  /ed 

in  one  storm  would  support  the  service  for  two  years, 
in  the  winter  of  1895-1896,  by  forecasting  six  very  g 
hundred   and   fifty  vessels,  valued   at   seventeen  million  dollars 
carrying  eighteen  hundred  persons,  were  held  safe      in  port         ' 
warnings/  ' 

1  Reprinted  from  HABKI^OTON'S  About  the  Weather,  copyright,  1899.,  by  D.  Appi. 
and  Company. 


INTRODUCTION  5 

5.  There  is  probably  no  subject  which  so  fully  occupies  our  attention 
and  is  of  so  much  importance  to  us  as  the  weather.     And  yet  there  is 
probably  no  subject  about  which  the  ordinary  person  knows  The  ^^^ 
less  and  where  ignorance  and  superstition  are  more  universal  bility  of  an 
than  in  connection  with  the  weather  and  the  causes  of  its  edj^ated 

public* 

changes.     Meteorology  has  long  since  dispelled  the  mystery 
connected  with  weather  and  weather  changes.     It  is  thus  the  duty  of  | 
our  schools  and  colleges  to  produce  an  educated  public  which  can  dis-  ' 
tinguish  between  truth  and  error,  fact  and  superstition.     It  is  also  a  real 
pleasure  to  know  the  causes  of  the  changes  and  to  appreciate  the  mechan- 
ism back  of  the  ever  shifting  panorama  which  constitutes  our  weather. 
There  are  many  facts  to  be  arranged  in  an  orderly  way  so  that  the 
relation  of  cause  and  effect  can  be  traced.     Many  observations  have  been 
made  so  that  generalizations  and  laws  can  be  derived.     Mete- 
orology thus  has  a  well-developed  descriptive  side  which  offers  tionai  value 
(training  in  exactness  of  statement  and  description  and  in  ofmeteor- 
logical  reasoning.     It  also  has  a  mathematical  side  which  is 
being  developed  more  and  more  at  the   present  time.     This   develops 
extreme  exactness  in  thought  and  expression  along  abstract  lines.     Ap- 
paratus must  be  tested  and  improved,  and  there  are  many  observations 
to  be  made.     It  thus  has  a  laboratory  side  which  gives  increased  familiar- 
ity with  the  details  of  a  subject,  sense  training,  and  skill  in  manipulation. 
Meteorology  thus  offers  to  the  student  training  along  three  different  lines, 
and  these  are  the  three  kinds  of  training  offered  by  any  one  of  the  natural 
sciences. 

6.  In  discussing  the  utility  of  meteorology  attention  should  be  called 
to  how  intimately  the  weather  enters  into  every  aspect  of  human  life. 
Our  plans  are  daily  made  and  unmade  on  account  of  the 
weather.     Our  very  moods  depend  upon  the  weather.     As  influences 
mentioned  above,  untold  damage  to  property  and  even  loss  of  of  th® 

life  may  be  caused  by  storms,  frosts,  hail,  lightning,  and 
sudden  changes  in  temperature.  The  student  of  criminology  tells  us  that 
certain  crimes  are  more  prevalent  at  one  season  of  the  year  than  at  an- 
other or  with  one  particular  type  of  weather.  Theft,  for  example,  is 
more  common  during  the  winter,  and  the  cause  is  not  far  to  seek.  During 
the  winter  there  is  less  employment  than  during  the  summer,  and  incomes 
are  smaller.  In  addition  the  cold  makes  food,  clothing,  and  shelter  more 
imperative.  The  result  is  a  decided  seasonal  variation  in  this  form  of 
crime.  The  physician  tells  us  that  certain  diseases  show  a  well-marked 
seasonal  variation  and  are  more  prevalent  witbt  one  kind  of  weather  than 


6  METEOROLOGY 

another.  Meteorological  statistics  are  being  made  use  of  more  and 
more  by  lawyers  not  only  in  damage  cases  but  also  in  criminal  cases.  A 
single  illustration  will  suffice.  A  certain  burglary  case  turned  largely  on 
the  certain  identification  of  a  person  seen  to  come  from  a  building  during 
the  early  hours  of  the  morning.  The  observer  was  in  a  near-by  building 
across  the  street.  It  was  put  in  evidence  by  the  defense  that  on  the  morn- 
ing in  question,  according  to  the  observations  of  the  Weather  Bureau, 
a  dense  fog  hung  over  the  city  so  that  positive  identification  at  the  dis- 
tance in  question  would  have  been  impossible. 

7.  Relation  of  meteorology  to    the    other    natural    sciences.  —  The 
relation  of  meteorology  to  physics  is  a  close  one.     Many  of  its  terms  and 
Relation  of     ^ac^s  are  used,  and  so  many  of  its  laws  and  principles  find  their 
meteorology    application  on  a  large  scale   in  the  atmosphere   that  mete- 
to  physics.      oroiOgy  is  sometimes  defined  as  the  physics  of  the  atmosphere. 
A  course  on  elementary  physics  at  least  should  precede  a  study  of  mete- 
orology.    Such  terms  as  the  following  must  be  used  from  time  to  time,| 
and  but  little  space  can  be  given  to  defining  or  illustrating  them  :  mass,  t 
volume,  density,  velocity,  acceleration,  rotation,  revolution,  force,  in- 
ertia,  centrifugal  force,  gravitation,  gravity,   weight,   pressure;   atom, 
molecule,   ether,  solid,  liquid,  gas;    sound;  heat,  temperature,  expan- 
sion, specific  heat,  latent    heat,    conduction;    light,    reflection,    trans- 
mission,   absorption,  radiant  energy;  magnetism,  electricity. 

But  two  groups  of  facts  are  borrowed  from  astronomy.  They  are,  first, 
the  facts  concerning  the  position  of  the  earth  with  reference  t9  the  sun 
Relation  of  at  different  times  of  year,  and,  secondly,  the  facts  concerning 
tTt'hTnatu-  tne  rotation  of  the  earth  on  its  axis.  These  will  be  presented 
rai  sciences,  at  the  appropriate  place.  (See  section  36.) 

Meteorology  touches  chemistry  in  but  one  place,  namely,  in  the  discus- 
sion of  the  composition  of  the  atmosphere. 

Almost  no  facts  or  principles  are  taken  from  biology  or  geology. 

THE  ATMOSPHERE 

8.  The  atmosphere  and  its  properties.  —  The  atmosphere  *  may  be 
defined  as  the  envelope  of  gas  surrounding  the  earth.     It  is  an  odorless, 
Definition      colorless,  tasteless  gas  and  when  at  rest  one  might  almost 
oftheatmos-  doubt  its  substantiality.     When  it  is  in  motion,  however, 
ph.ere>  in  the  form  of  wind,  and  hinders  walking  and  even  overturns 
trees  and  houses,  there  can  be  no  doubt  of  its  existence. 

1  From  the  Greek :  dr/*6s=  vapor  or  gas  ;  and  <r0cu/m=  sphere. 


INTRODUCTION 


Since  air  is  a  gas,  it  must  have  all  the  physical  properties  of  gases. 
Some  of  these  properties  which  will  be  met  with  later  are  the  following  : 
(1)  A  given  quantity  of  air  will  occupy  all  the  space  open  to  Properties 
it.     (2)  If  allowed  to  expand,  it  will  become  cooled ;  and  con-  °fasas- 
versely,  if  compressed,  it  will  be  heated.     (3)  If  the  temperature  is  kept- 
constant,  the  volume  of  a  given  quantity  of  air  will  vary  inversely  as  the 
pressure,  a  larger  pressure  thus  producing  a  smaller  volume.     (4)  If  the 
temperature  changes,  either  the  volume  or  the  pressure  will  change  ac- 
cording as  the  other*  is  kept^constant.1 

One  cubic  centimeter2  of  pure  dry  air  under  standard  conditions3  has 
a  mass  of  0.0012927  gram.  In  the  English  system  of  units  it  requires 
about  13  cubic  feet  for  1  pound. 

9.    Composition  of  the  atmosphere.  —  The  four  major  constituents  of 
pure  dry  air  in  order  of  amount  are  nitrogen  (N),  oxygen  (O),  argon  (Ar), 
and  carbon  dioxide  or  carbonic  acid  gas  (CO2).     The  f611ow7  The  four 
ing  table  shows  the  percentage  composition  bv  volume  and  bv  maJ°r  com~ 

J     ponents  of 

weight.     The  atomic  weights  are  also  added.  air. 


N 

O 

Ar 

C02 

Vol.  % 

78.04 

20.99 

0.94 

0.03 

Weight  % 

75.46  . 

23.19 

1.30 

0.05 

Atomic  weight 

14.04 

16.00 

39.9 

i 


The  various  determinations  made  by  different  investigators  with  different 
apparatus  and  in  different  parts  of  the  world  will,  of  course,  differ  slightly 
from  each  other  and  from  the  above  figures.  This  is  due  to  errors  of 
observation  and  perhaps  to  a  very  small  real'difference  in  the  composition 
of  the  air.  The  first  decimal  place  in  the  case  of  both*N  and  O  is  certainly 
correct.  •* 

Hydrogen  is  also  one  of  the  permanent  constituents  of  the  atmosphere, 
but  the  quantity  is  extremely  small  at  the  earth's  surface.     It  is  now 
usually  assumed  to  be  about  0.01  per  cent  by  volume,  but  it  The  rarer 
was  formerly  considered  to  be  less  than  this  and  thus  not   components, 
considered  among  the  major  components  of  the  air. 

1  The  relation  between  volume,  pressure,  and  temperature  is  expressed  by  the  formula 
PV  =  RT,  where  P  denotes  the  pressure,  V  the  volume,  and  T  the  absolute  temperature 
reckoned  from  273  below  zero  Centigrade  or  491  below  the  freezing  point  Fahrenheit.     R  is 
a  constant  whose  value  depends  upon  the  units  chosen,  and  the  starting  points. 

2  The  use  of  the  metric  system  cannot  be  avoided.     For  the  tables  and  equivalents  see 
Appendix  I. 

3  Standard  conditions  arp  r»™»««— -  **»*  —  ™  QM  standard  gravity. 


8  METEOROLOGY 

Argon  was  first  separated  from  nitrogen  by  Rayleigh  and  Ramsay  in 
1894,  and  it  is  now  known  that  what  was  formerly  considered  argon  was 
not  a  single  substance,  but  had  several  other  gases  mixed  with  it.  Helium, 
neon,  krypton,  and  xenon  have  been  separated  from  it.  The  amount 
of  these  rare  gases  in  the  atmosphere  is  about  4.1,  12,  .05,  and  .006 
parts  in  a  million  respectively. 

Air  is  a  mechanical  mixture  of  its  component  gases  and  not  a  chemical 
compound.  This  is  proved  by  the  following  considerations :  (1)  The 
Air  a  me-  relative  proportions  of  the  components  are  not  those  of  their 
chanical  combining  or  atomic  weights.  (2)  When  liquid  air  is  allowed 
to  boil  off,  some  components  leave  faster  than  others  so  that 
the  percentage  composition  changes.  {3)  When  the  components  are 
mixed  together,  no  change  in  temperature  or  volume  occurs.  (4)  The 
index  of  refraction  has  the  average  value  of  its  components,  and  not  a 
unique  value.  (5)  The  composition  of  air  dissolved  in  a  liquid  is  not  the 
same  as  that  of  the  free  air. 

10.   The  composition  of  the  air  is  remarkably  constant.     In  the  case 

of  oxygen,  for  example,  all  the  reliable  determinations  which  have  ever 

been  made  fall  between  20.81  and  21.00  vol.  per  cent.     In  the 

Constancy 

of  the  com-  case  of  carbon  dioxide  the  amount  varies  from  0.036  to  0.0304 
ponentsof  according  to  different  determinations.  In  closed  rooms  the 
amount  of  CO2  is  of  course  larger;  0.07  per  cent  is  usually 
considered  the  limit  of  good  ventilation.  In  closed  rooms  occupied  by 
many  people,  particularly  in  sleeping  rooms,  values  from  0.24  to  even 
0.95  have  been  found.  It  is  a  popular  mistake  to  think  that  the  air  ex- 
haled from  human  lungs  contains  a  very  large  amount  of  CO2.  The  fol- 
lowing table  gives  the  percent  composition  of  inhaled  and  exhaled  air : 


INHALED 

EXHALED 

N 

78.04 

78.04 

0 

20.99 

16.03" 

Ar 

0.94' 

0.94 

C02 

0.03 

4.40 

It  will  be  seen  that  exhaled  air  contains  less  than  five  per  cent  of  CO2. 

There  are  two  reasons  why  the  composition  of  the  air  remains  so 

Reasons  for    neal>ly  constant.     (1)  The  air  is  very  mobile.     It  is  being 

the  con-         constantly  mixed  and  transported  great  distances  by  the 

wind.     (2)   Gases   diffuse   easily,  so   that   any  irregularity 

would  quickly  eliminate  itself  even  if  there  were  no  wind. 


INTRODUCTION 


9 


1 1 .  Although  the  composition  of  the  air  is  so  uniform  all  over  the  earth's 
surface  and  to  any  height  at  which  man  can  live,  this  is  far  'from  the  case 
when  great  heights  above  the  earth's  surface  are  considered.  Com  og. 
Since  air  is  only  a  mechanical  mixture  of  its  gaseous  coin-  tion  at  great 
ponents,  each  behaves  as  if  the  others  were  not  present.^  This  heishts-  . 
means  that  the  heavier  gases  will  be  held  closer  to  the  earth's  surface,  and 
the  lighter  components  will  predominate  at  great  heights^  This  is,  of 
course,  based  on  the  assumption  that  the  atmosphere  is  not  mixed  by  the 
wind,  but  that  each  gas  is  free  to  distribute  itself  in  accordance  with  its 
density.  Accordingly,  from  a  knowledge  of  the  composition  of  air  at  the 
earth's  surface  and  the  temperature  at  different  elevations,  the  percentage 
composition  at  any  height  can  be  computed.  The  following  table  com- 
puted by  Humphreys l  gives  the  percentage  composition  at  various 
heights.  In  Fig.  1  these  results  are  shown  graphically. 


HEIGHT  IN 
KILOMETERS 

N 

0 

Ar 

C02 

H 

HELIUM 

5 

77.89 

20.95. 

0.94 

0.03, 

O.OL- 

o.oo   • 

15 

79.56 

19.66 

0.7l 

0.02 

o.oe 

0.00* 

30 

84.48 

15.10 

0.22 

0.00 

0.20 

0.00 

50 

86.16 

10.01 

0.08 

0.00 

3.72 

0.03 

80 

22.70 

1.38 

0.00 

0.00 

75.47 

0.45 

100 

1.63 

0.07 

0.00 

0.00 

97.84 

0.46 

150 

0.00 

0.00 

0.00 

0.00 

99.73 

0.27 

It  will  be  seen  that  at  the  height  of  150  Jdlometers,  or  less  than  100  miles, 
the  atmosphere  should  be  composed  almost  entirely  of  hydrogen  with  a 
little  helium.  This  is  in  good  agreement  with  the  observed  fact  that 
the  spectrum  of  shooting  stars' shows  prominently  the  hydrogen  and 
helium  lines  (section  24).  The  outside  of  the  atmosphere  of  the  sun  is 
also  composed  largely  of  hydrogen  and  helium. 

12.   The  minor  constituents  of  the  atmosphere,  often  spoken  of    as 
impurities,  are  water  vapor,  nitric  acid,  sulfuric  acid,  ozone,  organic 
and  inorganic  particles,  and  minute  traces  of  several  other  The  minor 
things.     Of  these  the  water  vapor  and  the4  particles  and  per-  £?  tile*!^*8 
haps  ozone  are  the  only  ones  which  deserve  attention.  mosphere. 

The  amount  of  water  vapor  in  the  atmosphere  is  always  small,  as  it 
never  exceeds  4  per  cent  and  the  amount  is  constantly  changing  with 
every  change  in  the  weather.      Yet  it  is  one  of  the  most  water 
inmortaTvt    components,    for    without    it    both    plant  and  vaP°r- 

1  M.Mint  Weather  Bulletin,  vol.  II,  p.  66,  1909. 


10 


METEOROLOGY 


animal  life  would  be  an  impossibility.  The  discussion  of  its  various 
forms,  such  as  dew,  frost,  fog,  cloud,  rain,  hail,  and  snow  must  form  a 
large  portion  of  every  book  on  meteorology. 


405 

10        20        30      ^40        60        80        70        80        90       10078° 
VOLUME  PER  CENT. 


FIG.  1.  —  Composition  of  the  Atmosphere  at  Various  Heights. 
(After  HUMPHREYS  in  Mount  Weather  Bulletin,  Vol.  II.)  \ 

13.  The  organic  particles  include  bacteria  and  the^BQres  of  plants,  and 
these  minute  organisms  are  scattered  throughout  t'he  atmosphere.  It  is 
Organic  estimated  that  even  on  high  mountains  and  over  the  oceans 
particles.  there  is  at  least  one  in  every  cubic  meter  of  air.  In  the  streets 
of  our  cities  the  number  probably  runs  up  to  3000  per  cubic  meter  and  in 
crowded  houses  and  hospital  wards  it  probably  reaches  at  least  80,000. 


INTRODUCTION 


11 


The  Inorganic  particles  are  usually  spoken  of  as  dust.     These  dust 
particles  are  much  more  numerous  than  the  organic  particles  and  are  for 
the  most  part  entirely  invisible  to  the  naked  eye.     A  few  of 
the  giant  members  of  the  family  may  be  seen  when  carried  up 
by  the  wind  from  the  earth's  surface  or  when  a  beam  of 
sunlight  falls  through  a  small  opening  into  a  darkened  room.     At  the 
present  time  the  number  of  these  particles  in  a  given  volume  can  be 
determined  by  means  of  Aitken's  dust  counter.     This  in-  jy^^ig 
genious  instrument,  as  illustrated  in  Fig.  2,  consists  of  a.  dust 
pump  P  by  means  of  which  the  air  in  the  box  B  may  be-  cc 
suddenly  rarefied.      This  is  provided  with  a  graduated  part  G  at  the 
lower  end  so  that  a  known  quantity  of  air  may 
be  withdrawn  from  B  if  desired.     The  box  B  is 
about  a  centimeter  thick  with  other  dimensions 
in  proportion.     At  the  bottom  is  a  glass  plate 
divided  by  lines  usually  into  square  millimeters 
and  illuminated  by  the  mirror  M.    At  the  top 
is  a  lens  L  for  viewing  and  magnifying  the  spaces 
on  the  glass  plate.     At  the  sides  are  pieces  of 
filter  paper  saturated  with  water.     There  are 
two    stop    cocks    at   C  and   Cf .     The  working 
principle  of  the  apparatus  is  this :    Whenever 
the  air  in  B  is  suddenly  rarefied,  it  becomes 
colder  and  can  no  longer  hold  all  the  water 
vapor  in  it.      (See   section   183.)     This  water 
vapor  collects  on  the  dust  particles  and  forms  a 
fog  which  slowly  settles  and  collects  on  the  glass 
plate.     The  number  of  these  small  water  drops 
can  then  be  counted,  and.  this  is  the  number  of 
dust  particles  which  were  present.     The  method 
of  conducting  this  experiment  is  at  once  ap- 
parent.    By  repeated  rarefactions  all  dust  must 
be  removed  from  the  box  B.     A  known  quantity 
of  the  air  to  be  investigated  is  then  introduced. 
The  instrument  is  shaken  so  as  to  mix  the  air 


FIG.  2.  —  Aitken's  Dust 
Counter. 


with  the  dust-free  air  already  in  the  box  and  saturate  it  with  moisture. 
The  next  rarefaction  will  coat  the  introduced  dust  particles  with  water 
and  cause  them  to  collect  on  the  glass  plate,  where  they  may  be 
counted. 

determinations  have  been  made  with  this  or  similar  instruments 


12 


METEOROLOGY 


in  all  parts  of  the  world  and  at  various  heights  on  mountains  and  under 
The  amount  many  conditions.  The  following  table  gives  the  results  ob- 
ofdustun-  tained  by  Fridlander  for  the  various  oceans  and  at  various 

der  various      .     .    ,  ,,      -r^.     , 

conditions,      heights  on  the  Bieshorn : 


DCST  PARTICLES 

DCST  PARTICLES 

PER  CUBIC 

ELEVATION 

PER  CUBIC 

CENTIMETER 

CENTIMETER 

Atlantic  Ocean      

2,053 

6,700 

950 

Pacific  Ocean                        .     . 

613 

8,200 

480 

Indian  Ocean         

512 

8,400 

513 

10,665 

406 

11,000 

257 

13,200 

219 

13,600 

157 

In  a  dusty  city  100,000  dust  particles  per  cubic  centimeter  is  by  no  means 
large v and  it  has  been  found  that  a  single  puff  of  cigarette  smoke  contains 
about  4,000,000,000  particles. 

There  are  four  chief  sources  of  the  dust  of  the  atmosphere.  (1)  It  is 
blown  up  from  the  earth  by  the  wind.  (2)  It  is  injected  into  the  atmos- 
Sources  of  phere  by  volcanoes  while  in  eruption.  During  the  volcanic 
dust.  explosion  in  Krakatoa  between  Sumatra  and  Java  in  1883  it  is 

estimated  that  dust  and  steam  were  thrown  into  the  air  to  a  height  of 
nearly  twenty  miles,  and  the  presence  of  this  dust  could  be  detected  in 
sunset  colors  all  over  the  world  for  more  than  three  years.  (3)  Shoot- 
ing stars  put  an  immense  quantity  of  dust  into  the  upper  atmosphere  as 
a  result  of  their  combustion  and  disintegration.  (4)  Ocean  spray  when 
evaporated  adds  dust  to  the  atmosphere,  especially  fine  particles  of  salt. 

Atmospheric  dust  plays  an  important  part  in  at  least  four  ways. 
(1)  It  is  one  of  the  chief  causes  of  haze.  (2)  It  probably  serves  as  cen- 
Effects  of  ters  of  condensation  for  all  fog  particles  and  raindrops.  It 
dust.  was  once  thought  that  condensation  was  impossible  without 

it.  (3)  It  is  the  cause  of  the  sunrise  and  sunset  colors  and  perhaps 
of  the  blue  color  of  the  sky.  (4)  It  is  the  cause  of  twilight.  These 
various  effects  of  the  dust  will  be  fully  considered  later. 

14.  Ozone  is  a  peculiar  form  of  oxygen,  in  that  its  molecule  is  composed 
of  three  atoms  of  oxygen  linked  together,  while  the  ordinary  oxygen  mole- 
Properties  of  cule  consists  of  two.  This  third  atom  has  a  tendency  to 
ozone.  leave  the  molecule,  and  to  this  is  due  the  oxidizing  power 

of  ozone  and  its  value  as  a  sanitary  agent.  Ozone  is  usually  con- 


INTRODUCTION  13 

sidered  a  powerful  disinfectant  and  is  supposed  to  be  particularly 
useful  where  organic  matter  is  decomposing.  It  is  even  stated  by 
some  that  the  bracing  effect  of  certain  climates  and  of  certain 
types  of  weather  is  due  to  a  slightly  larger  amount  of  ozone  present 
in  the  air.  In  this  last  instance,  however,  a  lower  temperature,  the 
absence  of  moisture,  and  an  increase  in  the  amount  of  electricity  in 
the  air  probably  play  a  far  more  important  part  in  determining  one's 
feelings  than  any  change  in  the  amount  of  ozone. 

The  discovery  of  ozone  is  usually  attributed  to  Schonbein  in  1848,  al- 
though a  few  investigators  probably  detected  its  peculiar  odor  earlier. 
It  is   this   peculiar   odor  which   gives   it  its   name.1     The  The  quan_ 
amount  present  in  the  atmosphere  is  extremely  small,  usually  tity  of 
about  one  part  in  a  million.     The  amount  shows  a  decided  ° 
daily  and  annual  variation  and  irregular  fluctuations  which  are  closely 
correlated  with  the  type  of  weather.     There  is  much  more  during  the 
winder  than  during  the  summer. 

The  quantity  of  ozone  present  in  the  air  is  determined  by  means  of  its 
oxidizing  power  on  certain  chemical  compounds.  Potassium  iodide 
is  ordinarily  used,  and  a  known  quantity  of  this  substance  is  added  to  a 
paste  formed  by  dissolving  a  known  quantity  of  starch  in  water.  This 
is  then  spread  on  pieces  of  paper  and  exposed  to  the  air  a  known  length  of 
time.  From  the  depth  of  the  blue  color  which  results  from  the  decom- 
position of  the  potassium  iodide,  the  amount  of  ozone  is  estimated.  The 
determinations  are  none  too  accurate,  even  when  a  standard  paste  and  a 
standard  color  scale  are  used,  as  other  things  affect  the  decomposition  of 
the  potassium  iodide  slightly. 

Ozone  is  formed  in  the  laboratory  by  allowing  electricity  to  discharge 
through  oxygen  or  air,  and  here  the  peculiar  odor  can  be  Formation 
readily  detected.      In  nature  it  may  be  formed  by  electrical  of  ozone- 
discharges,  or  by  the  action  of  ultra-violet  light  on  oxygen,  or  possibly 
in  connection  with  the  evaporation  of  water. 

15.    Offices  and  activities  of  the  atmosphere  and  its  components.  — 
The  atmosphere  as  a  whole  has  the  following  offices  and  activities: 
(1)  It  disseminates  bacteria  and  the  spores  of  plants.     It  also  carries 
the  seeds  of  some  plants,  particularly  those  provided  with  Activitiesof 
down  like  the  thistle,  long  distances.     (2)  It  makes  flight  theatmos- 
possible  in  the  case  qf  birds  and  insects  and  even  some  ani-  phere  as  a 
mals.     (3)  It  furnishes  power  to  sailing  vessels  and  windmills. 
(4)  It  transports  moisture,  thus  making  possible  animal  and  plant  life 

1  From  the  Greek  6fr  =  I  smell. 


14 


METEOROLOGY 


The  func- 
tions of  the 


theatmos- 


on  large  portions  of  the  earth's  surface.  (5)  It  produces  sana  mounus 
or  sand  dunes  and  is  the  cause  of  "  weathering."  (6)  It  produces 
waves  on  bodies  of  water.  (7)  It  makes  sound  possible. 

1 6.  The  various  components  of  the  atmosphere  all  have  their  own  in- 
dividual offices  and  functions. 

Nitrogen  is  the  inert  component.  When  taken  into  the  lungs  of  ani- 
mals, it  appears  to  have  no  physiological  effects.  Plants  also  are  unable 
to  use  it.  It  serves  simply  to  dilute  the  oxygen,  which  is  the  active  ele- 
ment in  the  atmosphere.  Nitrogen  does  not  readily  form  compounds, 
and  to  this  may  be  due  the  fact  that  it  makes  up  such  a  large  part  of  the 
atmosphere  and  is  found  to  such  a  slight  extent  in  the  compounds  in 
the  earth's  crust. 

^Oxygen  is  the  active,  energizing  component.  All  animals  derive  their 
energy  and  power  to  do  work  from  the  oxygen.  It  is  taken  into  the  lungs, 
makes  its  way  into  the  blood,  and  joins  with  the  tissues  of  the 
body,  liberating  energy.  As  a  result  of  this  process,  large 
amounts  of  CO2  are  added  to  the  atmosphere  by  animals. 
Oxygen  is  also  consumed  whenever  combustion  takes  place. 
A  small  amount  of  oxygen  is  also  lost  to  the  earth's  atmos- 
phere by  forming  compounds  with  certain  substances  in  the  earth's 
crust.  Oxygen  is  supplied  to  the  atmosphere  chiefly  by  green  plants. 
A  small  amount  comes  from  volcanoes  and  other  vents  in  the  earth. 

Carbon  dioxide  is  also  an  important  component  of  the  earth's  atmos- 
phere, for  without  it  plant  life  would  be  impossible.    The  sap  which  comes 
up  from  the  roots  consists  largely  of  water  with  certain  organic  and  in- 
organic substances  in  solution.     The  green  cells  in  the  leaves  in  the  pres- 
ence of  sunshine  have  the  power  of  taking  the  CO2  from  the  air  and  com- 
bining it  with  the  sap,  thus  building  the  complex  molecules  which  make 
up  its  own  tissues,  at  the  same  time  liberating  oxygen.     CO2  is  supplied 
to  the  atmosphere  from  many  sources.     It  comes  from  volcanoes  and 
vents  in  the  earth's  crust.     The  water  of-  the  ocean  contains  a  large 
amount  of  it.     Meteors  when  consumed  in  the  atmosphere  add  a  small 
quantity.     A  large  quantity  is  added  as  the  result  of  combustion,  but 
the  largest  quantity  is  put  into  the  atmosphere  by  animal  life  and  the  slow 
decay  of  vegetation.     It  is  estimated  that  nearly  a  billion  tons  of  coal 
are  now  burned  annually.     This  alone  would  put  into  the  atmosphere 
nearly  four  billion  tons  of  CO2  each  year.    Slightly  more  CO2  is  found  ove 
the  oceans  than  over  the  land,  more  in  the  southern  hemisphere  than  i 
the  northern,  more  over  cities  than  in  the  country,  and  more  at  night  tha 
during  the  day      TV»p  rpqsnns  for  this  are  apparent. 


INTRODUCTION 

Oxygen  and  carbon  dioxide  are  thus  held  in  a  state  oi  equilibrium  by 
-is  of  plant  and  animal  life.  The  animal  consumes  O  and  liberates 
CO2,  while  the  plant  consumes  CO2  and  liberates  O.  Should  the  quan- 
tity of  COo  increase,  plant  life  would  become  more  luxuriant  and  animal 
life  would  be  hindered.  Should  the  quantity  of  oxygen  increase,  animal 
life  would  become  more  active  and  exhilarated  and  plant  life  would  be 
stunted.  In  each  case  the  tendency  would  be  towards  a  restoration  of 
equilibrium. 

Argon,  hydrogen,  and  the  rare  gases  of  the  atmosphere, like  nitrogen^ 
have  no  individual  functions.  The  sources  and  effects  of  the  secondary 
components  of  the  atmosphere  have  already  been  considered. 

17.  Atmosphere  of    other    heavenly  bodies.  —  There    are  many  in- 
direct methods  of  determining  the  amount  of  atmosphere  possessed 
by  the  various  heavenly  bodies  which  make  up  our  solar 

system.     The  giant  planet  Jupiter,  with  an  equatorial  diam-  pheres  of  " 
eter  of  90,190  miles  and  a  temperature  certainly  above  that  the  various 
of  boiling  water,  has  an  immense  atmosphere  which  is  con-  bodies. y 
stantly  filled  with  clouds.     Saturn,  Uranus,   and  Neptune 
seem  to  be  in  about  the  same  condition.     Venus  has  an   atmosphere 
somewhat  less  bulky  than  that  of  the  earth.     Mars,  an  older,  colder 
planet  with  a  diameter  of  4352  miles,  has  only  one  twelfth  as  dense 
an  atmosphere  as  the  earth,  while  the  moon  with  a  diameter  of  2163  miles 
has  practically  no  atmosphere  at  all.     Thus  great  diversity  is  found  in  the 
amount  of  atmosphere.     The  determining  factors  seem  to  be  tempera- 
ture, size,  and  perhaps  age.     For  a  full  discussion  of  this  topic  the  reader 
must  be  referred  to  books  on  astronomy. 

1 8.  Evolution    of    the   atmosphere.  —  If    during  '  the   early    history 
of  the  earth  it  was  molten  to  any  extents,  or  even  if  the  surface  tem- 
peratures were   high,    the   water    of   .the    oceans    and    the  Early  his- 
volatile  mineral  substances  of  the  earth  itself  must  have  tory  of  the 
been  in  the  atmosphere  as  vapor  or  gas.     The  earth  must  a 

then  have  possessed  an  immense  atmosphere  lade^i  with  vapor  and 
clouds.  As  the  earth  cooled,  the  water  and  other  substances  would 
be  precipitated  upon  the  earth,  thus  reducing  the  atmosphere  to  its 
present  bulk.  * 

If.  however,  as  some  prefer  to  believe,  the  earth  grew  verjRlowly  by 

accretion,  each  small  mass  of  matter  as  it  was  joined  to  the  earth  bring- 

,ng  its  quantum  of  gas,  it  may  be  that  the  surface  temperatures  were  never 

au<)     :  "'   ;1        "~" ^We  screw  as  gradually  in  bulk  as  the  earth 


16  METEOROLOGY 

19.  Whatever  may  have  been  the  initial  bulk  or  condition  of  the  at- 
mosphere, there  are  several  reasons  for  believing  that  great  changes  in 

composition  have  taken  place  during  geological  times.  The 
composition  luxuriant  plant  growth  during  the  carboniferous  age  when 
during  geo-  the  great  coal  beds  were  laid  down  is  usually  explained  by 

assuming  a  much  larger  amount  of  CO2  in  the  atmosphere 
In  fact  some  go  so  far  as  to  state  that  there  was  probably  extremely  little 
oxygen  in  the  atmosphere  at  that  time,  as  it  had  all  been  used  in  forming 
compounds  such  as  water  and  the  oxides  of  the  earth's  crust.  They 
attribute  our  present  supply  of  oxygen  entirely  to  the  very  luxuriant 
plant  life  of  that  time.  Great  changes  have  also  taken  place  in  the  tem- 
peratures of  the  earth.  Glaciation  has  extended  down  the  Mississippi 
Valley  as  far  as  Kansas,  and  traces  of  luxuriant  vegetation  have  been 
found  in  northern  countries  where  it  is  at  present  impossible.  One  of 
the  explanations  sometimes  given  is  to  attribute  it  to  changes  in  the 
composition  of  the  atmosphere.  So  great  is  the  influence  of  composi- 
tion on  temperature  that  the  statement  is  sometimes  made  that  if  the 
amount  of  C02  in  the  atmosphere  were  multiplied  by  four,  the  vegeta- 
tion of  Florida  would  be  found  in  Greenland. 

20.  Future  of  the  atmosphere.  —  At  present  a  small  amount  of  gas 
is  lost  to  the  atmosphere  by  escape  into  space  beyond  the  gravitational 

control  of  the  earth.  A  small  amount,  particularly  oxygen, 
*s  a^so  ^os^  m  ^e  f°rmati°n  of  compounds  which  become 
constant  in  part  of  the  ocean  or  the  solid  earth.  The  atmosphere  gains 
composition.  a  small  amount  of  gas  from 'meteors  and  from  volcanoes  and 
other  vents  in  the  earth.  The  amount  of  0  and  CO2  in 
the  atmosphere  is  kept  constant  by  the  balance  between  plant  and 
animal  life.  It  thus  seems  that  the  future  will  see  no  such  changes 
in  bulk  or  composition  as  have  been  witnessed  in  the  past.  A  very 
gradual  diminution  in  the  amount  of  the  atmosphere  is,  however,  to  be 
expected. 

The  sun,  however,  may  eventually  grow  cold.     The  only  two  explana- 
tions of  the  maintenance  of  the  sun's  heat  which  have  withstood  mod- 
ern investigation  are  that  the  heat  is  maintained  by  slow 

The  future  .  .          , 

disappear-  contraction  or  by  the  presence  of  radio-active  material, 
anceofthe  However  the  outpour  of  heat  may  be  maintained,  it  must 
eventually  come  to  an  end.  As  the  sun  grows  cold,  so 
must  the  earth.  A  glance  at  the  accompanying  table  of  the  boiling 
points  of  the  components  of  the  atmosphere  shows  the  inevitable 
results. 


INTRODUCTION  17 

Water,    f  100°  C.  When  the  temperature  reaches  -  78°  C.  the  CO2 

CO2,      —    78,°  will  come  out  of  the  atmosphere,  and  plant  and  ani- 

O,          —  183°  mal  life  must  cease.   As  the  temperature  falls  lower, 

Ar,         —  187°  the  various  components  will  come  out  in  order  as 

N,          —  194°  their  boiling  points  are  reached,  until  finally  a  cold, 

H,          —  253°  dark,  airless  world  will  be  revolving  about  a  dying 

sun.  And  a  strange  world  it  will  indeed  be,  for  the  sky  will  be  black  in- 
stead of  blue,  the  stars  will  be  visible  in  the  daytime,  shadows  will  be 
entirely  black,  sound  will  be  impossible,  and  the  constant  bombardment 
by  meteors  will  make  life  in  the  open  more  dangerous  than  on  a  modern 
battlefield. 

THE  PRESSURE  AND  HEIGHT  OF  THE  ATMOSPHERE 

21.  Gravity  and  its  effects.  —  All  objects  near  the  earth's  surface  are 
pulled  downward  by  what  is  called  the  force  of  gravity.     Gravity  in  mag- 
nitude and  direction  is  in  reality  the  resultant  of  three  forces  ; 

the  attraction  of  the  earth  as  a  whole,  the  attraction  of  sur-  definition,5 
rounding  objects,  and  the  centrifugal  force  due  to  the  earth's  magnitude, 
rotation.     It  is  ordinarily  considered  that  the  direction  of  the  j^n 
force  of  gravity  is  towards  the  center  of  the  earth  and  that 
its  magnitude  is  constant  all  over  the  earth.     These  statements  are,  how- 
ever, only  approximately  true.     It  is  only  at  the  equator  and  poles  of  the 
earth  that  the  direction  of  gravity  is  directly  towards  the  earth's  center. 
At  other  places  it  is  nearly  but  not  exactly  towards  it.     The  magnitude 
also  varies  slightly  with  latitude  and  elevation.     Two  effects  of  this 
ever  present  force  deserve  attention.     (1)   All  masses,  since  they  are 
acted  on  by  gravity,  will  have  weight  and  exert  a  downward  pressure  on 
whatever  supports  them.     (2)  Since  a  fluid  does  not  resist  a  Effects  of 
change  of  form,  it  will  set  itself  with  its  lower  surface  con-  &&vli* 
forming  to  the  shape  of  the  containing  vessel  and  with  its  upper  surface 
at  right  angles  to  the  direction  of  gravity. 

22.  Geosphere,  hydrosphere,   atmosphere.      The  great  mass  of  the 
earth  is  solid  at  least  in  the  outer  crust  and  has  a  very  uneven  surface. 
It  is  covered  in  part  by  the  oceans,  which  under  the  influence  Qeosphere 
}f  gravity  conform  with  their  under  surfaces  to  the  irregular-  hydro- 
ties  of  the  solid  earth  and  have  an  upper  free  surface  at  right 

ingles  to  gravity.     Both  solid  earth  and  fluid  oceans  are  sur- 

ounded  by  an  envelope  of  gas.     These  three  are  often  spoken  of  as  the 

posphere,  the  hydrosphere,  and  the  atmosphere.1 

.=  gas  ;  <r<j>aipa  =  sphere. 


18  METEOROLOGY 

The  surface  of  the  hydrosphere  is  considered  a  level  surface,  and  all 
elevations  are  reckoned  from  it.  When  the  dimensions  of  the  earth  are 
stated,  it  is  always  the  dimensions  of  the  hydrosphere  that  are  given.  Its 
form  is  not  that  of  a  perfect  sphere,  but  that  of  a  sphere  flattened  at  the 
poles  and  known  as  an  oblate  spheroid.  The  equatorial  diameter  of 
the  hydrosphere  exceeds  the  polar  by  27  miles. 

The  following  are  a  few  numerical  facts  concerning  the  geosphere, 
hydrosphere,  and  atmosphere. 

Polar  diameter  of  hydrosphere 7899.580  miles 

Equatorial  diameter  of  hydrosphere 7926.592  miles 

Area  of  oceans I  of  earth's  surface 

Mass  of  oceans ?^  of  earth 

Mass  of  atmosphere vvkinsTS  °f  earth 

23.  Since  the  atmosphere  is  substantial,  that  is,  possesses  mass,  and  is 
acted  upon  by  gravity,  it  must  have  weight  and  exert  a  downward  pres- 
Pressure  of    sure-     The  pressure  of  the  atmosphere  is  simply  the  weight  of 
theatmos-      the  column  of  air  above  the  point  in  question.     With  eleva- 
tion above  the  earth's  surface  the  pressure  must  thus  grow 

less.  The  average  pressure  of  the  atmosphere  at  the  surface  of  the  hydro- 
sphere is  14.7  pounds  per  square  inch.  It  is  usually  measured,  however, 
not  in  pounds  per  square  inch,  but  by  the  length  of  the  balancing  or 
equivalent  mercury  column,  as  will  be  fully  discussed  in  Chapter  IV. 

24.  Height  of  the  atmosphere.  —  Since  a  gas  tends  to  expand  and 
occupy  all  the  space  open  to  it,  no  theoretical  limit  can  be  set  to  the 
Notheoreti-    height  of  the  atmosphere.     We  cannot  picture  the  free  sur- 
cai  limit.        face  of  a  gas>  an(j  must  think  of  the  earth's  atmosphere  as 
growing  gradually  thinnc**  and  thinner  with  elevation  until  it  merges  with 
that  mere  trace  of  gas  which  fills  interplanetary  space.     The  question  of 
the  height  of  the  atmosphere  may  be  put,  however,  in  a  more  practical 
form.     To  what  height  above  the  earth's  surface  does  a  sufficient  quanti- 
ty of  air  extend  to  give  us  any  indication  whatever  of  its  presence  ?     This 
is  sometimes  called  the  sensible  height  of  the  atmosphere,  and  there  are 
several  methods  of  detecting  the  presence  of  air  at  considerable  hei;. 

The  five         ^  Twilight  is  caused  by  the  reflection  of  sunlight   i 

ways  of  de-    the  dust  and  perhaps  moisture  particles  of  the  upper  air 

termining       after  the  sun  has  gone  below  the  horizon  of  the  place  in 

bie  height      question.     Diffraction  may  also  play  a  part.     The  cause  of 

I  the  twilight  is  illustrated  in  Fig.  3.     By  noting  the  duration  of 

twilight  and  knowing  the  dimensions  of  the  earth,  the  height 

of  the  air  producing  the  t.,lllte-ht  may  be  "etermined*    It  has  been  found 


INTRODUCTION 


19 


AIR    PRODUCING 
TWILIGHT 


FIG.  3.  —  The  Cause  of  Twilight. 


that  a  sufficient  quantity  of  air  for  deflecting  an  appreciable  amount  of 
sunlight  extends  to  a  height  of  63  kilometers.  (2)  The  ordinary  height 
of  clouds  is  from  a  few  meters  to  perhaps  15  kilometers,  but  on  certain 
rare  occasions,  at  night,  par- 
ticularly in  high  latitudes  in 
midsummer,  faint  luminous 
clouds  have  been  observed  as 
high  as  83  kilometers  l  above 
the  earth's  surface.  (3)  The 
Aurora  Borealis,  or  northern 
lights,  is  supposed  to  be  due 
to  the  electrical  discharges 
in  the  rarefied  gases  of  the 
upper  atmosphere.2  The 
height  of  the  aurora  has 

been  determined  and  is  found  in  some  cases  to  vary  from  60  to  200 
kilometers.1  (4)  Meteors  are  masses  of  matter  from  pinhead  size  up 
which  are  flying  haphazard  through  space  and  often  enter  the  earth's 
atmosphere  with  velocities  from  12  to  50  miles  per  second.  The  heat 
caused  by  the  resistance  of  the  air  raises  them  to  incandescence  and 
they  become  visible  as  shooting  stars.  The  height  at  which  these 
become  visible  has  often  been  determined,  and  the  larger  values  vary 
from  240  to  300  kilometers.1  Thus  there  is  sufficient  quantity  of  air 
above  this  height,  even,  to  make  a  meteor  incandescent.  (5)  From 
observations  of  eclipses  of  the  moon  it  is  also  possible  to  determine  the 
extent  of  the  atmosphere.  This  method  also  gives  elevations  as  great 
as  300  kilometers. 

As  a  general  conclusion,  then,  it  may  be  stated  that  a  sufficient  quantity 
)f  air  extends  to  a  height  of  300, kilometers  to  give  us  an  indication  of  its 
)resence.  It  should  be  noted  in  this  connection  that  the  highest  moun- 
tain does  not  rise  above  10  kilometers.  The  greatest  height  attained  by 
a  manned  balloon  is  not  over  11  kilometers  (10.3  kilometers  or  6J  miles 
by  Dr.  Berson  and  Professor  Siiring  in  1901),  and  unmanned  balloons  and 
kites  have  not  gone  above  29  and  8  kilometers  respectively. 

1  These  heights  are  determined  trigonometrically,  by  means  of  surveying  instruments. 

•<lar  problem  of  determining  the  height  of  an  inaccessible  object.     Simul- 

;ons  of  direction  and  altitude  from  the  two  ends  of  a  base  line  several 


-ary. 


aurora  is  so  little  understood  and  the  various  values  for  height 
1  are  so  discordant,  that  it  is  questionable  whether  observations 
the  extent  of  the  atmosphere. 


20  METEOROLOGY 

The  earth's  atmosphere  cannot  extend  more  than  21,000  miles  and 
turn  with  the  earth  as  it  rotates  on  its  axis.  At  this  distance  centrifugal 
force  due  to  the  rotation  and  gravitational  attraction  balance  so  that  the 
air  would  be  abandoned. 

The  middle  layer  of  the  atmosphere  is  at  a  height  of  3.6  miles.  That 
is  to  say,  there  is  as  much  air  above  as  below  this  level.  If  the  air  were  of 
the  same  density  throughout  as  at  the  earth's  surface,  it  would  have  a 
height  of  about  5  miles.  This  is  sometimes  called  the  height,  of  a  horaa- 
geneous  atmosphere. 

THE  METEOROLOGICAL  ELEMENTS 

25.  The  meteorological  elements.  —  The  condition  of  the  atmosphere 
at  any  particular  time  and  place  is  completely  determined  by  six  things. 
The  six  me-    These  are  called  the  meteorological  or  weather  elements, 
teoroiogical     They  are  temperature,  pressure,  wind,  humidity,  clouds,  and 

lts'  precipitation.  Dust  and  atmospheric  electricity  are  some- 
times included.  For  example,  at  8  A.M.,  July  4,  1908,  at  Williamstown, 
Mass.,  the  temperature  was  72°,  the  lowest  temperature  during  the  night 
had  been  65°  and  it  occurred  a  little  before  sunrise ;  the  pressure  was 
29.47  inches ;  the  wind  velocity  was  two  miles  per  hour  from  the  east ;  the 
air  contained  7.10vgrains  of  moisture  per  cubic  foot  and  held  86  per  cent 
of  what  it  could ;  the  clouds  were  stratus  and  Were  moving  slowly  from 
the  east ;  the  sky  was  totally  covered  with  clouds,;  no  rain  was  falling  or 
had  fallen  during  the  night. 

The  exact  condition  of  the  atmosphere  at  any  time  and  place  can  thus 

1  be  stated  by  giving  numerical  values.to  different  phases  of  the  m'eteoro- 

Hogical  elements.  f 

26.  Weather  and  climate.  —  Weather  is  defined  as  the  condition  of  the 
atmosphere  at  any  time  and  placeynd  is  thus  best  described  by  giving  the 
Definition      numerical  values  for  the  meteorological  elements.  (Climate  is 
of  weather      generalized  weatheYj     It  is  concerned  more  with  the  average 

lte>  rather  than  the  particular  values  of  the  meteorological  ele- 
ments. The  term  is  only  used  in  connection  with  larger  areas  and  longer 
periods  of  time.  Thus  one  should  speak  of  the  weather  on  December 
25,  1910,  in  New  York  City,  but  of  the  winter  climate  of  New  England. 

27.  Periodic  and  irregular  variation  and  normal  values.  —  The  numeri- 
Periodic         ca^  vames  °f  the  meteorological  elements  are  by  no  nu'tins  co^- 
and  irregu-     stant,  but  are  always tundergoing  change  or  variatio 

on'   are  two  kinds  of  change  or  variation,  periodic  r 
Whenever  in  the  course  of  a  variation  the  initial  va1 


NTRODUCTION  21 

after  the  lapse  of  approximately  equal  intervals  of  time,  the  variation  is 
said  to  be  periodic.  If  the  changes  are  irregular  or  haphazard  as  regards 
amount  or  time  of  occurrence,  the  variation  is  said  to  be  irregular.  Now 
the  meteorological  elements  are  undergoing  both  kinds  of  variation 
simultaneously,  and  sometimes  two  or  more  periodic  variations  are  present 
at  the  same  time.  For  example,  it  usually  grows  warmer  during  the 
morning  and  early  afternoon  and  then  cooler  during  the  rest  of  the  after- 
noon and  night.  This  is  aperiodic  variation  in  temperature.  A  sudden 
thunder  shower  may,  however,  lower  the  temperature  twenty  degrees  or 
more  during  a  few  minutes.  This  is  an  irregular  variation.  Sometimes 
the  irregular  variations  are  of  such  magnitude  as  to  cloak  or  render  almost 
imperceptible  the  periodic  variations.  (See  Figs.  4  and  5.)  There  are  no 
examples  in  the  realm  of  meteorology  of  a  pure  undisturbed  periodic 
variation  except  for  a  short  time.  Examples  of  this  must  be  taken  from 
mechanics  or  physics.  The  amount  of  snow  which  falls  during  the  win- 
ter, the  highest  or  lowest  temperature  which  occurs  on  a  definite  date  for 
successive  years,  the  number  of  thunder  showers  during  a  year,  are  all 
examples  of  an  irregular  variation  only.  In  all  the  changes  of  tempera- 
ture and  pressure,  and  in  fact  of  all  the  meteorological  elements  from 
moment  to  moment,  we  have  examples  of  periodic  and  irregular  variation 
combined. 

28.   Whenever  observations  of  any  phase  of  any  one  of  the  meteorologi- 
cal elements  have  been  made  for  a  considerable  time,  it  often  is  desirable 
to  summarize  the  observations.     Such  a  summary  ordinarily 
contains  four  things,  the  average  value,  usually  called  the 
normal  value,  the  greatest  value,  the  least  value,  and  the  insumma- 
average  departure  from  normal.     The  meaning  of  these  and  "aliens.  S 
the  method  of  determining  their  numerical  values  can  be  best 
illustrated  by  an  example.     The  total  number  of  thunder  showers  ob- 
served at  Albany,  N.Y.,  for  successive  years  is  shown  on  p.  22. 

The  average  number  of  thunder  showers  i§  22.  This  is  usually  spoken 
of  as  the  normal  number  of  thunder  showers  per  year.  The  largest  num- 
ber is  35  in  1910,  and  the  smallest  number  is' 7  in  1890.  The  year  1910, 
with  35,  shows  a  departure  from  normal  of  13.  The  departure  for  1909 
is  1,  for!908  is  9,  etc.  When  these  departures  are  averaged,  the  result  is 
called  the  average  departure  from  normal.  Its  value  here  is  6.  Thus  the 
normal  value  22,  the  greatest  value,  35,  the  least  value,  7,  and  the  average 
departure  from  normal,  6,  form  a  convenient  summary  of  the  observations 
and  give  a  complete  picture  of  what  may  be  expected  at  Albany  as  regard 
the  number  of  thunder  showers  during  a  year.  This,  by  the  way,  is  an 


22 


METEOROLOGY 


YEAR 

NUMBER  OF  THUN- 
DER SHOWERS 

YEAR 

NUMBER  OF  THUN- 
DER SHOWERS 

1884 

24 

1898 

26 

1885 

22 

1899 

22 

1886 

14 

1900 

28 

1887 

14 

1901 

30 

1888 

9 

1902 

28 

1889 

12 

1903 

22 

1890 

7 

1904 

28 

1891 

9 

1905 

21 

1892 

32 

1906 

32 

1893 

23 

1907 

25 

1894 

23 

1908 

31 

1895 

18 

1909 

21 

1896 

17 

1910 

35 

1897 

22 

illustration  of  irregular  variation,  and  most  of  the  observations  which  are 
summarized  in  this  way  are  examples  of  irregular  variation  only. 

29.  Graphical  representation.  —  The  variation  of  a  quantity,  whether 
periodic,  irregular,  or  both,  can  be  readily  pictured  by  plotting  its  various 
Graphical  values  to  scale.  This  is  called  graphical  representation,  and 
represen-  the  curve  which  represents  the  variation  is  called  the  graph. 
The  values  are  plotted  by  choosing  equal  distances  on  one  of 
the  axes,  usually  the  horizontal  or  X-axis,  to  represent  time  and  by  laying 
off  equal  distances  on  the  other  axis  to  represent  the  values  of  the  quan- 
tity. The  points  are  then  located  in  accordance  with  the  observations 
and  are  connected  by  a  broken  line  or  a  continuous  curve. 

The  following  examples  will  illustrate  this  method : 


Place  :  Williarastown,  Mass. 


JUNE  12,  1907 

JUNE  9,  1906 

JUNE  12,  1907 

JUNE  9,  1906 

Temperature 

Temperature 

Temperature 

Temperature 

2  A.M. 

48 

64 

6  P.M. 

70 

68 

4  A.M. 

44 

63 

8  P.M. 

62 

66 

6  A.M. 

43 

63 

10  P.M. 

55 

65 

8  A.M. 

51 

68 

Midnight 

50 

64 

10  A.M. 

62 

73 

2  A.M. 

47 

63 

Noon 

68 

80 

4  A.M. 

44 

62 

2  P.M. 

70 

83 

6  A.M. 

43 

60 

4  P.M. 

71 

72 

INTRODUCTION 


23 


Figure  4  represents  the  typical  undisturbed  daily  variation  in  tem- 
perature. Figure  5  represents  the  typical  variation  disturbed  by  a 
thunder  shower  between  3  and  5  o'clock  in  the  afternoon. 


70 
65 
60 
55 
50 
45 
40 

^~-~ 

•  —  s 

William: 

1 

town,  Mass. 

/ 

s 

\ 

June 

12,1 

907 

/ 

N 

\ 

1 

\ 

/ 

\ 

\ 

7 

x 

k  

y 

^-*, 

24         6         8        10      12        2         4         6 
A.M.  NOON  P.M. 


10       12         2         4 
MIDN.         A.M. 


FIG.  4.  —  Typical  Undisturbed  Daily  Variation  in  Temperature. 

N 

Many  other  examples  of  graphical  representation  will  be  found  scat- 
tered through  the  book.     Usually  it  is  the  variation  of  some  quantity 


FIG.  5.  —  Daily  Variation  Disturbed  by  a  Thunder  Shower. 

with  the  time  that  is  represented.  If  any  two  quantities  are  so  related 
that,  as  one  changes,  the  other  changes  also,  the  relation  can  be  rep- 
resented graphically  in  this  way. 


THE  THREE  METHODS   OF  INVESTIGATION 

30.   There  are  three  methods  of  reaching  conclusions  and  deriving 
general  laws.     These  are  known  as  the  inductive,  the  deduc-  The  induc- 
tive, and  the  experimental  method.     In  the  inductive  method  tlve  method- 
fa.nts  and  observations  are  generalized  to  get  the  underlying  law. 


24  METEOROLOGY 

It  proceeds  from  particulars  to  the  general.  All  general  statements 
based  upon  meteorological  observations  are  thus  examples  of  the  induc- 
tive method.  Meteorology  is  usually  presented  largely  from  the  induc- 
tive standpoint. 

In  the  deductive  method  one  starts  with  general  principles  or  laws  and 
determines  what  ought  to  take  place  in  a  particular  instance.  This  must 
The  deduc-  then  be  compared  with  the  observed  facts  and  an  agreement 
tive  method.  stamps  the  whole  work  as  correct.  In  Chapter  IV,  C,  will  be 
found  a  good  example  of  deductive  reasoning.  We  will  start  there  with 
two  fundamental  facts,  determine  what  ought  to  be  the  pressure  and  wind 
direction  at  different  points  on  the  earth's  surface  and  then  compare  this 
with  the  results  of  observation. 

The  experimental  method  is  essentially  the  laboratory  method.  Here 
the  conditions  of  the  experiment  are  changed  and  the  results  noted.  In 
The  expert-  ^is  way  new  facts,  relations,  and  laws  may  be  found,  and  all 
mental  assumptions  and  hypotheses  may  be  rigidly  tested.  The 
experimental  method  is  of  only  limited  application  in  mete- 
orology because  all  of  the  phenomena  furnished  by  nature  are  on  such  a 
stupendous  scale  that  man  is  powerless  to  change  the  conditions. 

THE  PLAN  OF  THE  BOOK 

31.  The  first  chapter  contains  introductory  material  and  a  discussion 
of  the  composition,  pressure,  and  height  of  the  atmosphere.  The  second 
The  con-  chapter  is  devoted  to  the  study  of  the  heating  and  cooling  of 
tents  of  the  the  atmosphere,  as  this  is  of  such  fundamental  importance. 
The  meteorological  elements  are  then  considered  in  order  in 
Chapters  III  to  V  inclusive.  Chapter  VI  is  given  up  to  the  study 
of  the  different  kinds  of  storms.  Two  chapters  are  then  devoted  to  the 
practical  side  of  meteorology,  namely,  weather  bureaus  and  their  work  and 
weather  prediction.  Part  II,  consisting  of  five  chapters,  is  devoted  to 
special  subjects  not  always  included  in  meteorology  proper.  They  could 
be  omitted  without  destroying  the  unity  or  completeness  of  the  book. 
These  subjects  are  climate,  floods  and  river  stages,  atmospheric  elec- 
tricity, atmospheric  optics,  atmospheric  acoustics. 

Since  the  book  is  intended  primarily  as  a  text-book,  the  syllabus  at  the 
beginning  of  each  chapter,  the  marginal  topics,  and  the  questions,  topics 
The  aim  of  for  investigation,  exercises,  and  references  at  the  end  of  each 
the  book.  chapter  have  been  added.  It  is  also  hoped  that  the  intelligent 
reader  who  has  not  had  special  training  in  this  science  will  be  able  to  find 


INTRODUCTION  25 

here  a  clear  and  concise  picture  of  the  modern  aspects  of  the  subje* 
This  volume  does  not  pretend  to  be  a  compendium  or  compilation  of 
existing  knowledge.     It  is  hoped,  however,  that  in  the  references  to  the 
literature  the  way  has  been  pointed  out  to  a  more  complete  knowledge 
of  the  subject  on  the  part  of  him  who  desires  it. 

QUESTIONS 

(1)  How  is  a  natural  science  defined?  (2)  Mention  several  natural  phenomena 
and  state  the  natural  science  to  which  each  belongs.  (3)  Enumerate  the  natural 
sciences.  (4)  What  is  the  province  of  each  ?  (5)  Why  are  physics  and  chem- 
istry fundamental  ?  Qtf  Define  meVy,  (7)  Of  what  does  it  treat  ?  (8)  Why 
is  met'y  onejafJ^he  oldest  sciences j*  (9)  What  is  the  origin  of  the  word  m^JejL? 
(10)  Wher?  where,  and  by  whom  was  the  first  treatise  written?  (11)  What 
did  it  contain  ?  (12)  Give  the  dates  of  the  four  periods  in  the  history  of  met'y. 
(13)  What  brought  about  each  new  period  ?  (14)  State  the  characteristics 
of  each  period.  (15)  Name  the  two  lines  of  usefulness  of  me^'y.  (16)  Discuss 
the  question  of  the  financial  saving.  (17)  What  is  the  duty  of  schools  and  col- 
leges as  regards  the  public  ?  (18)  Why  should  the  public  be  educated  in  met'y  ? 
(19)  Along  what  three  lines  does  a  natural  science  off er^ training?  (20)  State 
the  character  of  each  line  of  training.  (2i)  State  some  of  the  varied  in- 
fluences and  effects  of  the  weather,  (22)  State  the  relation  of  jnet'y  to  physics. 
(23)  State  its  relation  to  the  other  natural  sciences^  (24)  Define  the  atmos-  „  1  V 
phere.  (25)  What  is  the  origin  of  the  word  ?  (26)  What  are  the  character-  •  a* 
istics  of  air  ?  •  (27)  State  some  of  the  physical  projDer^ies  of  gases.  (28)  What 
are  the  four  major  constituents  of  pure  dry  air  ?  (29)  State%approximately  the 
percentage  composition,  »(30)  Tlow  much  hydrogen  is  found  in  the  atmosphere  ? 
(31)  Name  the  four  rare  gases  jn  the  atmosphere,  (32)  Give  the  proofs  that  air 
is  a  mechanical  jnixture  of  its  Components.  (33)  How  constant  is  the  composition 
of  the  air  ?  (3%)  How  much  GO2  does  exhaled  air  contain  ?  (35)  Why  is  the 
composition  so  constant  ?  (36)  What  change  takes  place  in  the  composition  at 
great  heights?  (37)  Why?  (38)  What  facts  tend  to  justify  this  theoretical 
conclusion  ?  (39)  Name  the  minor  constituents  of  the  atmosphere.  (40)  How 
much  water  vapor  is  preseiS??  (41)  Why  is  water  ''vapor  "important  ?  (42) 
How  many  organic  particles  are  in  the  atmosphere?  (43)  What  is  included 
under  this  head?  (44)  fre  the  inorganic  particles  visible?  (45)  Describe 
Aitken's  dust  counter.  (46)  State  the  working  principle.  (47)  How  is  a  deter- 
mination made?'  (48)  How  numerous  are  dust  particles?  (49)  Does  the 
number  vary?  (50)  What  are  the  sources  of  atmospheric  dust?  '(51)  What 
are  some  of  the  effects  of  this  dust  ?  (52)  What  is  the  nature  afid  use  of  ozone?  • 

(53)  What   are   the   offices   and   activities   of   the   atmosphere   as   a   whole? 

(54)  What  are  the  functions  of  the  nitrogen  ?     (55)  What  are  the  functions  'of 
the  oxygen  ?     (56)  How  is  it  lost  to  and  gained  by*the  atmosphere  ?     (57)  What 
are  the»functions  of  CO2  ?     (58)  How  is  it  lost  to  and  gained  by  the  atmosphere  ? 
(59)  Is  the  amount  constant  everywhere  ?     (60)  How  are  CO2  and  O  held  in 
equilibrium?     (61)  Describe   briefly   the   atmos^iere    of    the   other   heavenly 
bodies  in  our  solar  system.     (62)  What  determines  the  amount  ?     (63)  What 
was  the  early  history  of  the  atmosphere  ?     (64)  What  changes  have  taken  place 
in  geological  times?     (65)  Is  the  atmosphere  changing  now  as  regards  bulk 
and   composition?     (66)  What   is   the   probable   future   of   the   atmosphere? 
(67)  Define  gravity.     (68)  What  is  its  direction  ?     (69)  State  some  of  Its  effects 


26  METEOROLOGY 

on  matter.  (70)  Define  geosphere,  hydrosphere,  and  atmosphere.  (71) 
State  the  origin  of  the  words.  (72)  What  is  the  shape  of  the  hydrosphere? 
(73)  Why  does  the  atmosphere  exert  a  pressure?  (74)  How  much  is  the  pres- 
sure? (75)  Why  does  the  pressure  change  with  elevation?  (76)  Why  is  there 
no  theoretical  limit  to  the  atmosphere?  (77)  What  is  meant  by  the  "  sensible  " 
height  of  the  atmosphere?  (78)  Describe  the  five  methods  of  determining 
it.  (79)  What  heights  have  been  found?  (80)  How  high  have  balloons 
and  kites  ascended?  (81)  At  what  height  is  the  middle  layer  located?  (82) 
Define  the  met'l  or  weather  elements.  ,(83)  Name  the  six  met'l  elements. 
(84)  How  may  the  condition  of  the  atmosphere  at  any  time  and  place  be  de- 
scribed? (85)  Define  weather  and  climate.  (86)  Have  the  met'l  elements 
constant  values?  (87)  Name  and  define  the  two  kinds  of  variation  or  change. 

(88)  Illustrate  by  means  of  examples  the  various  kinds  of  change  or  variation. 

(89)  What   four   things   are    computed    when   observations    are    summarized? 

(90)  Define  what  is  meant  by  normal  values.          (91)  What  is  graphical  repre- 
sentation?    (92)  How  is  a  graph  constructed?     (93)  Name  the  three  methods 
of  deriving  general  laws.     (94)  Describe  and  illustrate  the  inductive  method. 
(95)  Describe  and  illustrate  the  deductive  method.     (96)  Describe  and  illus- 
trate the  experimental  method.      (97)  What  is  the  relative  importance  of  these 
three  methods  in  met'y?     (98)  What  is  the  order  of  the  subjects  treated  in  this 
book?     (99)  What  is  the  aim  of  the  book? 

TOPICS   FOR   INVESTIGATION 

(1)  The  financial  saving  caused  by  the  Weather  Bureau. 

(2)  The  relation  of  weather  to  disease. 

(3)  The  relation  of  weather  to  crime. 

(4)  The  extent  to  which  meteorology  is  taught  in  the  schools  and  colleges. 

(5)  The  agreement  of    the  furious  determinations  of    the  composition  of 
the  air. 

(6)  The  history  of  the  discovery  of  argon.          i 

(7)  The  history  of  the  discovery  of  the  rarer  gases  in  the  air!^ 

(8)  The  dust  of  the  atmosphere.  • 

(9)  Changes  in  the  earth's  atmosphere  during  geological  times. 

(10)  Meteors.     (See  any  text-book  on  astronomy.) 

(11)  Determination  of  the  maximum  and  minimum  of  graphs.    (The  usual 
method  is  to  draw  a  line  connecting  the  middle  points  of  the  chords  parallel  to 
the  X-axis  and  terminated  by  the  curve. )  % 

PRACTICAL   EXERCISES 

(1)  Copy  frojn  some  source  or  compile  a  few  tables  showing  the  relation 
between  the  prevalence  of  certain  kinds  of  disease  and  the  weather.     Plot  the 
graphs  representing  these  relations. 

(2)  If  physical  and  chemical  apparatus  is  available,  experiments  may  be 
performed  to  show  the  composition  and  pressure  of  the  atmosphere,  and  the 
properties  of  gases. 

(3)  Write  out  a  definite,  complete  description  of  the  weather  at  several  dif- 
ferent times  and  places. 

(4)  Copy  from  some  source  or  compile  observations  of  some  quantity  sub- 
ject to  irregular  variation  only  and  summarize  them  in  accordance  with  section 
28. 

(5)  Plot  several  graphs  from  series  of  observations  of  some  kind  and  state 
the  kinds  of  variation  depicted. 


INTRODUCTION  27 

REFERENCES  * 

,  :  Observations  of  Atmospheric  Dust,"  W.  B.  Bulletin  11,  p.  734 
ARRHENIUS,  Lehrbuch  der  kosmischen  Physik,  Composition  of  the  ati 

pp.  473-490. 
KASSNER,  Das  Welter  und  seine  Bedeutung  fur  das  praktische  Leben, 

to  practical  life,  pp.  114-144. 
MOORE,  JOHN  W.,  Meteorology,  2d  ed.,    The  Influence  of  Weather  or 

pp.  407-457. 
RAMSAY,  The  Gases  of  the  Atmosphere;    the  History  of  their  Discovery. 

1  For  further  information  concerning  the  books  and  for  further  references  see  Appen- 
dix IX. 


CHAPTER   II 

THE   HEATING   AND   COOLING   OF   THE   ATMOSPHERE 

THE  NATURE  OF  MATTER,  HEAT,  TEMPERATURE,  AND  RADIANT 

ENERGY,  32 

THE  SOURCES  OF  ATMOSPHERIC  HEAT,  33 
INSOLATION 

Amount,  34. 

Variation  with  latitude  and  time  of  year,  35-37. 

Distribution  over  the  earth,  38. 

THE  INTERRELATION  OF  MATTER  AND  RADIANT  ENERGY 

Reflection,  39. 

Transmission,  40. 

Absorption,  41,  42. 

Actinometry,  43,  44. 

Behavior  of  the  ocean  as  regards  reflection,  transmission,  and 

absorption,  45. 

Behavior  of  the  land  as  regards  reflection,  transmission,  and  absorption,  46. 
Behavior  of  the  atmosphere  as  regards  reflection,  transmission,  and 

absorption,  47. 

CONDUCTION  AND  CONVECTION 

Conduction,  48. 

Convection,  49. 

Convection  in  the  atmosphere,  50,  51. 

Evidences  of  convection,  52. 

TEMPERATURE  GRADIENTS 

Vertical  temperature  gradient,  53,  54. 

The  isothermal  layer,  55,  56. 

Inversion  of  temperature  —  nocturnal  stability,  57. 

Diurnal  instability  —  conditions  of  convection,  58. 

How  THE  ATMOSPHERE  is  HEATED  AND  COOLED,  59 

THE  NATURE  OF  MATTER,  HEAT,  TEMPERATURE,  AND  RADIANT  ENERGY 

32.  In  order  to  have  a  clear  conception  of  those  fundamental  processes 
which  are  operative  in  the  heating  and  cooling  of  the  atmosphere,  one 
must  understand  something  about  the  nature  of  matter,  heat,  tempera- 

28 


i*  HEATING  AND   COOLING   OF  THE  ATMOSPHEI 

ture,  and  radiant  energy.  This  subject  will  be  briefly  sketched  r 
the  reader  must  be  referred  to  text-books  on  physics  for  a  more 
treatment. 

A  piece  of  wood,  for  example,  may  be  divided  into  several  portions. 
These  portions  may  in  turn  be  subdivided,  and  the  process  may  be  con- 
tinued until  the  resulting  portions  are  scarcely  visible  in  a  Molecule 
powerful  microscope.     The  natural  question  at  once  arises  and 
as  to  whether  this  process  of  subdivision  could  be  carried  on  indefinitely 
provided  the  mechanical  means  were  at  hand  for  accomplishing   it. 
The  answer  is  that  there  is  a  smallest  particle  which  can  exist  and  still 
retain  the  properties  of  the  substance  in  question.     This  smallest  par-J 
tide  is  calle^the  moleculeT  "M^^uile^^  atojmsl 

which  may  be  like  or  unlike,  few  or  many.     There  are  known  to   , 
eighty  differdfiridncfioi :  atoms  corresponding  to  the  numberjoL^o-called 
elements.     It  is  now  thotfghtTthat  the  atom  in  turn  is  composed  of  per- 
haps thousands  of  particles  which  may  even,  under  certain  conditions, 
escape  from  the  atoms  themselves.jrr~Haese  particles  are  spoken  of  as 
corpuscles  or  electrons.     A4oms  are  held  together  by  the  force  of  chemiTfal  | 
affinity  to  form  molecules; — Molecules  are  held  together  by  cohesion  or| 
adhesion  to  form  masses-,  iand~aLl-maoooa  -exertra  gravitational  attraction! 
pea-each,  .other. 

The  whole  of  space  is  supposed  to  be  filled  with  a  substance  called 
luminiferous  ether,  having  certain  properties  that  account  for  the  facts 
which  are  observed.  The  planets  have  moved  through  it  for 
ages,  without  showing  appreciable  retardation  ;  it  must,  then, 
be  practically  frictionless.  Waves  are  transmitted  with  tremendous 
velocities ;  it  must,  therefore,  be  highly  elastic.  It  has  the  same  prop- 
erties at  all  points  and  in  all  directions ;  it  must,  then,  be  homogeneous. 
Ether  waves  come  to  us  from  the  remotest  depths  of  space;  it  must, 
therefore,  be  all-pervading.  We  picture,  then,  the  whole  universe  as 
filled  with  this  frictionless,  highly  elastic,  homogeneous,  all-pervading 
substance. 

The  molecules  are  not  in  contact  with  each  other,  but  are  embedded 
in  this  ether  as  dust  particles  are  suspended  in  dusty  air  or  as  sand  parti- 
cles are  suspended  in  muddy  water.     These  molecules  are  Heat  and 
not  at  rest  but  are  in  constant  motion,  colliding  with  their  temperature, 
neighbors,  rebounding,  and  quivering.    Since  the  molecules  are  in  motions 
possess  energy,  and  this  energy  of  the  molecules  is  called  heatl, 
Health*  -'.it  a  form  of  energy.    The  r. 

!v  possesses  is  thus  simply  the  sum  total  of  the  kinetic  er 


30  METEOROLOGY 

molecules.  A  careful  distinction  must  be  made  between  heat  and  tem- 
perature. Temperature  is  determined  by  the  velocity  of  motion  on  th 
part  of  the  individual  molecules.  If  a  body  is  heated,  the  molecules  mov 
more  rapidly  and  hit  harder.  There  is  now  a  more  active  motion  on  the 
part  of  the  molecules  and  the  temperature  is  thus  higher.  The  sum  total 
of  the  energy  of  the  molecules  is  also  greater  and  th§  body  thus  possesses 
a  greater  amount  of  heat.  The  following  numerical  data  for  hydrogen 
gas  under  normal  conditions  will  give  a  more  concrete  and  definite  picture 
of  the  construction  of  mattej :  The  number  of  molecules  per  cubic  centi- 
meter is  21  X  1018,  each  molecule  with  a  diameter  of  5.8  X  10~8  centimeter, 
and  a  mass  of  4.6  X  10~24  gram.  These  molecules  are  moving  with 
an  average  velocity  of  1.9  X  105  centimeters  per  second  over  a  path 
whose  average  length  is  9.7  X  10"6  centimeter,  before  colliding  with 
another  molecule.  The  average  number  of  collisions  per  second  is 
9,480,000,000. 

Whenever  a  molecule  collides  with  another  and  rebounds  and  quivers,  j 
it  becomes  the  centeFof  an  ether  wave  which  spreads  out  spherically  inl 
Radiant  every  direction.  These  waves  have  various  lengths,  that  is,' 
energy.  distances  from  crest  to  crest,  and  various  intensities.  They 
all  proceed  with  a  velocity  of  186,330  miles  per  second  in  ether  in  which 
no  material  molecules  are  embedded.  If  a  body  is  heated,  a  larger  num- 
ber of  waves  is  sent  out  and  shorter  waves  are  emitted  as  well  as  the  longer 
ones.  Terrestrial  bodies  at  ordinary  temperatures  are  emitting  waves 
having  lengths  from,  say,  0.001500  to  0.000270  centimeter.  The  waves 
sent  out  by  the  sun  are  shorter,  having  lengths  varying  from  0.000270  to 
0.000019.  Those  between  0.000270  and  0.000075  are  spoken  of  as  the 
infra-red  or  heat  rays ;  those  between  0.000075  and  0.000036  are  light 
waves  running  from  red  to  violet ;  those  between  0.000036  and  0.000019 
are  called  the  ultra-violet  rays.  The  limits  are  by  no  means  definite. 
The  figures  have  been  given  simply  to  indicate  approximately  the 
limits  between  which  the  great  majority  of  the  wave  lengths  lie.  The 
energy  conveyed  by  ether  waves  is  spoken  of  as  radiant  energy.  AH 
bodies,  then,  which  are  emitting  ether  waves  are  sending^but  radiant 
energy. 

The  statements  above  have  been  made  as  if  they  were  all  undoubted 

facts.     Many,  however,  are  merely  highly  probable  hypotheses.     The 

probability  of  the  correctness  of  an  hypothesis  increases  with 

the   number   and   complexity  of   the   facts  of   observation 

which  are  explained  and  correlated  by  it,  and  as  it  becomes  more  and 

more  evident  that  no  other  hypothesis  can  explain  as  much. 


OO)/  N'G   OF  THE  ATMOSPHERE 

THE  SOURCES  OF  ATMOSPHERIC  HEAT 

33.  ..There  are  uiree  sources.. from  which  heat  might  be  supplied  to  the 
Litmosphere ;    n;tmeJy^:Uhe_jSujv,the,  earthls-dntenor,  ancL.the-.ata.i>;  and 
|.Qthex.hfiaYenlyJaQdifia^ .  There  are'  two  ways  of  showing  that  Th,,  three 

the  amount  of  heat  supplied  to  the  atmosphere  by  any  other  possible 

urce  than  the  sun  is  relatively  negligible.     The  heat  sup-  s 
plied  by  the  earth's  interior  would  be  the  same  at  the  poles  as  at  the 
equator,  the  same  by  night  and  during  the  winter  as  by  day  and  during 
the  summer.     Thus  the  changes,  at  least,  in  the  temperature  ^  but  the 
of  the  atmosphere  cannot  be  accounted  for  by  heat  from  the  sun  negii- 
earth's  interior.     Again,  the  stars  shine  by  day  as  well  as  by  ^ 
night  and  on  all  portions  of  the  earth's  surface.'   Thus  again,  the  changes 
in  temperature  cannot  be  explained  on  the  basis  of  heat  received  from  the 
stars.     Direct  measurements  have  also  been  made  of  the  amount  of  heat 
received  from  these  sources,  and  the  general  conclusion  is  that  the  total 
amount  of  heat  supplied  by  all  other  sources  than  the  sun  is  not  suf- 
ficient to  change  atmospheric  temperatures  by  0.25°  Fahrenheit.     It  is 
the  sun,  then,  which  controls  absolutely  the  heating  of  the  atn   •-     ;*• 
and  we  must  look  to  it  for  an  explanation  of  all  the  varied  \.:' 
in  atmospheric  temperatures. 

INSOLATION 

34.  Amount.  —  The  radiant  energy,  that  is  thetenergy  in  the  form  of  | 
ether  waves,  received  from  the  sun  is  given  the  special  name  of  insolation,  [ 
Some  idea  of  the  amount  of  insolation  can  be  gained  by  con-  Definition  of 
sideringthe  size  and  condition  of  the  sun  itself.     The  sun  is  an  insolation, 
immense  globe,  having  a  diameter  of  866,000  miles  and  an  average 
surface  temperature  of  10,000  degrees  Fahrenheit.      More  than   325 
worlds  like  ours  could  be  strung  like  beads  on  a  string  around  the 
equator  of  the  sun.    The  best  pictures  of  the  intense  incan-  T 

Illustrations. 

descence  of  the  sun's  surface  can  be  gained  by  realizing  that 
each  square  yard  of  the  -Tin's  surface  is  constantly  emitting  140,000 
horse  power  of  energy.  The  earth  receives  but  a  minute  portion  (one 
two-billionth  part)  of  this,  and  yet  the  energy  received  in  the  course  of 
a  year  would  be  sufficient  to  melt  a  layer  of  ice  241  feet  thick  covering 
the  whole  earth. 

35.  "    '  latitude  and  time  of  year. — The  amount  oM»>- 
tioi  sun  at  any  particular  time  and  place  depe 

thro  arncss  to  the  sun.     The  amount  of  rad;aru    Tierg 


32 


METEOROLOGY 


The  three 
factors 
which  de- 
termine the 
amount  of 
insolation 
received. 


ved  from  any  hot  body  varies  inversely  as  the  square  of  the  distance 
of  the  point  in  question  from  the  body.  Thus  the  amount  of  insolation 
varies  inversely  as  the  square  of  the  distance  frofn  the  sun. 
(2)  Directness  of  the  rays.  When  the  sun's  rays  fall  upon  &r 
surface  obliquely  they  are  spread  out  over  a  larger  area  than 
when  they  fall  perpendicularly.  The  result  is  that  the 
amount  of  energy  received  by  each  unit  of  surface  is  far  less 
for  oblique  than  for  perpendicular  rays.  The  accompanying 
figure  will  illustrate  the  principle.  Let  AB  be  the  width  of  a  beam  falling 
perpendicularly  upon  the  surface  MN.  The  total  amount  of  energy  will 

be  concentrated  upon  the  space 
whose  width  is  CD.  Suppose  a 
beam  of  the  same  width  AB  falls 
obliquely  upon  the  surface.  Its 
energy  will  be  spread  out  over  a 
surface  having  a  width  of  CE. 
The  amount  of  energy  received 

FIG.  6.  —  Etoergy  Received  by  Perpendicular        .  ,.  r  ^»T>       •« 

ank  Oblique  incidence.  by  each  unit  of  area  of  CD  will 

be  seen  at  once  to  be  far  greater 

than  that  received  by  each  unit  of  area  of  the  surface  CE.  It  can  be 
readily  demonstrated  by  trigonometry  that  the  amount  of  energy  re- 
ceived by  a  unit  area  in  the  case  of  oblique  incidence  equals  the  amount 
received  by  perpendicular  incidence  multiplied  by  the  sine  of  the  angle 
of  elevation  of  the  sun.  By  angle  of  elevation  is  meant  the  distance  in 
degrees  of  the  sun  above  the  horizon.  (3)  Duration.  The  amount  of 
energy  received  from  a  radiating  body  is,  of  course,  directly  proportional 
to  the  duration  of  the  radiation. 

36.  In  order  to  understand  the  variations  in  these  three  factors 
which  determine  the  amount  of  insolation  received,  one  must  study  the 
revolution  of  the  earth  about  the  sun.  The  earth's  path 
around  the  sun  is  not  an  exact  circle,  but  an  ellipse  with  the 
sun  in  one  focus.  This  path  or  orbit  of  the  earth  lies  in  a 
plane  called  the  plane  of  the  ecliptic,  and  the  earth  completes 
the  circuit  in  365^  days.  The  average  distance  of  the  earth 
from  the  sun  is  92,900,000  miles,  but  the  actual  distance  varies  about 
1,500,000  miles  in  either  direction  in  the  course  of  a  year.  The 
earth  is  nearest  to  the  sun  on  January  1  and  at  its  greatest  distance  on 
July  1.  The  axis  of  the  earth  is  not  perpendicular  to  the  plane  of  the 
ecliptic,  but  makes  an  angle  of  66?°  with  it,  and  this  a.\is  remains  parallel  to 
itself  as  the  earth  revolves  about  the  sun.  Figure  7  illustrates  this  revo- 


The  char- 
acteristics 
of  the 

earth's  revo- 
lution about 
the  sun. 


THE  HEATING  AND  COOLING   OF  _THE  ATMOSPHERE        33 


Changes  in 
distance. 


mtion.  The  north  pole  and  the  northern  hemisphere  of  the  earth  are 
turned  most  directly  towards  the  sun  on  June  21  and  away  from  the 
sun  on  Dec.  21.  Figure  8,  which  indicates  the  position  of  the  earth 
on  June  21  and  Dec.  21,  shows 
this  change  in  presentation  to  the 
sun's  rays. 

Since     the     earth     is     nearly 
3,000,000    miles   nearer   the   sun 
on  January  1  than  on 
July  l,fnore  insolation 
must  be  received  in  January  than 
in  July.     The  difference  amounts 
to  about  7  per  cent. 

The  change  in  the  presentation 
of  the  earth  to  the  sun's  rays 
causes  an  apparent  Changes  in 
migration  of  the  sun  directness, 
northward  from  Dec.  21  until 
June  21  and  southward  from  June  21  until  Dec.  21.  The  sun  is 
directly  overhead  at  noon  on  March  21  at  the  equator,  on  June  21 
at  the  tropic  of  Cancer,  on  September  23  at  the  equator  again, 
and  on  December  21  at  the  tropic  of  Capricorn.  This  migration  of 


FIG.  7.  —  The  Revolution  of  the  Earth  around 
the  Sun. 


JUNE  21 


DECEMBER   21 


FIG.  8.  —  Til-  Presentation  of  the  Earth  to  the  Sun  on  June  21  and  December  21. 

the  sun  through  47°  (2X23J)  causes  decided  changes  in  the  directness 
of  the  sun's  rays  and  in  the  length  of  the  day.  The  elevation  of  the 
sun  at  noon  for  any  place  in  the  northern  h^rrji «.->••' 


METEOROLOGY 


is  <£  can  be 


shown 

° 


to  be  90°  -  <#>  on  March  21  and  September  23, 
90°  -  <t>  +  23J°  on  June  21,  and  90°  -  <£  -  23i°  on  December  21. 
Thus  the  noon  elevation  of  the  sun  changes  by  47°  during  the  year, 
and  this  causes  a  great  change  in  the  insolation  received  at  any  one  point. 
The  length  of  the  day  also  changes  markedly  throughout  the  year.  In 
the  latitude  of  New  York  State  the  length  of  the  day  varies  from  a  little 
Changes  in  more  than  fifteen  hours  during  the  summer  to  a  little  less  than 
Duration.  njne  ftours  during  the  winter.  The  accompanying  table  gives 
the  greatest  possible  duration  of  insolation  for  various  latitudes. 

41 


Latitude    0° 
Duration  12h 


17° 
13hr 


49° 
16h, 

(. 


63° 

20hr- 


66°  30' 

24hr- 


67°  21' 
Imo. 


69°  51'  78°  ft 
2  mo.     4  mo. 


90° 
6  mo. 


37.  Since  the  three  factors  that  determine  the  amount  of  insolation 
received  have  different  values  for  different  latitudes  and  times  of  year,! 
it  follows  that  the  insolation  received  varies  with  both  time  and! 

latitude.  For  the  same  place  it  is 
different  at  different  times  of  year ; 
for  the  same  date  it  is  different 
at  different  places,  that  is,  different 
latitudes,  on  the  earth's  surface. 

Figure  9  shows  the  amount  of 
insolation  received  for  the  various 
Variation  latitudes  at  five  differ- 
with  lati-  ent  times  between  the 
20th  of  March  and  the 
21st  of  June.  The  unit  of  insola- 
tion is  the  amount  received  in  a 
day  at  the  equator  on  March  21. 
On  the  21st  of  June,  for  example,  the  amount  of  insolation  Deceived  at 
the  North  Pole  is  greater  than  that  received  at  the  equator.  As  far  as 
nearness  to  the  sun  is  concerned,  the  amount  received  at  both  the 
equator  and  the  pole  would  be  the  same ;  as  far  as  directness  is  con- 
cerned, at  the  equator  the  sun  at  noon  has  an  elevation  of  66j°  while  at 
the  pole  its  elevation  is  but  23i°.  Due  to  this  cause,  much  more  energy 
would  be  received  at  the  equator.  The  duration  of  insolation,  how- 
ever, is  twelve  hours  at  the  equator  and  twenty-four  boars  at  the 
pole.  When  the  combined  effect  of  the  three  factors  is  determined 
it  is  found  that  the  amount  of  energy  received  at  the  pole  is  greater 
than  that  received  at  the  equator. 

Figure  10  shows  the  amount  of  energy  received  at  three  different  lati- 


jjj  20  30  10  .60  60  £0  80   50 

FIG.  9.  —  Variation  in  the  Insolation  with 
Latitude  at  five  Different  Dates,  (after 
WIENER.) 


HEATING  AND  COOLING   OF  THE  ATMOSPHERE        35 


tudes  on  the  earth's  surface  < 
two  maxima,  one  on  the  2L 
S(  ptembert     These  are  varia 
the  dates  when  the  sun  ^^ 
is  directly  over  head  at  noo 
the    insolation    thus    falls 
directly.     The   March  max 
is  greater  than  the  Septembe 
because  the  earth  is  nearer  1 
sun  at  that  time.     The  du 
and    directness   are   the   sai 
both  cases.     The   amount 
solation    received    at    the 
Pole  is  zero  until  the  21st  of  fl 
when  it  rises  rapidly  to  a  ma? 
on  the  2la/t  of  June,  then 

TV/ 

luring  the  y 
5t  of  March 

tion 
Lime. 

,             JAN. 

n  and        FEB. 
most      MAR. 

imum      APRIL 
ir  one       MAY 

,/                  JUNE 

$  the 
XT        JULY 

PtylOn             AUG. 

ae    in     SEPT. 
f   in-      OCT- 
North 

r       u         DEC' 

Larch,      JAN. 
mum 
drops      FlQ 

-ear.     At  the  equator  there  are 
and  the  other  on  the  23d  of 

p      P      P     *p      r      j-^ 

»o          *.           ea           oo          o          »                   \ 

\ 

\ 

FEB. 
MAR. 
APRIL 
MAY 
JUNE 
JULY 
AUG. 
SEPT. 
OCT. 
NOV. 
DEC. 
JAN. 

\ 

f 

~-      _ 

-~  —  .. 

-  —  -^ 

^ 

^-—  -, 

s^ 

}  ) 

\ 

S 

§ 



— 

—  — 

—  ^** 

.. 

^s' 

s\ 

f 

°l 

E 

( 

'( 

\ 

V 

£p          p          p 
1*.               0>              00 

10.  —  Annual  Vtfnatior 

9                   fed 

i  in  the  Insola- 
ent  Latitudes. 

back  toytftrQ  again  on  the  2 

3d  Of        tion  Received  at  thu^Diffei 

September. 

38.  Distribution  ove 
the  earth  is  shown  by 
the  values  of  insolation  for  fo 
ent  latitudes.  The  unit  is  t 
in  a  day  at  the  equator  on 


distribution  of  insolation  over 
ying  table,  which  gives 
^rent  dates  and  six  differ- 
nt  of  insolation  received  table. 
21st  of  March. 


LATITUDE                      ~*~S 

0° 

20° 

40° 

60° 

90° 

-  90° 

March  21    

1.000 

0.934 

0.763 

0.499 

0.000 

0.000 

June  21       

0.881 

1.040 

1.103 

1.090 

1.202 

0.000 

Sept.  23      

0.984 

0.938 

0.760 

0.299 

0.000 

0.000 

Dec  21       ..               .... 

0.942 

0.679 

0.352 

0000 

0000 

1  284 

Since  the  amount  of  insolation  varies  with  both  latitude  and  time,  this 


cannot  be  expressed  in  the  ordinary  way  by  means  o    a  £' 
Figure   11,  aowever,  from  DAVIS'S  Elementary  Meteorology, 
represents  tiis  relation.     The  time  is  plotted  along  the  hori- 
zontal  axis  aid  latitude  along  the  other  axis.     The  amount  of  tion  of  in- 
insolation  foi  any  definite  time  and  latitude  is  indicated  by  receded. 
the  length  of  ti?  perpendicular  to  the  plane  of  the  axes.     The 
surface  which  pisses  through  the  ends  of  these  perpendiculars  thus 
represents  fhe  vacation   in  insolation.     The  curves  given  in   FTP-    9 


36 


METEOROLOGY 


represent  the  intersection  with  this  surface,  of  planes  perpendicular  to 
the  time  axis  at  the  dates  in  question.  The  curves  given  in  Fig.  10  are 
the  intersection  with  this  surface  of  planes  passed  perpendicularly  to  the 
latitude  axis  at  the  latitudes  in  question. 

In  all  of  the  foregoing  sections,  35  to  38  inclusive,  it  is  the  distribution 
of  the  insolation  at  the  outer  limit  of  the  atmosphere  which  has  been 


FIG.  11.  —  Variation  in  the  Insolation  with  Latitude  and  Time. 
(From  DAVIS'S  Elementary  Meteorology.) 

considered.  The  distribution  over  the  earth's  surface  would  be  the  same 
provided  the  earth  had  no  atmosphere  or  provided  the  earth's  atmosphere 
allowed  all  of  the  insolation  to  pass  through  it.  The  question  as  to  what 
portion  is  absorbed  in  the  atmosphere  and  what  portion  actually  reaches 
the  earth's  surface  will  be  discussed  in  sections  43  and  44. 


THE  INTERRELATION  OF  MATTER  AND  RADIANT  ENERGY 


Reflection.  —  Whenever  ether  waves  strike  a  materiil  medium 
and  are  turned  back,  they  are  said  to  be  reflectel.  This  is 
entirely  analogous  to  the  reflection  of  a  sound  onvater  wave 
from  a  rigid  barrier.  There  are  two  kinds  of  reflation  ;  regu- 
lar or  mirror  reflection,  and  irregular  or  diffuse  Bflection.  In 
the  case  of  regular  reflection  the  angle  made  oy  the  incident 
ray  with  the  perpendicular  to  the  reflecting  surface  is  /qual  to  the  angle 


39- 

Definition. 


The  two 
kinds  of  re- 
flection. 


THE  HEATING  AND  COOLING   OF  THE  ATMOSPHERE        37 

'  made  by  the  reflected  ray.     In  the  case  of  diffuse  reflection  the  reflected 
rays  pass  off  in  every  direction  and  are  scattered.     Burnished  metal  and 
ground  glass  are  examples  of   these  two  kinds  of  reflectors.     The  best 
reflector  known  is  burnished  silver,  and  this  reflects  abj^98  per  cent  of 
the  incident  rays  when  these  are  perpendicular  to  the  surface.     The 
insolation  from  the  sun  on  reaching  the  earth  falls  upon  a  variety  of 
things,  such  as  pure  air,  dusty  air,  clouds,  water,  etc.     These  peflection 
things  arranged  in  order  of  reflective  power  are  as  follows :  of  insoia- 
water,  snow,  cloud,  dusty  air,  earth,  pure  air.     Of  these  pure 
air  reflects  practically  nothing,  while  snow  and- water  reflect  from  30  to 
50  per  cent. 

4«.    Transmission.  — Whenever  ether  waves  are  allowed  by  a  body- to 
pass  through  it  they  are  said  to  be  transmitted^    A  water  analogy  would 
be  the  passage  of  an  ocean  wave  through  the  meshes  of  a  fish  Definition 
net  suspended  in  it.     The  best  transmitter  known  is  rock  salt. 
All  transmission  is  selective  in  character ;  that  is,  each  body  allows  waves 
of  certain  lengths  to  pass  through  it  "more  readily  than  others.      In  the 
case  of  the  atmosphere,  the  longer  waves  are  the  ones  which  selective 
are  most  readily  transmitted.     Glass  transmits  well  the  light  transmis- 
waves,  but  does  not  transmit  as  readily  the  longer  and  shorter  s 
ether  waves.      I^ock  salt  is  the  best  transmitter  because  it  transmits  well 
ether  waves  of  practically  all  lengths.     The  various  things  upon  which 
insolation  may  fall  arranged  in  order  of  excellence  as  regards  The  ^ans_ 
transmission  are  as  follows  :  pure  air,  dusty  air,  water,  snow,  mission  of 
cloud,  earth.     Of  these  the  earth  transmits  practically  noth-  msolation- 

•ing,  while  pure  air  transmits  more  than  90  per  cent  of  the  insolation  which 
falls  upon  it. 

41.  Absorption. — Absorption  takes   place  whenever   an  ether  wave 
enters  a  body  and  is  destroyed  by  it.    A  water  analogy  for  this  process  can 
also  be  given.     Suppose  a  pond  to  be  covered  with  logs  which 

are  in  contact  with  each  other.     If  a  water  wave  strikes  these 
logs,  the  first  ones  are  set  in  motion,  and  the  wave  loses  intensity  and  soon 
ceases  to  exist.     The  logs  grind  against  each  other  and  by  friction  are 
soon  brought  to  rest.     The  energy  of  the  water  wave  has  Absorption 
been  transformed  into  energy  of  motion  on  the  part  of  the  logs,  of  insoia- 
The  best  absorber  known  is  carbon.     The  various  things  upon  * 
which  insolation  may  fall,  arranged  in  order  of  excellence  as  regards 
absorption,  are  as  follows  :  earth,  snow,  cloud,  water,  dusty  air,  pure  air. 

42.  There  are  four  effects  of  absorption.     (1)   It  may  heat  the  body. 
In  this  case  the  energy  of  the  ether  waves  has  been  used  up  in  exciting 


38  METEOROLOGY 

a  more  vigorous  motion  on  the  part  of  the  molecules.  (2)  It  may  cause 
vision.  Whenever  ether  waves  of  certain  wave  lengths  enter  the  eye 
The  four  an(^  ^  uPon  the  rods  and  cones  of  the  retina,  they  impart 
effects  of  a  stimulus  which  results  in  vision.  In  the  human  eye 
absorption.  waveg  of  greater  length  than  0.000075  centimeter  or  shorter 
than  0.000036  centimeter  produce  no  effect.  The  color  perceived 
ranges  from  red  to  violet,  depending  upon  the  length  of  the  wave. 
(3)  It  may  cause  chemical  reaction.  When  ether  waves  are  absorbed  by 
certain  compounds,  the  molecular  activity  which  is  excited  is  so  great 
that  the  molecules  break  apart  and  new  compounds  are  formed.  This  is 
the  basis  of  photography.  Certain  salts,  usually  silver  salts,  on  the 
photographic  plate  are  decomposed  by  the  light  rays,  and  thus  a  perma- 
nent impression  on  the  photographic  plate  is  made.  But  a  small 
amount  of  ether  energy  is  used  up  in  this  way,  however.  A  very  large 
amount  of  insolation  is  consumed  by  plants,  as  described  in  sec- 
tion 15,  when  the  green  cells  using  the  energy  of  insolation  com- 
bine the  sap  and  CO2  of  the  atmosphere,  making  the  complex 
molecules  from  which  its  own  tissues  are  built  up.  The  longer 
ether  waves  are  usually  more  efficacious  in  causing  heat,  the  inter- 
mediate ones  in  producing  vision,  and  the  shorter  ones  in  causing 
chemical  decomposition.  (4)  It  may  cause  change  of  state.  If  inso- 
lation falls  upon  a  body  of  water,  a  large  amount  of  energy  is  used  in 
evaporating  the  water,  that  is,  in  changing  it  from  the  liquid  to  the  gas- 
eous state  without  changing  its  temperature.  Energy  used  in  this  way 
is  called  latent  heat. 

43.    Actinometry.1  — An  actinometer  is  an  instrument  for  measuring 
the  insolation  received  from  the  sun.     Actinometry  treats  of  the  use 

Thefunda-      of     this 


mental         .  ment  in  making 

formula. 

the  measure- 
ments. The  fundamental 
formula  in  actinometry  is 

FIG.  12.  —  Showing  the  Contrast  in  the  Thickness  of      fj  =  Co?.     H  here  represents 
Air  passed  through  by  Vertical  and  Oblique  Rays. 

the  amount  of  energy  re- 
ceived on  a  unit  of  surface  at  right  angles  to  the  sun's  rays,  a  is  the 
percentage  transmitted  by  the  atmosphere  when  the  rays  fall  vertically. 
Its  value  would  be  unity  for  perfect  transmission.  I  is  the  thickness 
of  the  atmosphere,  considering  the  vertical  thickness  as  unity.  C  is^a 

1  For  a  bibliography  of  the  articles  on  solar  radiation  see  Bulletin  of  the  Mount  Weather 
Obs..  Vol.  3,  part  2,  pp.  118-126. 


1 


THE  HEATING  AND  COOLING   OF  THE  ATMOSPHERE        39 


constant,  ordinarily  known  as  the  "  solar  constant."  The  thickness  of 
the  layer  of  air  through  which  the  sun's  rays  pass  increases  rapidly  as 
the  sun  nears  the  horizon.  Figure  12  shows  well  the  contrast  in  the 
length  of  path  for  vertical  and  oblique  rays.  The  following  table  gives 
the  various  values  of  /  for  the  different  altitudes  of  the  sun :  — 

Altitude  of  sun  0°      5°      10°     20°    30°    50°    70°    90° 

Thickness  of  the  atmosphere  in  units      35.5  10.2  5.56  2.90  1.99  1.31  1.06  1.00 

It  will  be  seen  that  the  thickness  of  air  traversed  by  the  sun's  rays 
when  rising  or  setting  is  more  than  35  times  the   thickness  when  the  1  (jr 
sun  is  directly  overhead. 

The  term   a1  is  equal  to   one  if  I  is  zero  or  if  a  is  equal  to  one.     a 
equaling  one  means  that  the  atmosphere  is  transmitting  all  of  the  in- 
solation.    /  equaling  zero  corresponds  to  a  zero  thickness  of 
the  atmosphere,,  that  is,  to  no  atmosphere  at  all.     In  either  ing  of  the      •       • 
case  H  would  equal  C.    C  is  then  the  amount  of  energy  re-  solar  con~ 

»  Stflnt* 

ceived  by  a  given  unit  area  at  right  angles  to  the  sun's  rays, 

provided  there  were  no  absorption  in  the  atmosphere,  or  provided  the 

atmosphere  wore  not  present. 

If  the  elevation  of  the  sun  above  the  horizon  be  known,  I  can  at  once  be 
computed  from  the  table  given  above.     Absolute  or  relative  values  of 
H  can  be^ietermined  in  several  different  ways.     Seven  ways 
will  be  sirfaply  mentioned  here,  and  the  reader  must  be  re-  metho(is  of 
fericd  to  special  articles  on  the  subject  for  a  more  detailed  determining 
treatment.     The  various  methods  make  use  of  calorimetrical  received?7 
apparatus,  a  pyrheliometer,  a  thermoelectric  couple,  a  bolom- 
eter arrftngqment,  chemical  decomposition,  a  black  bulb  thermometer 
in  vacuo,  or  photographic  paper.     The  black  bulb  thermometer  will  be 
considered  later  HJL  section  70.     The  method  by  means  of  photographic 
paper,  although  not  especially  sensitive,  is  so  ingenious  as  to  be  worthy  of 
a  few  words  of  explanation.     The.  blackening  of  an  ordinary  piece  of 
photogra  phic  paper  when  exposed  to  the  rays  of  the  sun  depends  upon 
the  intensity  of  the  sun's  rays  and  the  time  of  exposure.     The  ratio  of 
the  times  of  exposure  required  to  produce  the  same  amount  of  blacken- 
ing for  two  different  values  of  intensity  gives  the  ratio  of  those  intensities. 
Thus  relative  values  of  H  can  be  readily  determined. 

If  H  has  tveeii  determined  for  two  different  values  of  I,  by  means  of 
the  equation  given  above,  C  and  a  can  be  computed.  There  The  method 
are  two  ways  of  getting  different  values  of  I  One  is  by  mininTc 

taking  simultaneous  observations  from  a  mountain  top  and  a  and  a. 


40 


METEOROLOGY 


near-by  valley,  another   is   to  take  observations  at  different  times  of 
day,  thus  getting  different  thicknesses  of  atmosphere. 

Numerous  measurements  have  been  made  under  the  most  varied  con- 
ditions and  at  many  places.  The  different  values  for  C  are  not  in  good 
The  values  agreement,  but  the  values  ordinarily  used  at  present  are 
of  C  and  a.  ^wo  or  three  calories  per  square  centimeter  per  minute.  A 
calorie  is  one  one-hundredth  of  the  amount  of  heat  required  to  raise  the 
temperature  of  one  cubic  centimeter  of  water  from  the  freezing  to  tho 

boiling  point.  When 
the  sky  is  clear,  the 
vahie  of  a,  the  trans- 
mission coefficient,  is 
about  75  per  cent. 
Under  various  condi- 
tions, howeve^^alues 
as  low  as  even  10  or 
20  per  cent  have  been 
observed.  The  best 
modern  values  of  the 
solar  constant  are 
without  doubt  those 
which  have  been  ob- 
tained since  1904  by 
Mr.  C.  G.  Abbot, 
Director  of  tH  e  Astro- 
physical  Observatory 
of  the  Smithsonian  In- 
stitution. His  results 
are  in  much  better 
agreement  and  would 
seem  to  indicate  a 


13  AUGUST, 1 1888 
Temp:  max.  21°0  C..  jmin. 
Bel.  Jiumidity.  M,  Sky  very  cle  ir 


VI     VII    VIII    IX       X      XI     NOON     I        II      HI     JV       V 

FIG.  13.  — -  Insolation  on  Mont  Ventoux. 
(From  HANN'S  Lehrbuch  der  Meteor ologie,) 


7 


MONTlPEU 


iv/n  i  rc.ui.ie.n,    fu 
1 13  AUGUST,  1888 

Tem'p-  iJx.  3ltO  C.,  ml     "' 

Rel.j  humidity;  70^. Sky 

Wind:  east,  moderate. 

Pressure- j  761. 7  mml 


VI     V.Il    Vlli   IX     X      XI    NOON    I        II      III      IV      V 

FIG.   14.  —  Insolation  at  Montpellier. 
(  From  HANN'S  Lehrbuch  der  Meteor  ologie.') 


The  effect 
of  atmos- 
pheric ab- 
sorption on 
the  insola- 
tion re- 
ceived dur- 
ing the  day. 


value  a  little  less  than  2 
(between  1.9  And  2.0). 
Figures  13  and  14  show  well  the  effects  of  atmospheric  .of  ^orption. 
The  amount  of  insolation  received  on  a  surface  having  an  area 
of  a  square  centimeter  and  kept  constantly  at  right  angles  to 
the  sun's  rays  is  here  given  for  the  same  day,  August  13,  1888, 
on  Mont  Ventoux  with  an  elevation  of  2000  meters,  and  at 
Montpellier  with  art  elevation  of  40  meters.     If  thn  atmos- 
phere had  transmitted  all  the  incident  insolation,  the  value 


VD  COOLING   OF  1:3JE  ATMOSPHERE        41 

irp  al  to  the  solar  constant  at  both  places  from  the 
n  •:.:  intii  sunset.      The  graphs  show  well  the  gradual 

rise  in  value  of  the  insolation  during  the  morning  and  the  gradual 
falling  off  during  the  afternoon,  due  to  the  varying  thickness*  of  the  at- 
mosphere through  which  the  rays  were  coming.     The  largest  value  on 
Mont  Ventoux  is  1.6,  while  the  largest  value  at  Montpellier  is  1.2.     This 
difference  is  due  to  the  absorption  of  the  1960  meters  of  air  which  is  their 
difference  in  elevation.     It  will  also  be  noticed  that  the  maximum  values 
were  received  several  hours  before  noon.     The  reason  for. this  is  that  the 
air  is  always  less  transparent  in  the  afternoon  than  during  the  morning. 
The  accompanying  table  from  Angot' gives  the  amount  of  insolation 
received  at  the  earth's  surface  in  the  course  of  a  year  at  various  latitudes, 
first  on  the  assumption  that  the  atmospher^  transmit?  a-lL--the  e^-ct 
of  the  insolation,  and  secondly  on  the  assumption  that  60  per  of  atmos- 
cent  is  transmitted.     The  unit  of  insolation  here  \fsed  is  the  so^tiorfon 
amount  of  insolation  received  at  the  equator  during  oi\e  day,  the  insoia- 
on  the  21st  of  March.     It  will  be  seen  at  once  that  the  "dim-  Reived  at 
inution  in  the  amount  of  insolation  at  the  pole  is  much  more  different 
marked  than  at  the  equator.         It  will  also  be  seen  from 
Figs.  13  and  14  and  the  table  that  not  more  than  one  third  of  the  in- 
solation received  from  the  sun  actually  reaches  the  earth's  surface  on 
any  particular  day.     The  transmission  coefficients  for  various  places  on 
the  earth's  surface  during  any  given  day  probably  vary  from  10  per  cent 
to  perhaps  90  per  cent.     Sixty  per  cent  would  seem  to  be  a  large  average 
for  the  whole  earth,  but  even  with  that  figure,  as  seen  from  the  table, 
barely  more  than  one  third  of  the  insolation  reaches  the  earth's  surface. 


a  =  1.0 
a  =  0.6 

EQUATOR 

10" 

20° 

30° 

40° 

50° 

60° 

70° 

80° 

POLE 

350.3 
170.2 

345.5 
166.5 

331.2 
155.1 

307.9 
137.6 

276.8 
115.2 

239.8 
90.6 

199.2 
67.4 

166.3 

47.7 

150.2 
33.5 

145.4 

28.4 

45.    Behavior  of  the  ocean  as  regards  reflection,  transmission,  and 
absorption. — The  ocean  reflects  about  40  per  cent  of  the  insolation  which 
falls  upon  it,  and  transmits  the  remainder  to  considerable  The  ocean 
depths,  but  eventually  absorbs    all    the    insolation    which  changes  but 
is  transmitted.     The  ocean  changes  extremely  little  in  tern-  perjure*5" 
perature  between  day  and  night,  and  there  are  five  reasons  for  between  day 
this  :   (l)  So  large  an  amount,  namely,  40  per  cent,  of  the  in-.  " 
solation  is  reflected  and  thus  lost  as  far  as  heating  the  water  is  concerned. 
(2)  The  insolation  which  is  absorbed  is  transmitted  to  considerable 


42  METEOROLOGY 

depths,  and  thus  it  is  not  a  thin  surface  layer,  but  a  layer  of  considerable 
depth  that  is  involved.  (3)  A  considerable  amount  of  evaporation  takes 
place  from  the  surface  of  the  ocean,  and  the  insolation  is  used  in  causing 
this  change  of  state  and  not  used  in  causing  a  rise  of  temperature  on  the 
part  of  che  water.  (4)  It  requires  a  larger  amount  of  heat  to  raise  the 
temperature  of  a  given  quantity  of  water  than  any  other  substance. 
Tlie  technical  expression  for  this  is  that  the  specific  heat  of  water  is  larger 
tl.an  that  of  any  other  substance.  For  this  reason  a  very  small  rise  in 
temperature  takes  place  as  the  result  of  absorbing  a  considerable  amount 
of  insolation.  (5)  The  water  of  the  ocean  is  in  continual  motion.  Thus 
again  it  is  not  a  small  surface  layer  which  is  involved,  but  a  considerable 
amount  of  water.  On  account  of  these  five  reasons,  the  temperature  of 
the  ocean  rises  very  little  during  the  day.  Because  it  is  in  continual  mo- 
tion, transmits  readily,  and  has  a  high  specific  heat,  it  cools  but  little  dur- 
ing the  night.  The  behavior  of  the  ocean  may  then  be  briefly  sum- 
marized by  stating  that  its  temperature  change  between  day  and  night 
is  extremely  small,  never  amounting  to  more  than  one  or  two  degrees. 

46.  Behavior  of  the  land  as  regards  reflection,  transmission,  and 
absorption.  —  Dry  ground  reflects  but  a  few  per  cent  of  the  insolation 
which  falls  upon  it,  transmits  practically  none,  and  thus  absorbs  nearly 

all.     The  rise  in  temperature  of  dry  ground  under  insolation 

perature         is  very  great.     In  the  first  place,  it  absorbs  nearly  all  of  the 

Choundbe-     ms°lati°n>  and,  being. opaque,  this  takes  place  in  a  thin  sur- 

tween  day      face  layer.    Its  specific  heat  is  far  les?  than  that  of  water,  and, 

being  a  solid,  there  can  be  no  mixture  as  in  the  case  of  the 

ocean.     Since  the  ground  is  a  good   Absorber,  it  is  also  a  good 

radiator  and  thus  for  similar  reasons  it  cools  rapidly  at  night.     If  the 

ground  is  wet  or  covered  with  vegetation,  the  change  in  temperature 

between  day  and  night  is  much  less  than  for  dry  ground,  but  always 

greater  than  for  the  ocean. 

47.  Behavior    of    the    atmosphere   as  regards  reflection,   transmis- 
sion, and  absorption.  —  Pure  air  reflects  but  a  minute  quantity  of  the 
The  chan  e    mso^a^^on  which  falls  upon  it,  absorbs  as  little,  and  transmits 
in  tempera-    practically   all.     As   a  result  it   changes   temperature  but 
I'tmo^here    very  little  between  day  and  night.     Dusty  air  reflects  more 
between        and  absorbs  more,  but  is  still  one  of  the  best  transmitters. 

The  atmosphere  contains  more  dust,  water  vapor,  and  carbon 
dioxide  near  the  earth's  surface,  and  thus  the  change  in  tem- 
perature between  day  and  night  increases  with  nearness  to  the  earth's  sur- 
face.    The  reflection  and  absorption  on  the  part  of  the  atmosphere  are 


THE  HEATING  AND  COOLING   OF  THE  ATMOSPHERE 


selective.  In  the  case  of  reflection  it  is  the  shorter  waves  which  are 
most  effectively  scattered,  while  in  the  case  of  absorption  it  is  the  longer 
ether  waves  which  are  chiefly  concerned,  and  this  is  particularly  the  case 
when  the  amount  of  water  vapor  and  carbon  dioxide  is  large.  The 
temperature  change  in  the  atmosphere  between  day  and  night  due  to 
its  behavior  as  regards  insolation  would  probably  be  larger  than  the 
temperature  change  of  the  ocean,  but  never  as  large  as  that  of  the 
ground. 

The  contrast  between  the  temperature  of  the  ground  and  the  tempera- 
ture of  the  air  above  it  is  well  brought  out  by  the  following  typical 
example.     The  observations  were  made  in  1895  at  Tiflis  with 
a  latitude  of.  41°  43'  and  an  elevation  of  410  meters.     The  trastbe- 
averages  of  the  temperature  observations  (centigrade)  made  tw<jen  air 
during  three  months  at  twelve  different  times  of  day  are  here 
given.     The  air  temperatures  were  taken  3  meters  above  the  ground. 


TIME           1  A.M. 

3 

5 

7 

9 

11 

1  P.M. 

3 

5 

7 

9 

11 

December 


January 


February 


Ground         0.2 

-0.2 

-0.5 

-0.8 

3.0 

10.3 

13.2 

10.9 

4.3 

1.9 

1.2 

0.6 

Air                 1.5 

1.1 

0.8 

0.5 

2.0 

4.6 

6.6 

7.3 

5.6 

3.8 

2.7 

2.1 

Difference  -1.3 

-1.3 

-1.3 

-1.3 

1.0 

5.7 

6.6 

3.6 

-1.3 

-1.9 

-1.5 

-1.5 

June 


July 


August 


Ground       19.2 

18.1 

17.6 

23.1 

34.7 

45.1 

49.0 

45.4 

35.8 

26.1 

22.3 

20.5 

Air               18.9 

18.0 

17.5 

19.4 

22.4 

24.8 

26.3 

26.9 

26.3 

23.8 

21.5 

20.1 

Difference    0.3 

0.1 

0.1 

3.7 

12.3 

20.3 

22.7 

18.5 

9.5 

2.3 

0.8 

0.4 

It  will  be  seen  that  during  the  (jay,  both  in  summer  and  in  winter,  the 
temperature  of  the  ground Js  much  fogjffi  thajn  the  temperature  of  the 
air.  The  summer  difference  is  two  or  three  times  the  winter  difference. 
During  the  nig^t  the  grounds  glightty  pnJrl^r  t^flj^thejiy  in  winter  and 
slightly  warmer  in  summer. 

The  contrast  between  the  temperature  (centigrade)  of  the  ocean  and 
the  temperature  of  the  air  above  it  is  well  shown  in  the  following  table, 
computed  by  Hann  for  the  Atlantic  Ocean  from  the  observa- 
tions  of  the  Challenger.      It  corresponds  to  about  30°  north  trast  be- 
latitude.     The  air  is  thus  slightly  warmer  than  the  ocean  by 
day  and  slightly  cooler  at  night.      The  ocean  can  thus  pla}' 
no  part  in  heating  the  atmosphere  by  day  or  in  cooling  it  at  night. 


44 


METEOROLOGY 


TIME           1  A.M. 

3 

5 

7 

9 

11 

1  P.M. 

3 

5 

7 

9 

11 

Ocean           19.8 

19.7 

19.8 

19.8 

20.0 

20.1 

20.1 

20.2 

20.1 

20.0 

19.9 

19.8 

Air                 18.9 

18.9 

19.0 

19.2 

19.6 

20.2 

20.6 

20.6 

20.3 

19.7 

19.3 

19.0 

Difference      0.9 

0.8 

0.8 

0.6 

0.4 

-0.1 

-0.5 

-0.4 

-0.2 

0.3 

0.6 

0.8 

Definition. 


The  two  typical  examples  which  have  just  been  given  hold  for  the 
interior  of  large  bodies  of  land  and  for  the  open  ocean.  Near  the  sea- 
shore a  combination  of  the  two  would  be  found. 

CONDUCTION  AND  CONVECTION 

48.  Conduction.  —  It  is  a  matter  of  everyday  experience  that  if  one 
end  of  a  bar  of  metal  is  heated,  the  other  end  also  becomes  hot.     The 

molecular  agitation  set  up  in  the  heated  end  of  the  bar  is 
transmitted  from  molecule  to  molecule  until  the  other  end 
is  reached.  This  transmission  of  heat  from  one  part  of  a  body  to  an- 
other or  from  one  body  to  another  by  means  of  the  molecules  is  known 
as  conduction.  All  of  the  metals,  particularly  silver  and  copper,  are  good 
conductors  of  heat.  All  of  the  substances  with  which  one  has  to  do  in 
meteorology,  such  as  air,  ground,  water,  cloud,  etc.,  are  very  poor  con- 
ductors. 

The  ground  during  the  day  may  become  very  warm  on'  account  of  the 

insolation  received.    At  night  it  cools  off  rapidly,  due  to  radiation.    This 

diurnal  oscillation  in  temperature,  however,  does  not  penetrate 

Air  RUG 

ground  to  a  depth  of  more  than  two  or  three  feet  and  requires  many 

poor  con-       hours  to  penetrate  to  even  this  depth.     The  temperature 

ductors. 

changes  between  winter  and  summer  do  not  penetrate  to  a 
depth  of  more  than  thirty  or  forty  feet,  and  nearly  six  months  elapse 
before  the  lower  layers  are  reached.  Thusjitji  depth  of  thirty  or  forty 
feet  the  highest  temperatures  occur  in  winter  and"  tHelowest  nVstfthmer. 
The  air  is  also  a  very  poor  conductor.  It  might  remain  in  immediate 
contact  with  the  ground,  which  is  many  degrees  hotter  than  itself,  for  a 
whole  day,  and  yet,  due  to  conduction  alone,  only  the  lower  two  or  three 
feet  would  become  perceptibly  heated. 

49.  Convection.  —  Convection  takes  place  only  in  liquids  and  gases. 
It  is  a  transference  of  heat  from  one  point  to  another  by  means  of  a  circu- 
lation of  the  liquid  or  gas  itself.     This  process  is  of  such 
great  importance  in  meteorology  that  it  deserves  careful  con- 
sideration.    Let  AB  in  Fig.  15  represent  a  long  tank  filled  with  fluid 
which  is  heated  in  the  middle  and  at  the  bottom.     The  layer  of  fluid  at 


Definition. 


THE  HEATING  AND  COOLING   OF  THE  ATMOSPHERE        45 

m  becomes  heated  by  conduction.      It  expands  and  thus  raises  the 
layers  of  fluid  directly  above  it.     This  causes  a  slight  bulg- 

.  .  ,,  ,  ~  ,.  Illustration. 

mg  of  the  upper  surface.      Gravity  acting  upon  this  causes 
the  fluid  to  flow  to  the  ends  of  the  tank,  thus  decreasing  the  pressure 
in  the  middle   and   in- 
creasing it  at  the  ends.   A| ^  ^ =* —       , — — — IB 

This  increase  of  pres- 
sure at  the  ends  drives 
in  the  colder  fluid  to- 
ward the  middle,  thus 
forcing  the  warmer  fluid 
to  rise.  If  the  supply  FIG.  15.  -  Diagram  Illustrating  Convection. 

of  heat  is  continued,  a 

permanent  convectional  circulation  will  be  established,  which  will  tend 

to  equalize  the  temperatures  throughout  the  tajik. 

50.    Convection  in  the  Atmosphere.  —  Since  the  atmosphere  is  fluid, 
convection  ought  to  take  place  in  it  provided  it  is  heated  at  the  bot- 
tom ami  at  one  place  more  than  at  another.     The  equator  is  Where  con_ 
constantly  heated  more  than  the  polar  regions.     There  ought  vection 
then  to  be  a  permanent  convectional  circulation  between  equa-  °^e  pf°ce 
tor  and  pole.     It  will  be  seen  later  that  this  is  the  cause  of  in  the  at- 
the  general  wind  system  of  the  globe.     The  continents  are  r 
warmer  than  the  adjacent  oceans  during  the  summer  and  colder  in 
winter.     There  ought  then  to  be  a  convectional  circulation  between  the 
continents  and  the  near-by  oceans.     It  will  be  seen  later  that  this  is  the 
cause  of  the  monsoons.  /The  land  is  heated  more  than  the  adjacent 
ocean  during  the  day,  while  at  night  the  land  is  colder  than  the  adjoining 
ocean.     There  ought  thus  *to  be  a  convectional  circulation  between  the 
land  and  the  adjoining.ocean.     It  will  be  seen  later  that  this  is  the  .cause 
of  the  land  and  sea  breeze.     If,  due  to  irregularities,  one  place  happens  to 
be  heated  more  than  another,  there  ought  to  be  a  minor  convectional 
circulation  between  this  heated  locality  and  surrounding  places.     Evi- 
dences of  such  local  circulations  will  be  given  in  section  52. 

51.   There  is  one  great  difference  between  convection  in  a  liquid  and  in 
a  gas.     If  a  given  quantity  of  liquid  (say  w^ater)  is  by  convec-  The  diff,._ 
tion  forced  to  rise,* no  temperature  change  takes  place,  pro-  e~ 
vided  the  rise  is  sufficiently  rapid  to  make  the  effect  of  condu 
tion  and  radiation  negligible.     If,  however,  a  given  qu? 
of  gas  rises,  itvarrives  at  regions  where  the  pressur 
It  therefore 


46  METEOROLOGY 

are  having  no  effect  upon  it.  In  order  to  make  the  illustration  more 
definite,  let  us  consider  a  cubic  foot  of  water  which  is  raised  three  thou- 
sand feet.  If  the  rise  has  been  sufficiently  rapid  so  that  conduction 
and  radiation  have  added  or  subtracted  only  a  negligible  amount  of  heat, 
then  there  has  been  no  change  in  temperature  whatever.  If  a  cubic  foot 
of  air  rises  three  thousand  feet,  it  has  expanded  and  grown  markedly 
cooler  without  the  addition  or  substraction  of  heat  by  radiation  or  con- 
duction. These  temperature  changes  are  called  adiabatic^  because  they 
take  place  without  thq  interchange  of  heat  with  the  surroundings.  The 
Air  cools  amount  of  cooling  for  air  is  1.6°  Fahrenheit  for  a  rise  of  300 
1.6°  F.  for  feet,  or  1°  centigrade  (more  exactly  0.993°)  for  100  meters. 
This  assumes  that  the  air  remains  unsaturated  with  moisture. 
Thus,  if  a  cubic  foot  of  air  rises  three  thousand  feet,  it  cools  sixteen 
degrees  without  gaining  heat  from  or  losing  heat  to  its  surroundings. 
In  .the  case  of  descending  air  the  converse  is  true.  The  air  is  com- 
pressed and  a  corresponding  rise  in  temperature  takes  place. 

52.    Evidences  of  convection.  —  There  are  three  natural  phenomena  of 
more  or  less  general  occurrence  which  are  evidences  of  convection  or  that 
the  conditions  suitable  for  convection  are  present.     These 
evidences       are  mirages,  dust  whirlwinds,  and  cumulus  clouds. 
of  local  The  mirage  is  most  common  in  hot,  dn^,  desert  countries. 

During  the  day  the  surface  of  the  ground  becomes  excessively 
heated  by  the  sun's  rays.     The  layer  of  air  in  immediate  contact  with  it, 
a  by  conduction  from  the  hot  ground,  also  becomes  consider- 

ably heated.    "The  heated  air  expands  and  thus  becomes 
lighter  than  the  layers  of  air  immediately  above.      If  an  observer  is 

DENSER  MR  «     located  at  a  modeP- 

ate  elevation  above 


this  heated  layer  of 

LIGHTER  AIR  ^*~^     ^^  air,    rays    of    light 

coming  to  him  from 
a  distant  object  will 
follow  the  curved 


FIG.  16.  — Diagram  Illustrating  Mirage.  path    *    °S    *n    **& 

16.     The  reason  for 

the  curved  path  is  that  the  rays  of  light  are  bent  to  or  from  the 

normal   as  they  enter  layers   of   different  density.      They   are   bent 

the   normal   if   the  density  is  greater,  and   away  from  the 

less.     The  observer  considers  the  object  to  be  in  the 

he  ray  as  it  enters  his  eye,  thus  sees  the  inverted  image 


THE  HEATING  AND  COOLING   OF  THE  ATMOSPHERE        47 

of  the  object,  and  imagines  it  to  be  the  reflection  of  the  object  in  a 
body  of  water  between  himself  and  the  object.  This  phenomenon  is 
called  mirage.  The  word  is  of  French  origin,  meaning  reflection. 
It  is  of  common  occurrence  in  hot,  dry,  dusty  regions  during  the 
hotter  parts  of  the  day.  It  does  not  indicate  that  convection  is  taking 
place,  but  that  conditions  suitable  for  convection  are  present. 

Dust  whirlwinds  are  also  common  in  hot,  dry,  dusty  countries.  A 
layer  of  heated  air  near  the  earth's  surface  is  forced  to  rise  at  sometpoint. 
The  whirl  is  due  to  the  fact  that  the  indraft  of  cooler  air  to-  Dust  whirl- 
ward  the  point  of  rise  is  not  aimed  directly  at  it.  This  is  due  winds- 
to  unevenness  in  the  surface  or  local  barriers.  The  result  is,  the  whirl 
is  formed,  which  readily  builds  itself  up  and  remains  permanent.  The 
direction  of  rotation  is  accidental,  perhaps  determined  to  a  slight  extent 
by  the  direction  of  the  rotation  of  the  earth  on  its  axis.  These  dust  whirl- 
winds sometimes  rise  to  a  height  of  a  thousand  or  more  feet.  They  sel- 
dom occur  over  a  very  uneven  surface  for  the  reason  that  the  mixing 
of  the  air  is  too  great  to  permit  a  definite  whirl.  These  whirlwinds  are 
also  seldom  seen  over  a  vegetation-covered  surface.  In  the  first  place, 
such  a  surface  is  not  heated  as  much  during  the  day  as  is  the  dry,  barren 
ground.  In  the  second  place,  on  account  of  the  absence  of  dust,  the 
whirl  is  less  readily  seen  should  it  occur.  Miniature  whirlwinds  of  this 
kind  are  often  seen  on  hot  summer  days  along  a  dusty  roadway. 

Cumulus  clouds  are  common  in  winter  as  well  as  in  summer  and  especially 
in  the  eastern  part  of  the  United  States.  They  are  masses  of  white  cloud, 
looking  like  exploded  cotton  bales,  usually  with  rounded,  cumulus 
domelike  summits  and  flat  bases.  They  occur  on  what  are  clouds- 
popularly  known  as  "  fair  north  air  days."  They  are  simply  the  heads 
of  rising  air  columns.  A  pocket  of  warm,  moist  air  is  forced  to  rise,  due 
to  convection,  and  as  it  rises,  it  expands  and  is  cooled.  If  the  rise  is  to 
a  sufficient  height,  it  may  become  cooled  to  such  a  degree  that  it  can  no 
longer  hold  the  moisture.  This  condenses  in  the  form  of  cloud.  The 
reason  for  the  flat  bases  is  that  the  pockets  of  rising  air  have  approxi- 
mately the  same  temperature  and  amount  of  moisture  over  consider- 
able areas,  and  as  a  result  they  reach  the  stage  at  which  they  can  no 
longer  hold  their  moisture  at  about  the  same  height.  By  watching 
cnrpfiiliv  tho  growth  and  change  in  form  of  a  cumulus  cloud,  one  can 
readily  see  that  it  is  the  top  of  a  risino-  «ir  ™ 


48  METEOROLOGY 

VERTICAL  TEMPERATURE  GRADIENTS 

53.   Vertical  temperature  gradient.  —  It  is  a  well-known   fact   that 
the  temperature  is  different  at  Different  heights  above  the  earth's  surface. 
.          This  change  in  temperature  with  elevation  is  called  the  verti- 
cal temperature  g|»^ieni^  a.nH  it.  is  nan  ally  expressed  as  a  cer- 
tain number  of  degrees  Fahrenheit  for  300  feet,  or  a  certain  number  of 
degrees  centigrade  for  100  meters.     The  facts  in  connection  with  the  ver- 
tical temperature  gradient  have  been  obtained  in  three  ways :  by  means 
of  mountain  observatories,  balloon  ascensions,  and  kites. 

The  vertical  temperature  gradient  may  be  obtained  by  taking  simulta- 
neous observations  in  a  valley  and  at  different  points  on  the  sides  and 
Mountain  ^°P  °^  a  mountain.  This  method  is  at  present  no  longer 
observa-  used,  because  the  motion  of  the  air  and  its  temperature  are 
influenced  by  the  mountain  itself.  The  temperaturexon 
the  top  of  a  mountain  is  not  the  same  as  in  the  open  air  at  the  same 
elevation. 

Balloon  ascensions  for  scientific  purposes  have  now  been  made  for 
more  than  a  century.     The  older  observations,  however,  are  of  compara- 
tively little  value  because  the  instruments  were  poor  and 

Observa- 

tions  ob-        chiefly  because  they  were  not  properly  exposed  and  venti- 
tainedfrom    iated.     A  new  era  commenced  with  the  year    1887.  when 

balloons.  . 

Assmann's  ventilated  thermometer  (see  section  67)  was  used 
for  the  first  time,  and  more  attention  was  paid  to  the  exposure  of  the 
instruments  in  general.  Balloon  ascensions  for  the  purpose  of  obtaining 
meteorological  observations  have  been  made,  particularly  in  Europe,  and 
in  France  and  Germany  more  than  in  any  other  countries.  In  the  years 
1888  to  1899  inclusive  75  balloon  ascensions  were  made  near  Berlin  and 
nearly  a  thousand  ascensions  have  now  been  made  near  Paris.  The 
Berlin  ascensions  are  fully  described  and  the  observations  carefully  dis- 
cussed in  a  three-volume  work  by  Assmann,  Berson,  and  others,  entitled 
Wissenschaftliche  Luftfahrten.  Figure  19  is  taken  from  page  188  of 
the  second  volume  of  this  work,  and  Fig.  17  shows  the  equipment  of  a 
balloon  used  for  these  purposes.  Smaller,  sounding  balloons  carrying 
self-registering  instruments  have  also  been  used  at  many  stations  for 
obtaining  observations  at  greater  heights  than  those  to  which  a  manned 
balloon  can  penetrate. 

Concerning  the  use  of  kites  for  making  meteorological  observations, 
Professor  Cleveland  Abbe,  in  his  Aims  and  Tethods  of  Meteorological 
Work,  writes: 


FIG.  17.  —  Balloon  Equipped  for  Meteorological  Observations. 
•     (From  ASSMANN'S  Wissenschaftliche  Luftfahrien.) 


FIG.  18.  —  Kite  Equipped  for  Meteorological  Observations. 


THE  HEATING  AND  COOLING   OF  THE  ATMOSPHERE        49 

» 

"  In  order  to  obtain  records  from  the  air  within  a  mile  or  two  of  the 
earth's  surface,  the  kite  was  first  employed  by  Alexander  Wilson  of  Glas- 
gow in  1749.  In  1885  the  writer  urged  the  renewed  application  The  use  of 
of  the  kite,  and  since  the  meeting  of  the  International  Confer-  kites- 
ence  on  Aerial  Navigation  at  Chicago,  in  1893,  it  has  become  an  impor- 
tant meteorological  apparatus.  Professor  Marvin's  construction  of  the 
Hargrave  cellular  kite,  or  box  kite,  is  fully  described  in  various  publica- 
tions of  the  United  States  Weather  Bureau ;  the  standard  size  used  by 
him  carries  about  68  square  feet  of  supporting  surface.  The  line  is  of 
the  best  music  wire,  whose  normal  tensile  strength  is  210  pounds.  The 
Marvin  reel,  on  which  the  wire  is  wound,  is  a  modification  of  the  Thomson 
and  Sigsbee  deep  sea  sounding  apparatus ;  it  keeps  an  automatic  record 
of  the  pull  on  the  wire,  both  as  to  its  intensity  and  direction  in  altitude 
and  azimuth.  The  reeling  of  the  wire  in  and  out  is  done  either  by  hand 
or  by  a  small  gas  engine.  The  meteorological  record  at  the  kite  is  kept 
on  one  sheet  of  paper  by  means  of  the  Marvin  Meteorograph.  This 
keeps  a  continuous  record  of  the  time  by  means  of  an  accurate  chrono- 
graph,  of  the  atmospheric  pressure  15 v  means  of  a  Bourdon  aneroid,  of 


c  pressure 
dr  bv  a  rr 


the  temperature  of  the  air  by  a  metallic  thermometer,  of  the  relative 
humidity  by  a  hair  hygrometer,  and  of  the  velocity  pf  the  wind  by  means 
of  a  small  Robinson  anemometer.  This  complete  apparatus  is  inclosed 
in  an  aluminum  case  to  protect  it  from  accident,  and  is  lashed  securely 
within  the  front  cell  of  the  kite  so  that  it  receives  the  full  force  of  the 
wind,  and  undoubtedly  gives  a  reliable  record  of  the  temperature.  The 
entire  meteorograph  weighs  about  two  pounds.  Although  kites  some- 
times break  away,  yet  no  injury  has  occurred  to  any  meteorograph  in 
the  course  of  1500  ascensions."  Figure  18  shows  a  Marvin  Meteoro- 
graph and  a  kite  fully  equipped  for  securing  observations. 

54.  The  observations  made  in  these  three  ways  all  show  that  the  ver- 
tical temperature  gradient  varies  markedly  with  the  time  of  day.  It  is 
also  somewhat  different  at  different  times  of  year  and  varies  _ 

The  char-  ' 

witH  the  type  of  weather  and  slightly  with  the  location  of  the  acteristics 
place  on  the  earth's  surface.     It  must  not  be  thought  that  the  °* the  era" 
change  in  temperature  with  altitude  is  always  a  regular  de- 
crease by  the  same  number  of  degrees  for  each  300  feet.     The  tempera- 
ture sometimes  increases  with  elevation.     Again,  it  may  remain  constant 
or  a  considerable  distance,  or  a  marked  change  in  temperature  may  take 
>lace  with  a  very  small  change  in  elevation.     Figure  19  shows  the  actual 
ertical  temperature  gradient  obtained  at  Berlin  on  the  19th  of  October, 
893.  bv  m^ano  '-f  "  K«n™«  oo™,™™      T-I^O  oooensiOn  took  place  at  10.18 


50 


METEOROLOGY 


6000 


5000 


4000 


.3000 


\ 


A.M.,  the  balloon  reached  the  highest  point  at  1.45  P.M.,  and  descended  to 
the  ground  at  4.20  P.M.  The  figure  thus  represents  the  vertical  temper- 
ature gradient  between  10.18  in  the  morning  and  1.45  in  the  afternoon. 

_^_r_^r__^r_T_^__        The  average  value  of  the 

vertical  temperature  gradient 

The  average  for  ail  times  of 
value.  dayt  for  all  gea. 

sons  of  the  year,  and  for  all 
places  and  weather,  is  ordina- 
rily considered  to  be  1°  Fah- 
renheit for  300  feet,  or  0.6° 
Centigrade  for  100  meters. 

In  order  to  understand  the 
change  in  the  vertical  tem- 
perature  gradient 

The  three        ^      . 

characteris-  during  the  day, 
tic  gra-  three  character- 

dients. 

istic  gradients 
must  be  first  considered. 
These  three  gradients  are 
the  average  vertical  tempera- 
ture gradient  which  occurs 
usually  about  9.00  in  the 
morning  and  8.00  in  the 
evening,  the  gradient  which 
exists  at  the  time  of  minimum 
or  lowest  temperature,  and 
the  gradient  which  occurs 
at  the  time  of  maximum  or 
highest  temperature.  These 

three  gradients  are  represented  graphically  in  Fig.  20  for  a  typical  October 
day.  The  maximum  temperature  during  this  day  is  assumed  as  60°*F., 
the  minimum  temperature  as  30°  F.,  and  the  average  temperature  for 
the  day  as  45°  F.  This  average  temperature  would  occur  about  9.00  in 
the  morning  and  8.00  in  the  evening.  The  straight  line  B  in  the  figure 
represents  the  average  vertical  temperature  gradient  and  is  plotted  with 
a  fall  of  1°  F.  for  300  feet.  Thus  at  a  height  of  9000  feet  above 
the  earth's  surface  the  temperature  would  be  30°  lower  than  at  the 
earth's  surface.  The  gradient  C,  which  represents  the  vertical  tempera- 
ture gradient  at  the  time  of  the  maximum  temperature,  follows  the 


2000 


1,0.00 


-28° 


-20' 


-10C 


10° 


TEMPERATURE.    CENTIGRADE 


FIG.  19.  —  Graph  Illustrating  Vertical  Temperature 
Gradient. 

(From  ASSMANN'S  Wissenschaftliche  Luftfahrten.) 


THE  HEATING  AND  COOLING   OF  THE  ATMOSPHERE        51 


average  vertical  temperature  gradient  closely  in  the  upper  atmosphere, 
but  departs  from  it  more  and  more  as  the  earth's  surface  is  approached. 
The  gradient  A,  which  represents  the  vertical  temperature  gradient  at 
the  time  of  the  minimum  temperature,  also  follows  the  average  gradient 


P4OUU 

11700 
10800 
9900 
9000 

8100 
fc 

Hi  7200 

Z 
g  6300 

gj  5400 
Ul 
4500 

3600 
2700 
1800 

•900 
0 

B\ 

1 

A\ 

Y 

\ 

V 

^ 

\ 

A 

\ 

- 

\ 

V 

\ 

\ 

3 

s 

\ 

\ 

\ 

\s 

S 

v. 

•» 

D\ 

e> 

A 

\ 

3 

\ 

\ 

\ 

Y 

i 

\ 
\ 

x\ 

\ 

^ 

\ 

\ 

\ 
\\ 

\ 

\ 

\, 

\ 

\ 

\N 

\ 
\ 

> 

\  ^ 
\ 

\ 

\ 

\\ 

\ 

\ 

2 

5 

s 

\ 

^ 

10  20  X)  40 

TEMPERATURE,  FAHRENHEIT 

FIG.  20.  —  Temperature  Gradients. 


50 


closely  in  the  upper  atmosphere  but  has  a  sharp  bend  at  low  altitudes, 
usually  less  than  2000  feet.     The  normal  change  in  the  vertical  tem- 
perature gradient  during  the  day  is  as  follows:  At  the  time  of  the  min- 
imum temperature  it  approximates  to  the  form  A.      It  then  The  daily 
straightens  out  and  at  about  9.00  in  the  morning  is  following  variation  in 
closely  the  average  vertical  temperature  gradient,  which  is  tempera-°a 
represented  graphically  by  the  straight  line  B.     It  then  ture  gra- 
curves  in  the  other  direction,  and  by  the  time*  the  maxi- 
mum temperature  is  reached  it  has  assumed  the  form  C.     It  then 


52  METEOROLOGY 

straightens  out  and  by  about  8.00  in  the  evening  has  reached  the 
average  vertical  temperature  gradient  again.  It  then  swings  over 
until,  when  the  time  of  minimum  temperature  is  again  reached,  it  has 
approximately  the  form  A. 

The  daily  range  of  temperature  is  defined  as  the  difference  between  the 
highest _and  lowest  temperature  which  occurs  during  the  day.  It  will  be 

seen  in  the  figure  That  the  daily  range  decreases  rapidly  with 

elevation.  At  the  earth's  surface  it  was  taken  as  30° ;  at  the 
height  of  a  mile,  as  indicated  at  H,  it  is  only  10°,  while  at  the  height  of 
two  miles,  as  indicated  at  G,  its  value  is  about  6°. 

It  is  not  necessary  to  take  observations  at  great  elevations  in  order  to 
ascertain  these  facts.  The  observations  taken  at  the  top  and  base  of  the 
Eiffel  Tower  in  Paris,  at  an  elevation  of  302  and  2  meters  respectively, 
show  the  same  results.  The  daily  range  decreases  by  62  per  cent  in 
winter  and  by  45  per  cent  in  summer  for  this  difference  in  elevation  of 
only  300  meters ;  and  for  12  hours  during  the  night  in  winter  and  for  8 
hours  during  the  night  in  summer  the  temperature  at  the  top  averages 
higher  than  at  the  base. 

The  average  vertical  fremperature  gradient  must  not  be  confused  with 
the  adHha*'n  rfllif  of  cooling  of  unsaturated  rising  air.  The  one  is  1° 

Fajirenheii  fatSDQJeet,  while  the  other  is  1.6°  Fahrenheit  for 
batLTraTe  ^00  ^f  The  vertical  temperature  gradient  states  that  on 
of  cooling  the  average  there  is  a  decrease  in  temperature  of  1°  F.  with 
withThe6  each  300  feet  of  elevation.  According  to  the  adiabatic  rate 
vertical  tem-  of  cooling,  rising  air  by  expansion  cools,  when  not  saturated 
gradient.  with  moisture,  1-6°  F.,  for  every  300  feet.  Should  air  with  a 

temperature  of  30°  rise,  it  would  cool  adiabatically  due  to  ex- 
pansion 1.6°  for  300  feet  and  the  behavior  of  a  quantity  of  air  rising  with 
this  initial  temperature  is  shown  graphically  by  D  in  Fig.  20.  The  be- 
havior of  quantities  of  air  rising  with  initial  temperatures  of  45°  or  60° 
is  shown  by  E  and  F  respectively.  H 

55.    The  isothermal  layer. — The  facts  which  have  just  been  stated 

concerning  the  vertical  temperature  gradient,   its  average 

value,  and  the  daily  change  in  it  apply  only  to  the  lower  por- 
verticai  tem-  tion  of  the  atmosphere,  say,  from  the  earth's  surface  to  a  height 
gradient  be-  °^  two  or  three  miles.  Between  the  height  of  two  miles  and 
tween  three  six  miles  or  somewhat  more,  the  characteristics  of  the  vertical 
miles!*  temperature  gradient  are  quite  different.  Here  it  is  much/ 

more  regular  and  the  average  value  is  somewhat  greater  than! 
near  the  earth's  suriace.  The  changes  in  the  gradient  between  day  and' 


THE  HEATING  AND  COOLING   OP  THE  ATMOSPHERE        53 

night,  summer  and  winter,  and  with  the  weather,  are  all  extremely 
small.  Of  course,  at  any  given  height,  the  temperatures  are  lower  in 
winter  than  in  summer,  but  the  gradient  is  almost  exactly  the  same. 
It  is  also  true  that,  at  the  same  height,  the  temperatures  are  usually 
higher  when  the  weather  is  fair  than  when  a  storm  or  area  of  low 
pressure  is  present,  but  the  gradient  changes  very  little  with  the  weather. 

After  a  height  of  six  miles,  or  in  some  cases  much  more  is  reached,  the 
temperature  seems  to  remain  constant  with  elevation,  or  may,  indeed, 
increase  slightly  with  altitude.  This  layer  in  which  the  tern-  The  iso_ 
perature  remains  so  nearly  constant  is  usually  spoken  of  as  thermal 
fthe  isothermal  layer  or  the  warm  stratum  of  the  atmosphere.  layer* 
'This  subject  has  been  investigated  especially  by  Teisserenc  de  Bort, 
Assmann,  and  in  this  country  by  Humphreys  and  Rotch,  the  director 
of  the  Blue  Hill  Observatory  near  Boston.  In  an  article  in  the  Monthly 
Weather  Review  of  May,  1908  Professor  Rotch  writes : 

"  This  in  version  of  temperature  was  first  discovered  by  M.  Teisserenc  de 
Bort  with  the  sounding  balloons  sent  up  from  his  observatory  at  Trappes, 
near  Paris,  France,  in  1901,  and  almost  simultaneously  by  Professor 
Assmann  from  similar  German  observations.  Since  then  almost  all 
the  balloons  which  have  risen  more  than  40,000  feet  above  central 
Europe  (that  is,  near  latitude  50°)  have  penetrated  this  stratum, 
without,  however,  determining  its  upper  limit.  Teisserenc  de  Bort 
early  showed  that  its  height  above  the  earth,  to  the  extent  of  8000 
feet,  varied  directly  with  the  barometric  pressure  at  the  ground.  Mr. 
Dines  gives  the  average  height  of  the  isothermal  layer  above  England 
as  35,000  feet,  with  extremes  of  nearly  50  per  cent  of  the  mean.  Ob- 
servations conducted  last  March  by  our  indefatigable  French  colleague, 
Teisserenc  de  Bort,  in  Sweden,  just  within  the  Arctic  Circle,  show  that 
the  minimum  temperature  occurred  at  nearly  the  same  height  as  at 
Trappes,  namely,  36,000  feet,  although  Professor  Hergesell,  who  made 
use  of  sounding  balloons  over  the  Arctic  Ocean  near  latitude  75°  N., 
during  the  summer  of  1906,  concluded  that  the  isothermal  stratum 
there  sank  as  low  as  23,000  feet. 

"  During  the  past  three  years  the  writer  has  dispatched  77  sounding  bal- 
loons from  St.  Louis,  Mo.,  U.S. A.,  latitude  38°  N.,  and  most  of  those  which 
rose  higher  than  43,000  feet  entered  the  inverted  stratum  of  temperature. 
This  was  found  to  be  somewhat  lower  in  summer,  but  the  following 
marked  inversions  were  noted  last  autumn:  October  8,  the  minimum 
temperature  of  —  90°  F.,  occurred  at  47,600  feet,  whereas  at  the  maxi- 
mum altitude  of  54,100  feet  the  temperature  had  risen  to  —  72°  ;  October 


54  METEOROLOGY 

10,  the  lowest  temperature  of  -  80°  was  found  at  39,700  feet,  while  -  69° 
was  recorded  at  42,200  feet,  showing  a  descent  of  nearly  8000  feet  in 
the  temperature  inversion  within  two  days.  The  expedition  sent  out 
jointly  by  M.  Teisserenc  de  Bort  and  the  writer,  on  the  former's  steam 
yacht  Otaria,  to  sound  the  atmosphere  over  the  tropical  Atlantic  during 
the  summer  of  1906,  launched  sounding  balloons  both  north  and  south  of 
the  equator  within  the  tropics,  and  although  some  of  these  balloons  rose 
to  nearly  50,000  feet,  they  gave  no  indication  of  an  isothermal  stratum. 
In  fact,  the  paradoxical  fact  was  established  that  in  summer  it  is  colder 
10  miles  above  the  thermal  equator  than  it  is  in  winter  at  the  same  height 
in  north  temperate  regions.  This  results  from  the  more  rapid  decrease 
of  temperature  in  the  tropics  and  the  absence  of  the  numerous  temporary 
inversions  which,  as  Mr.  Dines  has  pointed  out,  are  common  in  our  re- 
gions below  10,000  feet.  If,  therefore,  as  seems  probable,  the  isothermal 
or  relatively  warm  stratum  does  exist  in  the  tropical  and  equatorial 
regions,  it  must  lie  at  a  height  exceeding  50,000  feet,  from  which  height, 
as  the  data  quoted  show,  it  gradually  descends  toward  the  pole,  at  least 
in  the  northern  hemisphere." 

This  isothermal  layer  exists,  then,  at  a  great  height  over  the  equator,  has 
an  average  height  of  about  six  miles  over  the  middle  latitudes,  and  comes 
still  nearer  to  the  earth's  surface  in  polar  regions.  Its  average  tempera- 
ture during  the  summer  is  about  —  60°  F.,  while  its  average  winter  tem- 
perature is  about  —  71°  F.  When  the  weather -is  fair  and  is  c6ntrolled 
by  an  area  of  high  pressure,  it  is  at  a  greater  height  and  about  14°  F. 
colder,  as  an  average  for  all  the  quadrants  of  a  high  than  when  the 
weather  is  stormy  and  an  area  of  low  pressure  is  dominant.  In  this 
respect  its  temperate  is  the  opposite  of  the  air  temperature  between  a 
height  of  two  and  six  miles.  The  isothermal  layer  also  begins  at  a  lower 
altitude  in  winter  than  in  summer.  Its  upper  bounding  surface  has  never 
been  determined. 

56.  Many  articles  dealing  with  the  isothermal  layer  will  be  found  in 
the  periodical  literature  since  1900,  and  the  reader  must  be  referred  to 
A  possible  these  for  a  full  treatment  of  the  subject.  No  single  univer- 
expianation  sajiy  accepted  explanation  of  all  the  facts  in  connection 
thermal  with  the  isothermal  layer  has  yet  been  given.  In  what  fol- 
iayer.  iows  a  pOSSible  explanation  of  some  points  is  presented. 

Since  the  height  of  the  isothermal  layer  is  always  more  than  five  miles 
above  the  earth's  surface,  the  amount  of  water  vapor  present  must  be 
extremely  small,  if  not  entirely  negligible ;  furthermore  the  amount  of 
carbon  dioxide  is  much  less  than  at  the  earth's  surface.  Thus  of  the 


THE  HEATING  AND  COOLING   OF  THE  ATMOSPHERE        55 

three  ingredients  which  are  the  chief  causes  of  the  absorption  and  radia- 
tion on  the  part  of  the  atmosphere  the  dust  alone  remains.  At  a  height 
of  five  miles  or  more  the  dust  which  exists  in  the  atmosphere  can  come 
from  but  one  source.  The  dust  blown  up  from  the  earth  or  that  which 
comes  fro^the  evaporation  of  ocean  spray  or  from  volcanoes  cannot, 
except  Hinder  unusual  conditions,  penetrate  to  this  height.  The  dust 
which  exists  above  five  miles  must  come  primarily  from  the  meteors. 
This  is  put  into  the  atmosphere  at  a  height  of  50  miles  or  more  and  as  it 
slowly  settles  through  the  atmosphere  it  must  cause  the  distribution  of 
dust  to  be  uniform  below  this  height.  Now  the  one  process  which  is  op- 
erative in  heating  and  cooling  the  atmosphere  at  a  height  of  more  than  five 
miles  is  the  absorption  of  radiation  bythe  dust  particles  and  the*  radiation 
of  heat  by  them  to  space  above  and  to  the  earth  below.  No  account  is 
here  taken  of  the  adiabatic  changes  in  temperature  of  the  whole  layer  due 
to  its  rising  or  falling.  The  temperatures  would  be  different,'  but  the 
layer  would  still  remain  isothermal.  It  can  be  shown  mathematically 
that  the  radiation  received  by  a  body  at  a  given  distance  above  an  in- 
finite plane  (and  the  earth's  surface  may  be  considered  as  such)  is  inde- 
pendent of  its  height  above  that  plane.  Thus  the  amount  of  insolation 
from  the  sun  and  the  amount  of  radiation  from  the  earth  absorbed  by  the 
dust  pa¥ticles  will  be  independent  of  the  distance  above  the  earth's  sur- 
face, ^ne  should  thus  expect  that  each  given  volume  of  air  having  in  it 
the  same  number  of  dust  particles  would  gain  by  absorption  the  same 
amount  of  heat  and  would  lose  at  night  by  radiation  the  same  amount  of 
heat.  But  the  density  of  the  air  grows  steadily  less  with  elevation.  We 
have  thus  the  same  amount  of  heat  applied  to  a  smaller  quantity  of  air, 
and  one  would  thus  expect  an  increase  of  temperature  with  elevation  and 
decreasing  density.  The  dust  particles  are  probably  not  the  only 
absorbers  and  radiators.  Due  to  the  action  of  ultra-violet  light  and  the 
aurora  borealis,  the  ozone  content  of  the  upper  air  may  be  fairly  large. 
Ozone  absorbs  certain  wave  lengths  readily,  and  the  other  gaseous  con- 
stituents of  the  atmosphere  may  play  a  small  part. 
^57.  Inversion  of  temperature  —  nocturnal  stability.  —  It  was  a  fact 
of  early  observation  that  on  the  still,  clear  nights  of  winter  and  during  the 
clear,  frosty  nights  of  the  late  spring  or  early  fall,  the  tempera-  Definition 
ture  was  somewhat  higher  on  a  mountain  top  or  at  a  small  of  mversion- 
elevation  above  the  earth's  surface  than  on  the  earth's  surface  itself. 
When  the  temperature  increases  instead  of  decreases  with  elevation,  it  is 
spoken  of  as  an  inversion  of  temperature.  It  was  formerly  thought  that 
inversions  of  temperature  were  of  comparatively  rare  occurrence.  It  is 


56  METEOROLOGY 

now  known,  however,  that  they  occur  on  practically  all  nights.  When 
an  inversion  of  temperature  occurs,  the  vertical  temperature  gradient  has 
the  form  of  A  in  Fig.  20. 

During  the  night  the  atmosphere  is  entirely  stable.  If  a  quantity 
of  air  with  a  temperature  of,  say,  30°  on  a  night  when  an  inversion  of 
Wh  the  at  temperature  exists  should  be  pushed  up  a  short  distance, 
mosphere  is  it  would  expand  and  cool  at  the  rate  of  1,6°  F.  for  300  feet, 
stable  at  an(j  fin(j  itself  cooler  than  its  surroundings.  It  would  then 
immediately  drop  back  again  or,  more  accurately  stated,  it 
would  never  have  spontaneously  commenced  to  rise  at  all.  Thus  at  night 
the  atmosphere  is  entirely  stable. 

58.  Diurnal    instability  —  conditions    of    convection.  —  During    the 
day  the  atmosphere  may  find  itself  in  a  state  of  unstable  equilibrium. 

Suppose  when  the  vertical  temperature  gradient  has  the  form 
mohsyphereais  C  in  FiS-  20>  a  quantity  of  air  with  a  temperature  of  60° 
unstable  commences  to  rise.  It  will  expand  and  grow  cool  at  the  rate 

during  th          of  lQo  F    for  30Q  feet>      ^g   ghown   by  the  gtraight  Hne   F  in 

the  figure  it  will  continually  find  itself,  in  spite  of  the  cooling, 
warmer  than  its  surroundings,  and  will  continue  to  rise.  It  will  rise  until 
it  has  cooled  to  the  temperature  of  its  surroundings  ;  or,  expressed  graph- 
ically, it  will  rise  until  the  straight  line  F  intersects  the  vertical  tempera- 
ture gradient,  C. 

The  conditions  for  convection  may  now  be  summarized.      (1)   The 

atmosphere  must  be  heated  at  the  bottom.     (2)  The  atmos- 

1  ne  tnree 

conditions  pnere  must  be  heated  at  one  point  more  than  at  surround- 
ing places.  (3)  If  a  quantity  of  air  which  starts  to  rise  is 
to  continue  its  ascent,  the  average  vertical  temperature 

gradient  must  be  greater  than  1.6°  for  300  feet. 

How  THE  ATMOSPHERE  is  HEATED  AND  COOLED 

59.  The  present  chapter  may  be  summarized  and' the  principles  which 
have  been  stated  may  be  illustrated  by  considering  the  processes  which 
are  operative  in  the  diurnal  heating  and  cooling  of  the  atmosphere  near 
the  earth. 

In  the  heating  of  the  atmosphere  five  processes  must  be  considered. 
These  are  in  order :  the  absorption  of  insolation,  the  absorption  of  the 
The  five  radiant  energy  from  the  earth,  conduction  from  the  earth, 
heating  pro-  mixture  by  means  of  the  wind,  and  convection.  As  was  seen 
in  the  study  of  actinometry  under  the  most  favorable  con- 
ditions, when  the  sun's  rays  fall  vertically,  only  90  per  cent  of  the  insola- 


THE  HEATING  AND  COOLING   OF  THE  ATMOSPHERE        57 

tion  is  transmitted  and  the  remaining  10  per  cent  is  absorbed.  If  the 
rays  do  not  fall  vertically  and  under  unfavorable  atmospheric  condi- 
tions, a  much  larger  percentage  of  the  insolation  is  absorbed  by  the 
atmosphere.  The  dust,  water  vapor,  and  carbon  dioxide  are  the  chief 
absorbers,  and  they  increase  in  quantity  with  nearness  to  the  earth's 
surface.  As  a  result  a  rise  in  temperature  of  perhaps  a  degree  would 
occur  in  the  upper  atmosphere  and  a  rise  in  temperature  of  perhaps  3  or 
4°  would  occur  near  the  earth's  surface. 

The  earth  becomes  warmed  by  absorbing  the  insolation  which  reaches 
it  and  radiates  its  heat  in  the  form  of  long  ether  waves  to  space.  These 
longer  ether  waves  are  absorbed  even  more  readily  than  the  shorter  ether 
wavds  which  constituted  the  insolation.  This  constitutes  the  second 
source  of  heating,  and  here  again  the  amount  of  absorption  and  the  result- 
ing heating  would  be  greater  nearer  the  earth's  surface.  In  connection 
with  conduction  it  was  seen  that  only  a  layer  2  or  3  feet  deep  of  the  atmos- 
phere would  be  heated  by  being  in  contact  for  many  hours  with  the  warm 
ground.  This  process,  then,  would  be  effective  only  in  heating  the  layer 
of  air  close  to  the  ground.  The  wind,  however,  so  thoroughly  mixes  the 
various  layers  of  air  which  are  in  contact  with  each  other  that  the  heat 
imparted  by  conduction  to  the  2  or  3  feet  nearest  the  earth's  surface 
would  be  distributed  over  perhaps  two  or  three  thousand  ^feet  near  the 
ground.  Since  this  lower  layer  is  heated  by  conduction,  convection 


)rtit 

itloT 
iftti( 


would  take  place,  and  this  circulation  would  transfer  heat  fro 

surface  to  altitudes  of  from  one  to  several  miles.  ?  As  a  resultof  these  five 


heating  processes  the  temperature  of  the  air  is  raised  but  little  at  great 
altitudes  above  the  earth's  surface,  and  the  amount  of  heating  increases 
rapidly  as  one  approaches  the  earth's  surface.  The  vertical  temperature 
gradient  C  in  Fig.  20  has  thus  been  explained. 

There  are  three  processes  operative  in  the  cooling  of  the  atmosphere, 
namely,  radiation  to  the  cool  ground  and  to   space,   conduction,  and 
mixture  by  the  wind.     The  air,  filled  with  dust,  water  vapor,  The  three 
and  carbon  dioxide,  radiates  heat  as  well  as  it  absorbs  it.     The  cooling  pro- 
ground  at  night  cools  rapidly  and  the  air  loses  heat  by  radi-  c 
ation  both  to  the  open  sky  and  to  the  cooling  ground.    Conduction,  again, 
would  cool  the  two  or  three  feet  of  air  in  immediate  contact  with  the 
earth's  surface,  and  the  wind,  although  on  an  average  less  at  night  than 
during  the  day,  would  mix  this  layer  with  the  two  or  three  thousand  feet 
of  air  above  it.    Since  convection  is  not  operative  at  night,  the  marked  cool- 
ing must  take  place  very  near  the  earth's  surface.     The  vertical  tempera- 
ture gradient  A  at  the  time  of  lowest  temperature  has  thus  been  explained. 


58  METEOROLOGY 


QUESTIONS 

(1)  Describe  the  molecular  structure  of  matter.  (2)  How  many  kinds  of 
atoms  are  there?  (3)  Describe  the  structure  of  the  molecule  and  atom.  (4) 
Describe  the  luminiferous  ether.  (5)  What  properties  must  it  have  ?  (6)  How 
are  molecules  related  to  each  other?  (7)  Define  heat  and  temperature/ 
(8^  How  are  ether  waves  caused?  (9)  Define  radiant  energy."  (10)  Dis- 
tinguish between  an  hypothesis  and  a  fact.  (11)  Name  the  three  possible 
sources  of  atmospheric  heat.  (12)  Prove  in  two  ways  that  all  other  sources 
of  heat  than  the  sun  are  relatively  negligible.  (13)  Describe  the  size  and  condi- 
tion of  the  sun  itself.  (14)  Give  illustrations  of  the  amount  of  energy  received 
from  the  sun.  (15)  Define  insolation.  (16)  Upon  what  three  things  does  the 
amount  of  insolation  received  depend?  (17)  Explain  how  the  amount  of 
insolation  received  depends  upon  the  obliqueness  of  the  rays.  (18)  Describe 
the  earth's  ormfr ground  the  sun.  (19)  What  is  the  effect  of  the  change  in  dis- 
tance of  the  earth  from  the  sun?  (20)  What  are  the  two  effects  of  the  change 
in  presentation  of  the  earth  to  the  sun's  rays?  (21)  How  great  is  the  change 
in  directness  during  the  year  ?  (22)  State  the  change  in  duration  at  various 
places  during  the  year.  (23)  Explain  why  insolation  varies  with  latitude  and 
time.  (24)  How  may  this  relation  of  insolation  to  latitude  and  time  be  expressed 
graphically  ?  (25)  Define  reflection.  (26)  Describe  the  two  kinds  of  reflection. 
(27)  Arrange  the  substances  upon  which  insolation  may  fall  in  order  of  excellence 
as  regards  reflection.  (28)  Define  transmission.  (29)  What  is  meant  by  selec- 
tive transmission  ?  (30)  Contrast  rock  salt  and  glass  as  transmitters.  (31)  Ar- 
range the  substances  upon  which  insolation  may  fall  in  order  of  excellence  as  re- 
gards transmission  and  absorption.  (32)  Give  the  water  analogy  for  the  three 
processes,  reflection,  transmission,  and  absorption.  (33)  What  are  the  four  effects 
of  absorption?  (34) » Define  actinometry.  (35)  State  the  fundamental  formula 
in  actinometry.  (36)  What  is  represented  by  each  letter  in  the  formula?  (37) 
Define  the  solar  constant.  -(38)  In  how  many  ways  may  absolute  and  relative 
values  of  H  be  determined?  (39)  Describe  the  photograph  paper  method  of  de- 
termining relative  values  of  H.  (40)  Describe  the  method  of  determining  C  and 
a.  (41)  What  values  have  been  found  for  C  and  a?  s(42)  State  the  effect  of 
atmospheric  absorption  on  the  insolation  received  at  the  earth's  surface  in  dif-* 
ferent  latitudes.  (43)  What  is  the  behavior  of  the  ocean  as  regards  reflection, 
transmission,  and  absorption?  (44)  Why  is  the  temperature  rise  during  the  day 
extremely  small?  (45)  Why  does  the  ocean  cool  but  little  during  the  night? 
(46)  State  the  behavior  of  dry  ground  as  regards  reflection,  transmission,  and 
absorption.  (47)  Why  is  the  rise  in  the  temperature  of  dry  ground  under  in- 
solation so  large?  (48)  .What  is  the  effect  of  dampness  or  vegetation  on  the 
temperature  change?  (49)  State  the  behavior  of  the  atmosphere  as  regards 
reflection,  transmission,  and  absorption.  (50)  Name  the  three  chief  absorbing 
components  of  the  atmosphere.  (51)  How  do  the  temperatures  of  the  air  and 
the  ground  compare  during  the  day  and  at  night?  (52)  How  do  the  tempera- 
tures of  the  air  and  the  ocean  compare  during  the  day  and  at  night  ?  (53)  Define 
conduction.  (54)  Name  several  poor  and  several  good  conductors.  (55)  To 
what  height  would  the  air  be  heated  by  conduction  alone?  (56)  Describe  and 
explain  convection  in  a  liquid.  (57)  Where  should  convection  be  expected  to 
take  place  in  the  atmosphere?  (58)  State  the  difference  between  convection 
in  a  liquid  and  in  a  gas?  (59)  What  is  meant  by  the  adiabatic  rate  of  cooling 
of  rising  air?  (60)  Name  the  three  evidences  of  local  convection.  (61)  De- 
scribe and  explain  the  mirage.  (62)  Describe  and  explain  dust  whirlwinds. 
(63)  Describe  and  explain  cumulus  clouds.  (64)  What  is  meant  by  vertical 


THE  HEATING  AND  COOLING   OF  THE  ATMOSPHERE        5<) 

mperature  gradient?     (65)  Name  the  three  ways  of  obtaining  the  vertical 
mperature  gradient.     (66)  Why  are  mountain  observations  no  longer  used? 
•7)  Describe  the  use  of  balloons  for  obtaining  observations.     (68)  Describe 
le  use  of  kites  for  making  meteorological  observations.      (69)  With  what  does 
le  vertical  temperature  gradient  vary?     (70)  What  is  the  numerical  value  of 
ae  average  vertical  temperature  gradient?     (71)  Describe  the  three  character- 
;tic  gradients  which  occur  during  the  day.     (72)  Define  daily  range.      (73)  How 
may  the  daily  range  be  represented  graphically  ?     (74)  How  may  the  adiabatic 
rate  of  cooling  be  represented  graphically  ?     (75)  What  is  meant  by  the  isothermal 
layer?     (76)  At  what  height  does  it  occur?     (77)  What  are  its  characteristics ? 
(78)  Define  inversion  of  temperature.     (79)  Explain  why  the  atmosphere  is  un- 
stable during  the  day.     (80)  State  the  three  conditions  of  convection.     (81) 
Name  the  five  processes  which  are  operative  in  the  heating  of  -the  atmosphere. 
(82)  Describe  in  detail  the  effect  of  each  process.     (83)  Name  the  three/processes 
operative  in  the  cooling  of  the  atmosphere.     (84)  State  the  effect  of  each  of  the 
three  processes.  % 


TOPICS   FOR   INVESTIGATION 

(1)  The  nature  and  characteristics  of  the  atom. 

(2)  The  amount  reflected,  transmitted,  and  absorbed  by  the  different  things 
upon  which  insolation  may  fall. 

(3)  The  methods  of  obtaining  the  value  of  the  solar  constant. 

(4)  Changes  in  the  temperature  of  the  ocean  between  day  and  night. 

(5)  Changes  in  the  temperature  of  the  ground  between  day  and  night/ 

(6)  The  method  of  computing  the  adiabatic  rate  of  cooling  of  air. 

(7)  Mountain  observatories. 

(8)  Balloon  ascensions  for  scientific  purposes. 

(9)  The  use  of  kites  in  meteorology. 

(10)  The  variation  in  the  vertical  temperature  gradient. 

(11)  The  isothermal  layer. 

PRACTICAL   EXERCISES 

(1)  Contrast  the  amount  of  insolation  received  at  several  different  places  and 
at  several  different  times  by  working  out  the  exact  value  of  the  three  factors. 

(2)  Determine  by  the  photographic  paper  method  the  transmission  coefficient 
of  the  atmosphere  on  some  cloudless  day. 

(3)  Observe  carefully,  or  better  photograph,  some  cumulus  clouds. 

(4)  Note  the  time  when  the  cumulus  clouds  disappear  in  the  late  afternoon. 

(5)  If  possible,  compare  the  observations  of  temperature  made  on  some  near-by 
mountain  top  and  at  some  valley  station. 

REFERENCES 

ASSMANN,  BBRSON,  and  others,  Wissenschaftliche  Luftfahrten  (3  vols.). 
ROTCH,  Sounding  the  Ocean  of  Air. 

For  recent  articles  and  references  on  the  isothermal  layer  see : 

GOLD,  E.,  and  HARWOOD,  W.  A.,  The  present  state  of  our   Knowledge   of  the 
upper  Atmosphere  as  obtained  by  the   Use  of   Kites,   Balloons,  and  Pilot 
Balloons,  8°,  54  pp.,  London,  1909. 

The  Mount  Weather  Bulletin  (particularly  articles  by  Humphreys). 
See  Appendix  IX  for  other  books  on  upper  air  investigation. 


CHAPTER   III 

THE  OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE 
THE  DETERMINATION  OF  TEMPERATURE 

Thermometry,  60,  61. 

Thermometers,  62,  63. 

The  real  air  temperature,  64. 

Thermometer  shelter,  65. 

Sling  thermometer,  66. 

Ventilated  thermometer,  67. 

Thermographs,  68. 

Maximum  and  minimum  thermometers,  69. 

Black  bulb  thermometer,  70. 

Thermometers  for  special  purposes,  71. 

THE  RESULTS  OF  OBSERVATION 

The  observations,  72. 

Normal  hourly,  daily,  monthly,  and  yearly  temperature,  73-76. 

Diurnal,  annual,  and  irregular  variation,  77,  78. 

Temperature  data,  79,  80. 

Differences  of  temperature  with  altitude,  81. 

Temperature  differences  over  a  limited  area,  82. 

THE  DISTRIBUTION  OF  TEMPERATURE  OVER  THE  EARTH 

Construction  of  isothermal  charts,  83. 
Isothermal  lines  for  the  year,  84,  85. 
Isotherms  for  January  and  July,  86,  87. 
Poleward  temperature  gradient,  88. 
Thermal  anomalies,  89. 
Annual  range  of  temperature,  90. 
Extremes  of  temperature,  91. 
Other  temperature  charts,  92. 
Polar  temperatures,  93. 

THE  TEMPERATURE  OF  LAND  AND  WATER 

Ocean  temperatures,  94. 

Lake  temperatures,  95. 

River  temperatures,  96. 

Temperature  below  the  surface  of  the  land,  97. 

THE  DETERMINATION  OF  TEMPERATURE 

60.    Thermometry. — Thermometry,  as  the  word  indicates,  has  to  do 

with  the  determination  of  temperature,  and  the  instrument 
Definition. 

used  is  called  a  thermometer.1   There  are  three  systems  of  ther- 

1  6tp/Jir)  =  heat;  ^rpov  =  measure. 
60 


OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE     61 

mometry  or  thermometric  scales,  namely,  the  Fahrenheit,  the  Centigrade, 
and  the  Reaumur.     The  two  standard  temperatures  in  each  system  are 
the  same,  namely,  the  melting  point  of  ice  and  the  boiling  point 
of  water.    In  each  case  the  ice  and  water  must  be  pure  and  the 
pressure  must  have  the  standard  value.     These  two  standard  grade,  and 
temperatures  are  numbered  differently  in  the  three  systems, 
In  the  Fahrenheit  system  the  melting  point  is  numbered  3u 
and  the  boiling  point  212,;  in  the  Centigrade  system  the  melting  point  is 
numbered  0  and  the  boiling  point  100  ;  in  the  Reaumur  system  the  melt- 
ing point  is  numbered  0  and  the  boiling  point  80.     The  inter- 
vals between  the  two  standard  temperatures  are  thus  180,  reiationTe- 
100,  and  80  respectively  and  in  the  ratio  of  9  to  5  to  4.     The  tween  the 
following  formula  thus  indicates  the  relation  between  the  three 
systems  where  F,  C,  and  R  denote  the  three  corresponding  temperatures  : 

F  -  32  =  C  =  R 
9  54' 

F  —  32  represents  the  F.  interval  above  the  melting  point.  Since  the 
C.  and  R.  thermometers  have  the  melting  point  indicated  by  zero  the 
intervals  are  expressed  directly  by  C  and  R.  A  temperature  expressed 
in  any  one  of  the  three  systems  can  be  immediately  changed  into  the  cor- 
responding temperature  in  the  other  systems  by  means  of  this  formula.1 
For  example,  Let  F  =  60°  ;  the  equations  then  become  : 

60  —  32      C  _  R,  28  __  C  ._  /£  .  /nr  _  i  cs  «nr|  #  _  1 94  2 
~9~     ~5  ~4'  ¥      5      4!  ( 

All  temperatures  below  the  zero  of  the  scale  are  indicated  by  a  minus 
sign  and  are  read  "  so  many  degrees  below  zero."  Thus  7  degrees  below 
zero  Fahrenheit  is  indicated  by  —  7°  F. 

Jjl  OO  /"Y 

1  The  formula  —  — ^=  =  —  may  be  modified  so  as  to  make  the  mental  computation  of 

9  5 

the  Centigrade  temperature  for  the  Fahrenheit  easier. 

C  =  |(F-32)  =  (F-  32)  -555...  =  (F-32)  (i  +  A-J  +  iJo  •*•••). 

The  rule  would  be :  subtract  32  from  the  Fahrenheit  temperature ;  take  one  half  of  it  ; 
add  to  this  one  tenth  of  itself,  one  one-hundredth  of  itself,  etc.  Thus  for  60°  Fahrenheit : 

60  -  32  =  28 
*  of  28  =  14 
C=14 
1.4 
.14 

.01  etc. 
C  =  15.55  + 

2  In  Appendix  II  is  given  a  table  for  converting  Centigrade  into  Fahrenheit  and  vice 
versa.     In  Appendix  III,  a  graphic  comparison  of  the  two  scales  is  given. 


62 


METEOROLOGY 


The  inven- 
tion of  the 
thermom- 
eter. 


6 1 .  The  thermometer  was  invented  by  Galileo  and  Sanctorius  of  Padua 
in  1590.  Sanctorius  had  been  a  pupil  of  Galileo,  and  later  became  pro- 
fessor of  medicine  at  the  same  university.  Figure  21  repre- 
sents the  thermometer  used  by  Sanctorius  for  determining 
the  feverishness  of  his  patients.  It  consists  simply  of  a  glass 
globe  opening  into  a  narrow  tube  and  partly  filled  with  fluid. 
This  is  then  inverted  and  dipped  into  a  vessel  of  fluid.  The  bulb  is  taken 
in  the  hand  and  the  feverishness  is  indicated  by  the  height  at  which  the 
liquid  stands  in  the  tube.  If  this  early  form  of 
the  thermometer  was  invented  by  Galileo  and 
adopted  by  Sanctorius,  or  whether  it  was  the 
combined  invention  of  Galileo  and  Sanctorius,  is 
uncertain.  Within  a  few  years,  however,  the 
instrument  was  inverted,  and  it  then  became  a 
thermometer  of  essentially  the  same  form  as  at 
present.  For  nearly  100  years  after 

Confusion        f        .  J 

and  uncer-     the  invention  of  the  thermometer  all 
taintyfor       was    COnfusion    and    uncertainty,   for 

many  years.  . 

various  fluids  were  used  and  various 
systems  of  graduation  were  employed.  In  order 
to  get  a  constant  temperature  various  devices 
were  used.  Some  used  the  temperature  of  the 
water  of  a  certain  spring  as  constant.  The  tem- 
perature of  the  subcellar  of  the  Observatory  of 
Out  of  this  chaos  the  three  systems  of  ther- 
mometry  emerged,  because  the  founders  of  the  systems  made  instru- 
ments of  such  high  quality  and  in  such  large  numbers  that  they  came 
to  be  recognized  as  standard  instruments. 

The  Fahrenheit  thermometer  originated  at  Dantzig  about  1714.  The 
two  great  improvements  introduced  by  Fahrenheit  were  the  use  of  mer- 
The  Fahr-  cury  as  the  fluid,  and  the  use  of  two  known  temperatures  for 
enheit  ther-  the  graduation.  The  reason  for  choosing  32  as  the  melting 
mometer.  pOmt  of  ice  and  212  as  the  boiling  point  of  water  is  uncertain. 
It  is  known  that  he  had  traveled  in  Iceland,  and  it  may  be  that  the  zero 
of  his  thermometer  scale  was  the  lowest  temperature  which  he  had  ever 
experienced.  It  may  be  that  he  used  the  ^temperature  of  the  human 
body  for  standardizing  his  instrument,  intending  to  have  this  100°. 
The  average  temperature  of  the  l  '>ody,  however,  is  only  98.6°. 

A  mixture  of  salt  and  snow  !  by  the  Italian  thermometer 

n  akers  for  getting  the  zero  their  instruments.     The  temper- 


FIG.  21.— The  Original 
Thermometer  of  Sanc- 
torius. 

Paris  was  also  used. 


OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE      63 

ature  of  this  is  nearly  zero,  and  it  may  be  that  Fahrenheit  used  this 
for  determining  the  zero.  Fahrenheit  was  also  a  maker  of  astronomical 
instruments,  and  it  may  be  that  in  order  to  use  his  machine  for  dividing 
circles  for  graduating  the  stem  of  the  thermometer,  it  was  necessary  to 
have  the  interval  between  the  melting  and  boiling  points  180. 

The  Centigrade  thermometer  was  invented  by  Celsius  and  Linnseus  at 
the  University  of  Upsala  in  Sweden  in  1742.   The  two  fixed  points  are  num- 
bered 0  and  100  respectively.     In  the  original  thermometers  The  Centi_ 
the  freezing  point  may  have  been  numbered  100  and  the  boil-  grade  ther- 
ing  point  0,  so  that  a  distinction  should  be  made  between  the  ' 
Celsius  and  the  Centigrade  thermometer.     Within  a  few  years,  however, 
the  Centigrade  system  of  numbering  the  fixed  points  came  into  general  use. 

The  Reaumur  thermometer  was  invented  in  1731  by  the  French  physicist 
bearing  that  name.    The  Fahrenheit  and  Centigrade  scales  are  both  used  in 
scientific  work.     The  Fahrenheit  scale  is  used  popularly  in  all  The  Rgau_ 
English-speaking  countries.     The  Centigrade  thermometer  is  mur  ther- 
used  popularly  in  France  and  portions  of  Germany.   The  Reau-  * 
mur  thermometer  is  used  popularly  in  Russia  and  portions  of  Germany. 
No  thermometer  is  used  popularly  in  the  country  in  which  it  was  invented. 

62.  Thermometers.  —  A  thermometer  consists  essentially  of  a  glass 
capillary  tube  of  uniform  diameter  or  bore  called  the  stem,  which  is  at- 
tached to  a  bulb.  The  bulb  is  usually  cylindrical  in  form 
and  of  about  the  same  diameter  as  the  stem.  This  is  a  matter  tion  and 
of  convenience,  however,  and  many  good  thermometers  have  ^ncipie  Of 
nearly  spherical  bulbs.  The  bulb  and  part  of  the  stem  are  ather- 
filled  with  some  fluid,  usually  mercury,  and  above  this  there  is  z 
a  vacuum.  The  principle  of  a  thermometer  is  that  for  a  given  increase 
in^temperature_the  fluid  in  the  bulb  expands  many  times  (in  the  case  of 
mercury  seven  times)  as  much  as  the  glass  itself^  Thus  if  the  bore  is 
uniform,  to  equal  increments  of  temperature  will  correspond  equal 
changes  in  the  length  of  the  column  of  fluid  in  the  stem.  The  thermom- 
eter proper  is  attached  to  a  case  in  order  to  protect  it  from  injury  and 
to  facilitate  its  being  attached  to  various  objects.  The  presence  of  the 
milk  glass  at  the  back  of  the  thermometer  is  to  furnish  a  better  reflector 
for  reading  the  instrument.  The  curved  front  of  the  stem  serves  to 
magnify  the  liquid  within,  thus  making  it  more  easily  visible.  Figure  22 
represents  thermometers  of  various  forms  for  determining  temperature.1 

1  The  electrotypes  for  this  figure  and  for  many  other  illustrations  of  meteorological 
apparatus  appearing  in  this  book  were  very  kindly  furnished  by  Mr.  Henry  J.  Green, 
1191  Bedford  Avenue,  Brooklyn,  N.  Y.  <f 


64 


METEOROLOGY 


The  various 
steps  in  the 


The  first  step  in  the  construction  of  a  thermometer  is  the  making  of 
the  stem  by  a  glass  blower.  This  must  then  be  tested  in  order  to  deter- 
mine whether  the  cross  section  of  the  bore  is  uni- 
form. This  is  done  by  inserting  a  small 
thread  of  mercury,  and  observing  its 
construction  length  as  it  is  moved  up  and  down  the 
stem.  If  it  changes  in  length,  it  in- 
dicates a  larger  or  smaller  bore.  If 
the  stem  has  stood  this  test,  the  thermometer 
bulb,  and  the  bulb  and  tube  for  filling  it,  are 
next  added  by  the  glass  blower.  It  then  ap- 
pears as  indicated  in  Fig.  23.  In  order  to  fill  it, 
the  upper  bulb  is  filled  with  mercury.  By  alter- 
nately heating  ancl  cooling  the  lower  bulb  the 
fluid  is  drawn  down  into  the  thermometer  and 
it  is  then  sealed  at  the  top  at  the  appropriate 
temperature.  Next  the  strains  in  the  glass 
incident  to  the  construction  of  the  thermometer 
must  be  removed.  Formerly  the  thermometers 
were  laid  away  for  from  three  to  ten  years  in 
order  to  allow  the  strains  to  disappear 
with  time.  At  present,  however,  the 
strains  are  removed  by  an  annealing 
process  which  consists  in  alternately 
heating  and  cooling  the  thermometer. 
The  thermometer  is  then  exposed  to 

FIG.  22.  —  Thermometers    ,.  , 

of  Various  Forms.         the  two  standard  temperatures,  and 
the  indications  noted.     The  interval 

between  the  two  fixed  points  is  then  divided  into  the 
appropriate  number  of  degrees  by  means  of  a  dividing 
machine,  and  the  scale  may  be  extended  in  either  direction 
from  the  fixed  points. 

There  are  several  essentials  in  a  good  thermometer.     In 
the  first  place,  a  suitable^  fluid  mustbe  used.     This  fluid   Flo  23. —  A 
must  not  freeze  at  ordinary  temperatures,  and 

The  essen-  -.  \         — 7.  , 

tiais  in  a  it  must  not  be  decomposed_by  the  action  01 
good  ther-  light  or  at  moderately  high  temperatures,  and 

mometer.          .  "  — — - 

it  must  not  vaporize  or  boil  at  any  ordinary  temperature. 
Mercury  is  almost  universally  used  tor  the  higher  temperatures.  For 
the  lower  temperatures,  since  mercury  freezes  at  —39.4  F.,  alcohol 


Thermom- 
eter in  the 
Making. 


OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE      65 


or  some  light  oil  is  ordinarily  used.  The  larger  thebulb  the  more 
sensitive  the  thermometer,  but  the  time  required  for  the  thermom- 
eter to  indicate  a  newTempe^ature  is,  however,  increased  by  increasing 
the  size  of  the  bulb.  The  size  of  the  bulb  should  be  such  as  to 
make  the  thermometer  sufficiently  sensitive  but  not  unduly  sluggish, 
bore  must  be  uniform  throughout  and  the  fixed  points  must 
have  been  accurately  determined.  If  the  graduation  is  placed  on  the 
backing  of  the  thermometer  and  the  thermometer  is  held  against  this 
by  a  wire  fastening,  these  wires  are  apt  to  become  loosened  with 
time  and  the  thermometer  sags  in  its  fastening,  thus  causing  error. 
Every  good  thermometer  is  graduated  on  the  stem  itself. 

63.  The  inaccuracies  in  determining  a  temperature  (more  especially 
an  air  temperature)  may  be  divided  into  two  groups:  first,  avoidable 
blunders ;  and  secondly,  errors  in  the  construction  and  use  of 
the  thermometer  which  cannot  be  eliminated  without  careful 
testing  and  elaborate  experimental  methods.  There  are  four  determining 
avoidable  blunders  ;  (1)  _a_  good_t 

in  determining  the  temperatureT    It  is  a  waste  of  time  to 
attempt  to  make  an  accurate  determination  of  the  temperature  with  an 
inferior  instrument.     (2)    The  prror  of  parallax 
must   be   avoided.      By   parallax   in   general   is 
meant  the  displacement  of  one  object  with  refer- 
ence to  another  as  the  position  of  the  eye  of 
the  observer  changes.     In  Fig.  24  let  A  repre- 
sent   the   thread   of   mercury  in   the  The  &vo-^_ 
thermometer  stem,  and  B  the  gradu-  able 
ated  scale.     The  eye  of  the  observer 
placed  at  m  will  see  the  mercury  thread  at  x  on 
the  scale,  while  the  eye  of  the  observer  placed 
at  n  would  see  it  at  y.     The  correct  position  of 
the  eye  is  such  that  the  line  of  vision  is  at  right 
angles  to  the  stem  of  the  thermometer.     (3)  The 
observer  must  avoid   heating   the   thermometer 
by  the  presence  of  his  own  body.     When  the* 
difference  in  temperature  between  the  indication  of  the  thermometer  and 
the  observer's  body  is  large,  a  considerable  error  may  result  if  the  tem- 
perature is  not  read  quickly.     On  a  cold  zero  morning  in  winter,  an 
observer  standing  within  one  foot  of  an  exposed  thermometer  for  thirty 
seconds  would  probably  increase  its  indication  by  at  least  two  degrees. 
(4)  A  thermometer  must  be  given  time  to  indicate  a  new  temperature. 


blunders- 


FIG.  24.  —  The  Error  of 
Parallax  in  reading  a 
Thermometer. 


66  METEOROLOGY 

It  is  necessary  to  allow  at  least  five  minutes  for  the  thermometer  to  take 
up  any  considerable  change  in  temperature. 

The  following  are  errors  in  the  construction  and  use  of  the  thermome- 
ter :  (1)  The  fixed  points  may  be  slightly  misplaced.  (2)  The  bore  may 
The  errors  no^  ^e  absolutely  uniform.  (3)  Decomposition  may  have 
of  construe-  taken  place  in  the  fluid  of  the  thermometer.  (4)  The  bulb 
lse*  and  stem  may  not  be  at  the  same  temperature.  (5)  Pressure 
has  a  slight  influence  on  the  indication  of  a  thermometer.  The  indication 
of  a  thermometer  in  vacuo  and  when  exposed  to  the  atmospheric  pres- 
sure may  change  as  much  as  one  half  a  degree.  The  slight  changes, 
therefore,  in  the  pressure  of  the  atmosphere  which  are  continually  taking 
place  have  a  slight  influence  on  the  indications  of  a  thermometer.  (6)  The 
thermometer  must  not  be  strained  by  exposing  it  to  unusually  high  or  low 
temperatures.  If  the  temperature  of  a  certain  fluid  were  taken  and 
the  thermometer  then  exposed  to  a  temperature  of  100°,  or  more, 
and  then  placed  in  the  same  fluid  again,  it  would  probably  not  give  the 
same  indication.  A  thermometer  should  therefore  not  be  exposed  to 
direct  sunlight  any  more  than  is  necessary  on  account  of  the  high 
temperature  which  it  would  indicate  and  on  account  of  the  possible  de- 
composition of  the  fluid. 

A  standard  thermometer  costs  from  $5  to  $25.     This  would  indicate 
accurately  the  temperature  within  one  fourth  of  a  degree  if  the  four  blun- 

ders  are  avoided,  but  without  taking  account  of  the  errors  in 
racy  of  a  the  construction  and  use  of  the  thermometer.  A  thermom- 
thermom-  e^er  COsting  from  $.75  to  $3  would  usually  indicate  the  tem- 

perature correct  to  within  1°  F.  A  cheaper  thermometer 
may  be  in  error  anywhere  from  1°  to  even  10°  or  15°. 

64.    The  real  air  temperature.  —  It  is  not  easy  to  determine  the  tem- 
perature of  a  gas,  because  the  thermometer  does  not  indicate  the  temper- 

ature of  the  surrounding  medium,  but  the  temperature  of  its 
hard  to  get  own  bulb.  If  a  thermometer  is  surrounded  by  an  opaque 


the  real  air  fl^  or  so\i^  it  takes  up,  by  conduction,  the  temperature  of 
the  surrounding  medium,  and  thus  indicates  the  temperature  of 
that  medium.  If  the  bulb  of  a  thermometer  is  exposed  in  a  gas,  however, 
its  temperature  is  determined  both  by  the  conduction  of  heat  to  or 
from  the  surrounding  medium  and  the  difference  between  the  radiant 
energy  absorbed  and  emitted  by  the  thermometer.  A  thermometer 
exposed  in  the  open  air  gives  neither  during  the  day  nor  at  night 
the  real  air  temperature,  even  if  it  is  an  accurate,  standard  instru- 
ment. During  the  day  it  will  indicate  a  temperature  anywhere  from 


OBSERVATION  AND   DISTRIBUTION   OF  TEMPERATURE      67 


20  to  60°  above  the  real  air  temperature,  because  the  excess  of  the  in- 
solation received  over  the  radiant  energy  emitted  is  large.     During  the 
night  the  thermometer  may  indicate  a  temperature  from  xheindica- 
1  to  7°  below  the  real  air  temperature,  because  it  is  radiat-  tions  of  a 
ing  more  heat  to  the  sky  and  earth  than  it  is  receiving.     A  ^JgrTn  the 
close  approximation  to  the  real  air  temperature  can  be  deter-  open  by  day 
mined  in  summer  by  exposing  the  thermometer  in  the  shade  and  at  mght' 
of  a  small  tree,  and  in  winter  by  placing  it  on  the  inside  of  a  north  piazza 
post.     The  indications  of  the  instrument  in  these  positions  will  give 
approximately  the  real  air  tempera- 
ture,   although    they    may    differ    in 
certain  cases  by  as  much  as  three  or 
four  degrees. 

In  meteorological  work  the  real  air 
temperature  is  determined  in  one  of 
three    ways;    namely,    by  The  ^^ 
means    of    a   thermometer  methods  of 
shelter,  sling  thermometer,  ^1? ah-    ° 
or  ventilated  thermometer,   tempera- 

65.    Thermometer    shel-  t 
ter.  - —  The    thermometer    shelter    as 
used  by  the   U.  S.   Weather   Bureau 
consists  of   a   cubical   box  xhether- 
about  three  feet  on  a  side,  mometer 
with  a  double  sloping  roof,   ^6  IT.™. 
closed  bottom,  and  latticed  Weather 
sides.      Such    a   shelter   is  I 
represented  in  Fig.  25.      The  purpose 
of   the  double   roof   is    to  shield   the 
thermometers  from  the   insolation  of 
the  sun.     The  outer  portion  of   the 
roof  absorbs  the  insolation,  and   the 

circulation  of  air  between  the  two  portions  of  the  roof  prevents  the 
lower  layer  from  becoming  heated.  The  tight  bottom  excludes  rising 
air  currents  and  intercepts  any  radiations  from  the  earth,  while  the 
latticed  sides  allow  a  free  circulation  of  the  air.  The  shelter  must  be 
located  at  least  five  feet  above  the  ground.  In  the  country  or 
in  a  village  the  best  location  is  in  the  open,  over  sod,  although  it  may 
be  attached  to  the  north  side  of  an  unheated  building.  A  sufficient 
space  must  be  left  between  the  shelter  and  building  to  allow  a  free  cir- 


FIG.  25.  —  The  Thermometer  Shelter 
of  the  U.  S.  Weather  Bureau. 


68 


METEOROLOGY 


culation  of  air.  In  a  city,  since  the  air  stagnates  in  a  narrow  street,  the 
best  location  is  the  roof  of  some  high  building.  This  does  not,  of  course, 
give  the  temperature  of  the  streets  where  the  people ^paust  live,  but  it 
probably  does  give  a  close  approximation  to  the  real  air  temperature  in 
the  country  immediately  surrounding  the  city.  It  has  been  found  as  a 
result  of  many  experiments  that  the  indica- 
tion of  the  thermometer  in  such  a  shelter 


FIG.  26.  —  The  French  Thermometer  Shelter. 
(From  ANGOT'S  Instructions  Meteor  ologiques.) 


FIG.  27.  —  The  English  Ther- 
mometer Shelter. 


usually  gives  the  real  air  temperature  correct  to  within  half  a  degree. 

It  is  most  in  error  on  still,  clear,  cold  nights,  and  on  still,  hot,  summer 

days.     In  both  cases  the  indications  of  the  sheltered  thermometer  are 

too  conservative. 

The  form  of  the  thermometer  shelter  as  used  by  the 
various  countries  is  somewhat  different,  although  the 
underlying  principle  is  always  the  same.  Figures  26,  27, 
and  28  illustrate  tb^  bhelters  used  by  the  French,  English, 
and  Russian  governments  respectively. 


The  ther- 
mometer 
shelter  of 
other  coun- 
tries. 


OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE      69 


FIG.  28.  —  The  Russian  Thermometer  Shelter. 
(From  WALDO'S  Modern  Meteorology.) 


The  princi- 
ger  quan-  pie  of  the 
tityof  air   sling  ther- 
mometer. 

in    con- 


tact with  the  bulb, 
and  the  exchange  of  heat  with  the  surrounding  medium 
by  means  of  conduction  is  thus  emphasized.  The  under- 
lying principle  in  the  use  of  the  instrument  in  getting 
real  air  temperature  is  to  so  emphasize  conduction  that 
the  effect  of  the  radiant  energy  received  or  emitted 
becomes  negligible  in  comparison.  Several  minor  errors 
are  introduced  by  whirling  the  thermometer.  The  centrif- 
ugal force  causes  a  slight  lowering  in  the  indication  of  the 
instrument,  and  the  friction  of  the  bulb  with  jthe  air  would 
raise  the  temperature  slightly,  but  H&£se -»small  errors 
counterbalance  each  other  for  the  most -part.  It  is 


66.    Sling  thermometer.  —  Another  method  of  getting  the  real  air  tem- 
perature is  by  means  of  the  sling  thermometer,  which 
was  devised  by  Arago  in  1830.     Since  a  ther- 
mometer shelter  is  not  portable,  a  sling  ther- 
mometer    is    particularly    advantageous    for 
scientific  expeditions  and  for  those  observa- 
tions of  air  temperature  which  are  not  made  constantly 
in  the  same  place.     The  sling  thermometer  (see  Fig.  29) 

consists  usually  of 
two  thermometers 
attached  to  a  board 
hich  is  provided 
with  a  chain  or 
string  so  that  it 
can  be  whirled 
rapidly.  In  some 
cases  the  bulbs  of 
the  thermometers 
are  covered  by  wire 
netting  in  order  to 
protect  them  from 
injury. 

Whirling  the  ther- 
mometer brings  a 
muchlar- 


FIG.  29.  —  The 
Sling  Ther- 
mometer. 


70 


METEOROLOGY 


supposed  that  the  sling  thermometer  will  give  the  real  air  temperature 
accurately  to  within  a  half  a  degree  Fahrenheit  under  almost  all  cir- 
cumstances. 

\/  67.    Ventilated  thermometer.  -  The  ventilated  thermometer,  which  is 

the  best  instrument  for  determining  the  real  air  temperature,  was  invented 

by  Assmann  at  Berlin  in  1887.     Figure  30  gives  a  picture 

Description  —  , 

oftheven-     and  a  cross  section  of  this  instrument.     It  consists  essen- 

tiiated  ther-,  tially  of  two  sensitive,  accurate  thermometers  (i  and  t'  in  the 

figure)  whose  bulbs  are  located  within  a  double  jacket.     The 

instrument  is  provided  with  a  motor  A  driven  by  a  spring  which  draws 


THERMOMETER 


SUN  SHIELD 


FIG.  30.  —  Assmann's  Ventilated  Thermometer. 
(From  BORNSTEIN'S  Leitfaden  der  Wetter  kunde.) 

the  air  with  a  velocity  of  two  or  three  feet  per  second  past  the  bulbs  of 
the  thermometers  up  through   a   central    column  g   and  ejects  it   at 


OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE      71 


the  top.  The  instrument  is  made  of  burnished  silver  which  reflects 
more  than  90  per  cent  of  the  insolation,  and  thus  is  but  little 
heated  even  in  bright  sunshine.  It  is  provided  with  shields  for  keep- 
ing the  insolation  from  the  stems  of  the  thermometers,  and  the 
two  ivory  rings  prevent  the  conveyance  of  heat  by  conduction 
from  the  rest  of  the  apparatus  to  the  jackets  which  surround 
the  thermometer  ^ulbs.  The  jackets  are  double,  so  that  any  insola- 
tion which  might  be  absorbed  by  the  outer  jacket  would  be  communi- 
cated to  the  air  between  the  two  jackets  and  not  directly  to  the  air 
which  surrounds  the  thermometer  bulbs.  Numerous  tests  of  the  ac- 
curacy of  this  instrument  have  been  made,  and  it  is  stated  that  it 
will  determine  the  real  air  temperature  correctly  to  a  tenth  of  a  degree 
Fahrenheit  Accuracy  of 
under  any  theinstru- 

i  •  ,  •  ment. 

conditions 
whatever.  The  instru- 
ment is  extremely  port- 
able, and  is  by  far  the 
most  convenient  for  de- 
termining the  real  air 
temperature.  The  only 
drawback  is  the  expense, 
the  cost  being  about  $60. 

68.  Thermographs.  — 
For  many  purposes  con- 
tinuous records  of  the 
temperature  are  desir- 
able .  These  are  obtained 
by  means,  of  the  thermo- 
graph, of  which  there  are 
two  forms  in  ordinary 
use,  the  Draper  and  the 
Richard  Freres. 

The  Draper  thermo- 
graph (Fig.  31)  is  an 
American  in-  The  Draper 

Strumentand    thermo-  FlG    3L_  The  Draper  Thermograph. 

has  a  metallic  ***• 

thermometer,  one  end  of  which  is  fixed  while  the  other  end  is  attached 

to  a  series  of  levers  which  magnify  the  small  movements  due  to  tern- 


72 


METEOROLOGY 


perature  changes,  and  transmit  them  to  a  pen  which  moves  in  and  out 
across  a  dial.  As  the  temperature  rises  the  pen  moves  outward  from 
the  center,  and  as  the  temperature  falls  the  pen  moves  inward.  The 
pen  contains  a  non-freezing  glycerine  ink,  and  rests  against  the  dial, 
which  is  turned  by  clockwork.  The  dial  is  divided  into  days  and  hours 
by  curved  radial  lines,  and  into  degrees  by  concentric  circles.  The  dial 
makes  one  complete  revolution  in  a  week,  and  a  continuous  record  of 
the  temperature  is  thus  kept. 

The  Richard  Freres  thermograph  (Fig.  32)  is  made  in  Paris  and  has 
The  Richard  ^een  adopted  by  the^U.  S.  Weather  Bureau.     The  thermome- 
Freresther-    ter  is  here  either  a  metallic  thermometer  or  a  bent  tube  of 
metal  containing  a  non-freezing  liquid.     If  a  metallic  ther- 
mometer is  used,  it  consists  of  two  curved  strips  of  metal  soldered  to- 


FIG.  32.  —  The  Richard  Freres  Thermograph. 

gether.  In  either  case,  as  the  temperature  rises  the  bent  ther- 
mometer tends  to  straighten;  if  the  temperature  falls  the  elasticity 
bends  the  thermometer  into  a  sharper  curve.  These  small  motions 
are  magnified  and  communicated  by  a  system  of  levers  to  the 
pen,  which  moves  up  and  down  over  the  paper  which  is  attached  to  the 
outside  of  the  revolving  drum.  The  drum  contains  the  clockwork 
which  drives  it,  and  makes  one  complete  revolution  in  a  week  or,  if  so 
ordered,  in  a  day.  The  pen  rises  and  falls  with  temperature  changes,  and 
thus  the  continuous  record  of  temperature  is  kept.  The  thermograph  is 
at  best  not  an  accurate  instrument.  In  order  to  get  the  best  results  it 


OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE      73 

must  be  standardized  at  least  twice  a  day  by  comparing  it  with  the 
indications  of,  preferably,  a  maximum  and  minimum  thermometer  kept 
near  it. 

Other  forms  of  thermographs  are  in  use  for  special  purposes  or  for 
research  work,  but  the  two  described  above  are  the  only  ones  com- 
mercially on  the  market.  The  Draper  instrument  costs  about  $20 
and  the  Richard  Freres  costs  about  $50,  although  much  less  if  it  can  be 
imported  free  of  duty.  \ 

69.    Maximum  and  minimum  thermometers.  —  The  purpose  of  maxi- 
mum and  minimum  thermometers  is  to  record  the  highest  and  lowest 
temperatures  which  have  occurred  during  the  interval  since 
the  thermometers  were  set.     Figure  33  represents  the  maxi-  0f6tSh  "maxi- 
mum and  minimum  thermometers  used  by  the  U.  S.  Weather  mum  and 
Bureau.     The  maximum  thermometer,  which  is  represented 


at  the  bottom  of  the  figure,'is  a  mercurial  thermometer  with  a  ters  of  the 
constriction  in  the  stem  just  above  the  bulb.     As  the  tern-  er  Bureau*  " 
perature  rises  the  mercury  is  forced  past  the  constriction  ;  if 
the  temperature  falls,  the  mercury  thread  breaks  at  the  constriction,  leav- 
ing the  stem  filled  with  mercury.     The  highest  temperature  is  thus 


FIG.  33.  —  Weather  Bureau  Maximum  and  Minimum  Thermometers. 

indicated  by  the  upper  end  of  the  thread  of  mercury  in  the  thermometev 
stem.  It  is  set  by  rapidly  whirling  the  instrument,  thus  forcing  back  the 
mercury  by  means  of  centrifugal  force. 

The  minimum  thermometer  is  an  alcohol  thermometer  and  contains  a 
small  dumbbell-shaped  glass  index.  As  the  temperature  falls  this 
index  is  dragged  down  the  stem  of  the  thermometer  by  surface  tension. 
It  the  temperature  rises,  the  fluid  flows  past  the  index,  leaving  it  at  its 
lowest  position.  The  minimum  temperature  is  thus  indicated  by  the 
forward  end  of  the  glass  index.  The  instrument  is  set  by  lifting  the 
bulb  end  of  the  thermometer,  thus  allowing  the  index  to  slide  down  to 
the  end  of  the  column  of  fluid. 

The  Six  maximum  and  minimum  thermometer,  which  combines  both 


74 


METEOROLOGY 


thermometers  in  the  same  instrument,  has  come  within  the  last  few  years 
into  very  general  use.     Although  known  for  more  than  a  century,  it  was 
not  until  within  the  last  few  years  that  sufficient  accuracy  has 
been  attained  to  make  its  indications  of  value.     Figure  34 
gives  an  illustration  and  cross  section  of  the  instrument.     It 
consists  of  a  long  bulb,  containing  a  non-freezing  fluid,  a 
U-shaped  stem  containing  a  thread  of  mercury,  and  a  bulb  at 
the  top  containing  compressed  air.     As  the  temperature  rises  the  fluid  in 


The  Six 
maximum 
and  mini- 
mum ther- 
mometers. 


COMPRESSED  /    \ 

AIR — A  i 


MlN. 


130 


\J 


V     M 

\     ^  THE  INDEX 

MAX. 


130 


FIG.  34.  —  Six  Maximum  and  Minimum  Thermometer. 

the  bulb  expands,  forcing  the  mercury  to  rise  on  the  right,  .thus  raising 
the  index  and  compressing  the  air  in  the  bulb  above.  'If  the  temperature 
falls,  the  fluid  in  the  bulb  contracts  and  the  compressed  air  forces  the 
mercury  thread  around,  thus  preventing  the  formation  of  any  vacant 
space  in  the  bulb.  The  index  on  the  left  is  now  raised,  and  it  will  thus 
indicate  the  lowest  temperature.  The  lower  ends  of  the  two  indices  thus 
register  the  highest  and  lowest  temperatures  respectively.  The  glass 
index  consists  of  a  small  piece  of  steel  wire  surrounded  by  a  dumbbell- 
shaped  glass  covering  provided  with  two  hairlike  appendages  which  pre- 
vent the  index  slipping  in  the  stem  of  the  thermometer.  They  are  set 


OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE      75 

by  means  of  a  magnet  which  acts  on  the  steel  wire  in  the  center  of  each 
index. 

Other  forms  of  maximum  and  minimum  thermometers  have  been 
devised,  but  have  not  come  into  general  use  either  because  they  are  hard 
to  transport  from  place  to  place,  or  because  they  are  liable  to  become 
deranged  while  in  use. 

70.    Black   bulb    thermometer.  —  The   black   bulb   thermometer   as 
illustrated  in  Fig.  35  is  a  mercurial  maximum  thermometer,  the  bulb 
of  which  has  been  coated  with  lampblack  or  platinum  black.  The  black 
The  whole  is  inclosed  in  a  glass  jacket  from  which  the  air  and  bulb  ther- 
moisture  have  been  extracted.     The  transference  of  heat  by  r 
conduction  to  or  from  surrounding  objects  is  thus  eliminated.     The  tem- 
perature of  the  bulb  will  rise  to  such  a  point  that  the  energy  given  out  in 


FIG.  35.  —  Black  Bulb  Thermometer. 

the  form  of  radiation  exactly  balances  the  radiant  energy  absorbed. 
If  such  an  instrument  is  surrounded  by  a  case  kept  at  a  constant  tempera- 
ture, and  if  the  insolation  of  the  sun  is  allowed  to  fall  upon  it,  the  tem- 
perature recorded  will  give  relative  values  of  the  insolation.  The  instru- 
ment is  thus  an  actinometer  rather  than  a  thermometer.  Without  the 
use  of  the  case,  which  is  kept  at  a  constant  temperature,  the  instrument 
will  give  only  rough  relative  values  of  the  insolation  received. 

71.    Thermometers  for  special  purposes.  —  The  forms  given  to  ther- 
mometers are  many  and  varied,  thus  suiting  them  to  a  large  variety  of 
special  purposes.     As  far  as  meteorology  is  concerned,  but  Thermome 
three  of  these  need  be  mentioned.     These  are  thermometers  ters  for  three 
for  determining   the»  temperature    of   the    soil   at   various  sPecial 
depths_beneath  the  surface,  thermometers  for  determining 
the  temperature  of   water  at  variousF~depths   and   telethermometers. 
The  first  two  thermometers  will  be  briefly  mentioned  when  the  results 
of  the  observations  of  soil  and  water  temperature  are  presented.     The 
purpose  of  a  telethermometer  is  to  indicate  within  a  building  the  real 
air  temperature  outside.     There  are  several  forms  of  the  instrument. 
It    -  vr'v  t.  <.,!••.   nienl   bill   expensive  ;.-•  -;  Is  '•>         in  onlj)  a  £  w  of  the 


Weather  Bureau  stations. 


76  METEOROLOGY 


THE  RESULTS  OF  OBSERVATION 

72.  The  Observations.  —  The  stations  of  the  U.  S.  Weather  Bureau 
are  of  three  kinds:    regular  stations,  cooperative  stations,  andjspecial 

stations.  The  regular  weatrieT~t)ureau  stations,  of  which 
kinds  of6u  there  are  from  180  to  200,  are  located  ordinarily  in  the  larger 
S.  Weather  cities,  since  they  usually  publish  a  daily  weather  map  which 

must  be  distributed  as  soon  as  possible.     There  are  from  3600 

to  4000  cooperative  stations,  and  from  300  to  500  special  sta- 
tions. The  observations  of  temperature  taken  at  a  regular  station  are 
the  real  air  temperature  at  8  A.M.  and  8  P.M.,  the  highest  and  lowest 

temperatures  during  the  preceding  12  hours,  and  a  continu- 
Theinstru-  ous  thermograph  record.  The  instruments  for  determining 

ments  used 

and  the  these  temperatures  are,  a  good  mercury  thermometer,  maxi- 
temperature  mum  an(j  minimum  thermometers  of  the  regular  Weather 
taken  at  Bureau  form,  and  a  Richard  Freres  thermograph.  These 
each  kind  instruments  are  located  in  a  thermometer  shelter  which  is  or- 
dinarily placed  from  6  to  10  feet  above  the  roof  of  some  high 
building  in  the  city.  At  a  cooperative  station  the  highest  and  lowest 
temperatures  during  a  day  are  determined,  and  also  the  reading  of 
the  maximum  thermometer  just  after  it  has  been  set.  The  purpose 
of  taking  this  observation  is  to  make  sure  that  the  maximum  ther- 
mometer has  been  set  and  also  to  give  the  real  air  temperature  at 
the  time  of  observation.  Maximum  and  minimum  thermometers 
only  are  necessary  for  these  observations,  and  the  shelter  which 
contains  the  thermometers  is  ordinarily  located  from  5  to  10  feet 
above  sod  in  the  open  or  on  the  north  side  of  some  building.  Special 
stations  take  those  observations  for  which  the  stations  were 
established. 

73.  Normal  hourly,  daily,  monthly,  and  yearly  temperature.  —  From 
these  observations  of  temperature  which  are  made  at  the  various  Weather 

Bureau  stations,  certain  normal  temperatures  and  other  tem- 
tion  between  Perature  data  may  be  computed.  At  the  very  outset  the 
average,  three  words  "  average,"  "  normal,"  and  "  mean  "  must  be 
mean*1'  "  carefully  defined.  By  an  average  is  meant  simply  the  sum 

of  a  number  of  observations  divided  by  the  number  of  the 
observations.  If  the  observations  have  been  extended  over  a  sufficient 
length  of  time  so  that  the  accidental  irregulariti  iminated  by 

taking  the  average,  then  the  average  value  may  be  spoken  of  as  a 
normal.  Usually  at  least  twenty  years  of  observations  are  required 


'    OBSERVATION  AND   DISTRIBUTION  OF  TEMPERATURE      77 

for  a  normal.      The  word  "  mean  "  is  used  by  various  writers  to  cover 
both  average  and  normal. 

If  a  good  continuous  thermograph  record  for  at  least  twenty  years  is 
available,  the  normal  hourly  temperatures  for  the  various  days  of  the 
year  may  be  computed.     For  example,  the  normal  9  A.M.    Normal 
temperature  for  October  28  would  be  found  by  averaging  the  hourly  tem- 
twenty  or  more  values  which  had  been  recorded  for  this  partic-  peri 
ular  hour  on  the  date  in  question.     Similarly,  the  normal  hourly  tem- 
peratures for  all  the  hours  of  all  the  days  in  the  year  may  be  computed. 
Usually  this  is  not  done  for  every  day  in  the  year,  but  the  days  are  grouped 
by  months. 

The  average  temperature  for  a  day  is  found  by  averaging  the  24  values 
of  hourly  temperature  observed  during  that  day.     This  requires  a  ther- 
mograph, and  since  thermographs  have  not  been  in  general  The 
use  even  at  regular  Weather  Bureau  stations  for  many  years,  methods  of 
various  combinations  have  been  sought  such  that  the  average  fh^av^ra"? 
temperature  for  the  day  might  be  computed  from  the  tern-  daily  tem- 
peratures observed  at  certain  definite  times  during  the  day.  peri 
Some  of  the  various  combinations  which  have  been  used  are  the  fol- 
lowing:   \  (8  A.M.  +  8  P.M.)  ;     I  (7  A.M.+  2  P.M.  +  9  P.M.  +  9  P.M.)  ; 
4  (maximum  +  minimum).     Of   these  \  (7  A.M.  +  2  P.M.  -f-  9  P.M.  + 
9  P.M.)  was  formerly  used  by  many  Weather  Bureau  stations  for  comput- 
ing the  average  daily  temperature.     At  the  present  time  J  (maximum + 
minimum)  is  used  at  all  U.  S.  Weather  Bureau  stations  for  computing  the 
average  daily  temperature.     The  reasons  are  :  the  apparatus  required  is 
simple,  the  computation  is  very  easy,  and  in  the  long  run  the  average 
daily  temperatures  computed  in  this  way  approximate  very  closely  the 
average  daily  temperatures  found  by  taking  one  twenty-fourth  of  the  sum 
of  the  twenty-four  hourly  values.     The  normal  daily  temperature  is 
found  by  averaging  the  average  dailies  for  the  date  in  question  for  a  suffi- 
cient number  of  years  to  eliminate  the  accidental  irregularities.     If  the 
normals  are  based  on  twenty  years  of  observations,  it  will  be  formal 
found  that  there  is  not  an  even  transition  from  day  to  day,   daUy  tem- 
but  jumps  in  temperature  of  even  two  or  three  degrees  occur.   * 
It  might  seem  that  the  time  were  not  sufficiently  long  to  eliminate  acci- 
dental irregularities.     It  is  found,  however,  that  these  abrupt  changes 
do  not  disappear  even  if  the  normal  is  based  on  a  hundred  years  of  obser- 
vations.    It  may  be  that  these  abrupt  changes  are  not  due  to  the  fact 
that  the  normal  is  based  on  too  short  a  time,  but  to  the  fact  that  there 
are  actual  abrupt  advances  and  recessions  of  temperature  which  take 


78 


METEOROLOGY 


place  on  nearly  the  same  date  each  year.  Ordinarily,  however,  the  nor- 
mal daily  temperatures  are  "  adjusted  "  before  they  are  published.  That 
is,  the  irregularities  are  smoothed  out  and  there  are  no  jumps  in  temper- 
ature from  day  to  day.  The  accompanying  table  gives  the  adjusted  nor- 
mal daily  temperatures  for  every  day  in  the  year  at  Albany,  N.Y.  This 
station  at  Albany,  N.Y.,  has  been  chosen  for  most  of  the  data  in  this  book 
because  the  record  is  a  long  one,  the  observations  have  been  well  taken, 
and  it  is  typical  of  New  England  and  the  Middle  Atlantic  States.1  The 
larger  cities,  as  New  York  and  Boston,  are  too  near  the  ocean,  which  is 
always  a  disturbing  factor,  to  be  typical.  In  the  case  of  an  average 

ADJUSTED  NORMAL  DAILY  TEMPERATURES  AT  ALBANY,  N.Y. 

(Temp.  F.) 


DATE 

JA¥. 

FEB. 

MAR. 

APR. 

MAY 

JUNE 

JULY 

AUG. 

SEPT. 

OCT. 

Nov. 

DEC. 

1 

24 

22 

27 

38 

53 

64 

71 

72 

66 

57 

44 

32 

2 

24 

22 

27 

39 

53 

65 

71 

72 

66 

57 

44 

32 

3 

24 

22 

28 

40 

54 

65 

71 

72 

66 

56 

43 

31 

4 

24 

22 

28 

40 

54 

65 

71 

72 

66 

56 

43 

31 

5 

23 

22 

28 

41 

55 

66 

71 

71 

66 

55 

42 

30 

6 

23 

22 

29 

41 

55 

66 

71 

71 

65 

55 

42 

30 

7 

23 

22 

29 

42 

55 

66 

71 

71 

65 

54 

42 

30 

8 

23 

22 

29 

42 

56 

66 

72 

71 

65 

54 

41 

30 

9 

23 

22 

30 

43 

56 

66 

72 

71 

64 

53 

41 

29 

10 

23 

23 

30 

43 

57 

67 

72 

71 

64 

53 

40 

29 

11 

23 

23 

30 

44 

57 

67 

72 

70 

64 

52 

40 

29 

12 

23 

23 

30 

44 

58 

67 

72 

70 

64 

52 

40 

28 

13 

22 

23 

31 

45 

58 

68 

72 

70 

63 

52 

39 

28 

14 

22 

23 

31 

45 

58 

68 

72 

70 

63 

51 

39 

28 

15 

22 

23 

32 

46 

59 

68 

72 

70 

63 

50 

39 

28 

16 

22 

24 

32 

46 

59 

68 

72 

70 

62 

50 

38 

27 

17 

22 

24 

32 

47 

60 

68 

72 

69 

62 

50 

38 

27 

18 

22 

24 

33 

47 

60 

69 

72 

69 

62 

49 

37 

27 

19 

22 

24 

33 

48 

60 

69 

72 

69 

61 

49 

37 

27 

20 

22 

24 

33 

48 

61 

69 

72 

69 

61 

48 

37 

26 

21 

22 

25 

34 

48 

61 

69 

73 

69 

61 

48 

36 

26 

22 

22 

25 

34 

49 

61 

69 

73 

68 

60 

48 

36 

26 

23 

22 

25 

34 

49 

62 

70 

73 

68 

60 

48 

35 

26 

24 

22 

26 

35 

50 

62 

70 

73 

68 

60 

47 

35 

25 

25 

22 

.26 

35 

50 

62 

70 

73 

68 

59 

47 

35 

25 

26 

22 

'  26 

36 

51 

63 

70 

73 

68 

59 

46 

34 

25 

27 

22 

26 

36 

51 

63 

70 

73 

68 

58 

46 

34 

25 

28 

22 

27 

36 

52 

63 

70 

72 

67 

58 

45 

34 

24 

29 

22 

37 

52 

64 

70 

72 

67 

58 

45 

33 

24 

30 

22 

38 

52 

64 

71 

72 

67 

57 

44 

33 

24 

31 

22 

38 

64 

72 

67 

44 

24 

1  At  the  Albany  station,  Mr.  George  T.  Todd  is  local  forecaster  and  Mr.  Herbert  E. 
Vail  is  assistant.  It  is  to  the  courtesy,  kindly  interest,  and  willing  assistance  of  these 
gentlemen  that  the  data  for  Albany  in  this  book  are  due. 


OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE      79 


daily  temperature,  a  departure  from  normal  of  10°  is  common,  of  20°  is 
unusual,  and  of  30°  is  almost  certainly  record  breaking. 

74.   The  average  monthly  temperature  is  found  by  averaging  the  daily 
temperatures  for  the  various  days  of  the  month.     An  average  Average  and 
monthly  temperature  is  practically  independent  of  the  way  normal 
in  which  the  average  daily  temperatures  from  which  it  is  tempera- 
computed  were  found.     A  normal  monthly  temperature  is  tures- 
found  by  averaging  the  average  monthlies  for  a  sufficient  number  of 

(  \ 

AVERAGE  AND  NORMAL  MONTHLY  AND  YEARLY  TEMPERATURES  AT 
ALBANY,  N.Y. 

(Temp.  F.) 


YEAR 

JAN. 

FEB. 

MAR. 

APR. 

MAY 

JUNE 

JULY 

Atra. 

SEPT. 

OCT. 

Nov. 

DEC. 

AN. 

1874 

28 

22 

30 

36 

56 

66 

70 

66 

63 

49 

36 

26 

46 

1875 

14 

15 

25 

38 

56 

65 

69 

69 

58 

47 

31 

36 

43 

1876 

29 

24 

29 

42 

55 

70 

73 

72 

60 

47 

40 

17 

47 

1877 

16 

29 

30 

46 

56 

67 

72 

72 

64 

49 

41 

32 

48 

1878 

22 

24 

38 

52 

56 

64 

74 

70 

64 

53 

38 

28 

49 

1879 

18 

20 

30 

42 

60 

66 

71 

68 

60 

56 

38 

29 

47 

1880 

30 

28 

33 

50 

66 

71 

75 

71 

65 

52 

39 

25 

50 

1881 

19 

26 

37 

47 

65 

65 

74 

73 

71 

55 

43 

39 

51 

1882 

26 

31 

38 

46 

55 

69 

74 

72 

65 

56 

41 

30 

50 

1883 

21 

28 

29 

46 

58 

71 

73 

74 

61 

51 

44 

30 

49 

1884 

23 

32 

36 

47 

59 

71 

71 

72 

67 

51 

39 

28 

50 

1885 

23 

15 

24 

46 

57 

67 

73 

68 

61 

51 

41 

31 

46 

1886 

21 

23 

33 

51 

59 

66 

72 

71 

64 

53 

40 

24 

48 

1887 

21 

25 

28 

43 

65 

69 

77 

69 

60 

50 

38 

27 

48 

1888 

15 

22 

26 

50 

58 

69 

71 

71 

61 

46 

41 

30 

47 

1889 

31 

20 

37 

44 

62 

68 

72 

70 

64 

49 

43 

35 

50 

1890 

31 

31 

31 

47 

57 

68 

71 

71 

62 

51 

38 

20 

48 

1891 

25 

28 

32 

49 

57 

68 

69 

71 

67 

50 

39 

37 

49 

1892 

24 

26 

30 

46 

56 

71 

73 

72 

62 

51 

38 

26 

48 

1893 

17 

22 

31 

44 

58 

70 

72 

72 

59 

54 

39 

26 

47 

1894 

27 

21 

40 

48 

60 

70 

75 

69 

67 

53 

36 

29 

50 

1895 

23 

19 

30 

47 

62 

73 

70 

72 

67 

47 

41 

32 

48 

1896 

20 

25 

27 

50 

64 

68 

74 

73 

62 

48 

44 

26 

48 

1897 

25 

26 

35 

48 

59 

65 

75 

70 

63 

53 

39 

30 

49 

1898 

24 

28 

42 

46 

58 

70 

75 

73 

67 

53 

39 

29 

50 

1899 

23 

22 

31 

48 

60 

71 

73 

72 

61 

53 

40 

32 

49 

1900 

26 

26 

28 

48 

58 

70 

74 

75 

68 

58 

42 

29 

50 

1901 

24 

19 

33 

50 

59 

70 

76 

73 

65 

52 

34 

27 

48 

1902 

23 

24 

40 

48 

57 

64 

70 

68 

63 

51 

43 

23 

48 

1903 

24 

27 

43 

48 

62 

63 

71 

65 

64 

53 

36 

23 

48 

1904 

15 

17 

31 

44 

63 

69 

72 

69 

61 

49 

35 

20 

45 

1905 

21 

18 

33 

46 

59 

67 

74 

69 

63 

52 

38 

32 

48 

1906 

32 

22 

28 

47 

59 

69 

73 

73 

66 

52 

39 

24 

49 

1907 

22 

17 

37 

43 

53 

66 

73 

69 

64 

48 

39 

32 

47 

1908 

25 

20 

35 

46 

61 

69 

75 

70 

66 

54 

40 

29 

49 

Normal 

23 

24 

33 

46 

59 

68 

73 

71 

64 

51 

39 

29 

48 

80 


METEOROLOGY 


years  to  eliminate  accidental  irregularities.     It  should  also  be  equal  to 
the  average  of  the  normal  daily  temperatures  for  that  month. 

The  average  yearly  temperature  is  found  either  by  averaging  the  aver- 
age monthly  temperatures  for  the  year,  taking  account  of  the  number  of 
days  in  each  month,  or  by  averaging  the  average  daily  tem- 
nonnai          peratures  for  the  year.     A  normal  yearly  temperature  is 
yearly  tem-     found  by  averaging  the  average  yearlies  for  a  sufficient  num- 
ber of  years.     It  should  also  equal  the  average  of  the  normal 
monthlies,  taking  count  of  the  number  of  days  in  each  month,  or  the  aver- 


/  J 
70 
65 
60 
55 
50 
45 
40 
35 
30 
25 
20 
15 
10 
5 

n 

\ 

7** 

/ 

\ 

/ 

i 

\ 

\ 

i 

\ 

\. 

/ 

1 

\ 

/ 

NORM 

AL  YEA 

LY 

\ 

/ 

\ 

A 

P 

•-NORM/ 
ORMAL 

L  MON1 
DAILY 

HLY 

V 

/ 

\ 

\ 

/ 

\ 

\ 

/ 

/ 

/ 

\ 

\ 

^z 

7 

JAN.         FEB.       MAR.      APRIL        MAY       JUNE       JULY       AUG.       SEPT.     OCT.       NOV.       DEC. 

FIG.  36.  —  Graphical  Representation  of  the  Station  Normals  of  Temperature 
at  Albany,  N.Y. 


age  of  the  normal  dailies  for  all  the  days  in  the  year.  The  table  on 
page  79  gives  the  average  monthly  temperatures  and  the  average  yearly 
temperatures  for  several  years,  and  also  the  normal  monthly  and  yearly 


OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE      81 


temperatures  for  Albany,  N. Y.  It  will  be  seen  in  the  case  of  the  average 
monthly  temperatures  that  a  departure  from  normal  of  3°  is  common,  of 
5°  is  unusual  and  of  7°  is  almost  certainly  record  breaking  The  de- 
partures in  winter  are  usually  larger  than  those  in  summer.  In  the  case 
of  an  average  yearly  temperature,  a  departure  from  normal  of  0.5°  is 
common,  of  1.5°  is  unusual,  and  2.5°  almost  certainly  record  breaking. 
In  the  table  on  pages  82  and  83  are  given  the  normal  monthly  and  annual 
temperatures  for  20  cities  in  the  United  States  and  twenty-five  for- 
eign places  together  with  certain  facts  in  connection  with  their  location. 


DEC. 
MID. 


3    I 


6  i         9  NOON 

SUNhlSE  SUNSET 

FIG.  37.  —  Thermo-isopleths,  Centigrade,  at  Berlin,  Germany.     (After  BORNSTEIN.) 


The  data  for  the  United  States  have  been  taken  from  the  publications 
of  the  United  States  Weather  Bureau  while  the  data  for  the  foreign 
places  have  been  derived  from  HANN'S  Lehrbuch  der  Meteor ologie. 

75.  The  normal  daily,  monthly,  and  yearly  temperatures  are  often 
spoken  of  as  station  normals  of  temperature,  and  these  may  Graphical 
be  expressed  graphically,  as  is  shown  by  Fig.  36.  representa- 

76.  If  the  normal  hourly  temperatures   for  the  various  Jon  normals 
hours  of  each  day  in  the  year  are  known,  these  may  also  be  of  tempera- 
expressed  graph'     lly,  jjs  is  shown  for  the  city  of  Berlin,  Ger- 
many, in  Fig.  37.     Months  are  here  plotted  along  the  F-axis  and  hours 


82 


METEOROLOGY 


5 

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OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE     83 


C^J  i-H  CO 

CO  CO  CO  CO  CO 


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IO  i-H  CO  i-H  FH 

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84 


METEOROLOGY 


Graphical 
representa- 
tion of  nor- 
mal hourly 
tempera- 
tures. 


of  the  day  along  the  X-axis.  Lines  joining  points  having  the  same  values 
of  .temperature  have  also  been  drawn,  and  these  are  ordinarily  known  in 
Europe  as  thermo-isopleths.  They  are  also  sometimes  called 
chronoisotherms.  This  method  of  graphical  representation 
was  first  introduced  by  Lalanne  about  1843.  In  order  to  note 
the  relation  between  the  time  of  minimum  temperature  and 
the  time  of  sunrise,  dotted  curves  representing  the  time  of 
sunrise  and  sunset  have  been  added.  From  this  figure  the  normal  tem- 
perature at  any  hour  of  any  day  may  at  once  be  found,  also  the  time  of 
the  minimum  temperature  each  day,  the  time  of  the  maximum  tempera- 
ture, and  the  normal  value  of  daily  range.  Similar  charts  for  other  cities 
will  be  found  in  various  books  and  publications  •  for  Greenwich  in  SCOTT, 
Elementary  Meteorology,  p.  48 ;  for  Miinchen  in  ARRHENIUS,  Cosmische 
Physik,  p.  556 ;  for  Aachen  in  Meteor ologische  Zeitschrift,  April,  1904, 
p.  179 ;  for  Baltimore  in  FASSIG,  The  Climate  and  Weather  of  Balti- 
more, p.  62 ;  for  Chicago  in  WILLIS  L.  MOORE,  Descriptive  Meteorology, 
page  183. 

77.  Diurnal,  annual,  and  irregular  variation. — The  graph  which  repre- 
sents the  daily  variation  in  temperature  is  found  by  plotting  to  scale  the 
The  daily  normal'  hourly  temperatures.  If  the  values  of  the  normal 
variation  in  hourly  temperatures  are  not  known,  an  idea  of  the  form 
temperature.  ^  ^  curve  mav  j^  obtained  by  noting  the  variation 
on  some  typical  day,  that  is,  some  day  when  the  other  meteoro- 


FIG.  38.  —  Thermograph  Record  showing  Typical  Daily  Variation  of  Temperature 
at  Albany,  N.Y.,  October  14-18,  1908.     (U.  S.  Weather  Bureau.) 

logical  elements  have  remained  constant  or  followed  as  nearly 
normal  courses  as  possible.  Figure  38,  which  is  a  copy  of  the  ther- 
mograph record  at  Albany,  N.Y.,  for  the  five  days  ending  October  18, 


OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE      85 

1908,  slows  a  series  of  very  typical  daily  variations.  Figure  4  also 
shows  a  typical  daily  variation  in  temperature  at  Williamstown,  Mass. 
The  lowt  st  temperature  usually  comes  at  about  the  time  of  sunrise,  and 
the  highest  temperature  from  2.00  to  4.30  P.M.,  depending  upon  the 
season  of  the  year.  The  maximum  occurs  early  in  winter  and  later  in 
summer.  The  average  temperature  for  a  day  occurs  at  about  9  A.M. 
and  8  P.M  the  rise  during  the  morning  and  early  afternoon  is  sharp,  the 
curve  being  convex ;  the  drop  during  the  afternoon  and  night  is  long  and 
slow,  giving  a  concave  curve.  This  curve,  which  represents  the  daily 
variation,  varies  slightly  with  the  time  of  the  year,  the  elevation,  and  the 
immediate  surroundings.  It  varies  markedly  with  latitude  and  with 
nearness  to  the  ocean.  The  fact  that  the  highest  temperature  does 
not  occur  at  noon  when  receipts  of  energy  from  the  sun  are  largest,  but 
several  hours  later,  needs  explanation.  In  Fig.  39  let  A  represent  the 
insolation  received  from  the  sun  on  a  horizontal  surface  dur-  Why  the 
ing  some  day  in  September.  The  curve,  which  represents  highest  tem- 
the  receipts,  begins  at  about  6  A.M.,  rises  rapidly  to  a  maxi-  JoeVnot 
mum  at  noon,  and  drops  down  again  to  nothing  at  6  P.M.  occur  at 
This  represents  the  amount  of  energy  received  by  a  hori- 
zontal surface  provided  the  earth  had  no  atmosphere^  or  provided  the 
atmosphere  transmitted  all  of 
the  insolation  which  falls  upon 
it.  No  question  is  here  raised 
as  to  where  the  insolation  is 
absorbed;  whether  in  the  at- 
mosphere, at  the  surface  of  MID.  2  4  6  8  10  NOON  2 4  e  s^Vo  MID. 

the    ground,    Or    in    a    body  of    FlG-  39.  — Diagram    Illustrating  the    Energy   re- 
......  »   ,,        ceived  and  given  off  by  the  Earth  during  a  Day. 

water  upon  which  it  may  fall. 

The  amount  of  energy  given  off  by  the  earth  to  space  depends 'upon 
the  temperature  of  the  earth ;  thus  the  largest  amount  would  be  given 
off  at  the  time  of  highest  temperature  and  the  least  amount  at  the 
time  of  lowest  temperature.  The  graph  which  represents  the  energy 
given  off  is  indicated  by  B.  During  the  early  hours  of  the  afternoon, 
although  receipts  have  already  commenced  to  fall  off,  it  will  be  seen 
that  they  are  still  slightly  in  excess  of  expenditures,  that  is,  a  rise  in 
temperature  is  still  continuing  and  will  continue  until  the  receipts  and 
expenditures  become  equal,  which  takes  place  at  the  time  of  highest 
temperature. 

78.   The  curve  which  represents  the  annual  variation  in  temperature 
is  found  by  plotting  the  normal  daily  temperatures  to  scale,  and  this  is 


86 


METEOROLOGY 


pictured  for  Albany,  N.Y.,  in  Fig.  36.  The  time  of  lowest  temperature 
is  during  the  last  part  of  January,  and  the  time  of  highest  temperature  is 
The  annual  during  the  last  part  of  July,  in  each  case  nearly  forty  days 
variation  in  after  the  time  of  least  and  greatest  receipts  of  energy  from 
ure"  the  sun.  The  rise  from  February  until  July  is  slow  and  regular, 
and  the  fall  from  the  last  of  July  until  the  first  of  February  is  of  the  same 
nature.  The  reason  that  the  highest  and  lowest  temperatures  do  not 
come  at  the  time  of  greatest  or  least  receipts  of  energy  is  the  same  as  that 

given  for  the  time  of  occurrence 
of  the  maximum  temperature 
during  the  day.  The  annual 
variation  in  temperature  varies 
slightly  with  elevation  and  with 
the  immediate  surroundings  of  the 
station,  and  markedly  with  near- 
ness to  the  ocean  and  with  latitude. 
Figure  40  gives  the  annual  varia- 
tion at  five  different  places  on  the 
earth's  surface.  The  contrast  be- 
tween these  five  curves  shows  we11 
the  effect  of  latitude.  At  St.  Ann 
Trinidad,  which  has  a  tropic* 
location,  there  are  two  maxim 
and  two  minima  and  an  extremely 
small  change  during  the  year. 
With  increasing  latitude  there  is 
but  one  maximum  and  the  change 
during  the  year  increases  steadily. 
Werchojansk  with  its  high  latitude 
and  continental  location  has  an 

immense  change  in  temperature  during  the  year.  The  graphs  repre- 
senting the  annual  variation  in  temperature  can  be  constructed  for 
many  other  places  by  plotting  the  data  given  in  section  74. 

The  irregular  changes  of  temperature  are  sometimes  greater  than  the 

daily  variation  ;  that  is,  the  temperature  may  fall  steadily  during  the  day 

t  instead  of  rising,  or  it  may  rise  during  the  night  instead  of 

fluctuations    falling.     The  irregular  variations,  however,  are  never  larger 

in  tempera-    than  the  annual  variation. 

ture. 

79.    Temperature  data.  —  From  the  temperature  observa- 
tions which  have  been  made  at  the  different  weather  bureau  stations, 


FIG.  40.  —  Annual  Variation  in  Temperature 
at  five  different  Places. —  (1)  St.  Anns, 
Trinidad;  (2)  Palermo;  (3)  Berlin;  (4)  St. 
Petersburg;  (5)  Werchojansk,  Siberia. 


OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE     87 


various  temperature  data  in  addition  to  the  normals  already 
cussed  may  be  computed.  Among  these  are  the  following : 
(1)  Average  and  normal  daily  range  for  the  various  months  and 
for  the  year.  The  daily  range  has  been  denned  as  the  differ- 
ence between  the  highest  and  lowest  temperatures  during 
the  day. ,  This  range  may  be  found,  and  normals  may  be 
computed,  in  exactly  the  same  way  as  for  observations  of 
temperature.  The  accompanying  table  gives  the  average 


fully  dis- 

Tempera- 
ture  data 
which  may 
be  com- 
puted from 
the  observa- 
tions taken 
at  Weather 
Bureau 
stations. 


AVERAGE  AND  NORMAL  VALUES  OF  DAILY  RANGE  OF  TEMPERATURES 
AT  ALBANY,  N.Y. 

(Temp.  F.) 


YEAR 

JAN. 

FEB. 

MAR. 

APR. 

MAY 

JUNE 

JULY 

AUG. 

SEPT. 

OCT. 

Nov. 

DEC. 

AN- 
NUAL 

1874 

1875 
1876 

1877 
1878 

20.3 

17.7 
18.5 
174 

17.8 
19.5 
17.1 
15.1 

17.4 
17.7 
17.5 
14.8 

15.2 
16.8 
18.5 

18.7 

21.7 
22.4 

20.8 
19.8 

18.9 
22.5 
19.2 
17.4 

18.4 
20.7 
20.1 
18.1 

20.0 

18.5 
23.3 

18.7 

19.7 
20.6 
15.6 
19.1 

16.8 
17.3 
15.5 
15.1 

15.3 
16.1 
11.5 
14.2 

16.5 
15.3 
15.2 
14.6 

18.2 
18.8 
17.7 
169 

173 

17.9 

17.2 

14.2 

17.8 

18.9 

18.1 

17.6 

17.8 

18.4 

11.2 

12.5 

165 

1879 

20.2 

17.4 

16.2 

16.7 

21.5 

17.7 

18.4 

16.8 

16.2 

17.7 

14.6 

17.7 

17.6 

1880 

16.6 

18.6 

14.6 

18.3 

18.0 

17.2 

15.8 

16.8 

14.8 

15.7 

13.0 

10.8 

15.8 

1881 

16.5 

14.1 

11.1 

14.4 

16.6 

15.7 

15.0 

15.5 

14.3 

15.8 

12.3 

12.5 

14.5 

1882 

15.5 

16.4 

13.8 

16.4 

15.9 

16.8 

16.0 

16.7 

13.8 

15.4 

12.7 

10.8 

15.0 

1883 

13.7 

14.3 

15.5 

15.8 

17.1 

16.5 

16.9 

17.5 

17.0 

15.0 

13.4 

13.8 

16.4 

1884 

16.3 

13.3 

11.5 

13.9 

16.1 

20.0 

15.6 

16.9 

16.5 

17.8 

14.9 

14.7 

15.6 

1885 

1G.7 

20.3 

17.4 

22.2 

21.1 

21.5 

20.9 

17.3 

21.1 

17.4 

12.2 

14.5 

18.6 

1886 

15.2 

17.6 

14.7 

20.1 

20.6 

18.8 

21.3 

22.1 

20.1 

19.3 

16.9 

16.5 

18.6 

1887 

21.5 

17.3 

16.7 

18.6 

22.2 

19.9 

17.8 

18.5 

18.5 

16.4 

16.0 

12.4 

18.0 

1888 

15.9 

19.6 

16.3 

20.0 

18.3 

20.4 

22.3 

19.5 

16.8 

14.5 

14.3 

14.0 

17.6 

1889 

13.2 

13.9 

14.6 

18.9 

21.6 

18.1 

16.8 

20.9 

16.8 

18.3 

14.3 

14.9 

17.0 

1890 

14.9 

15.2 

14.9 

21.8 

19.4 

20.9 

20.7 

17.1 

15.7 

18.1 

14.1 

14.1 

16.8 

1891 

14.5 

14.0 

16.0 

18.5 

22.1 

20.7 

18.0 

17.2 

17.7 

16.4 

15.1 

14.7 

17.1 

1892 

13.9 

14.4 

13.8 

18.4 

15.6 

17.9 

20.9 

17.5 

18.5 

16.6 

11.1 

10.9 

15.8 

1893 

13.6 

15.2 

14.3 

17.5 

18.3 

19.4 

21.6 

20.0 

17.3 

17.6 

15.0 

13.9 

17.0 

1894 

14.5 

16.5 

16.7 

17.4 

19.1 

19.4 

20.9 

19.8 

18.6 

15.3 

12.4 

13.2 

17.0 

1895 

15.9 

19.0 

14.4 

17.5 

20.9 

20.1 

18.9 

19.7 

21.5 

17.5 

15.2 

16.8 

17.8 

1896 

13.5 

15.7 

16.2 

20.8 

20.9 

20.0 

19.2 

19.6 

19.5 

15.2 

14.7 

14.4 

17.5 

1897 

14.4 

15.8 

16.4 

19.9 

19.2 

19.6 

17.2 

19.4 

21.5 

22.9 

13.8 

12.4 

17.7 

1898 

15.5 

14.6 

17.5 

16.6 

15.9 

19.2 

20.7 

2(^.7 

20.2 

15.7 

14.2 

14.9 

17.0 

1899 

17.2 

14.4 

12.9 

19.6 

20.6 

21.7 

19.3 

20.3 

19.5 

16.5 

13.1 

13.0 

17.3 

1900 

17.7 

16.2 

17.4 

19.1 

24.3 

21.7 

22.0 

20.5 

19.5 

18.0 

13.0 

14.5 

18.7 

1901 

15.4 

13.9 

14.9 

16.2 

16.5 

20.2 

19.7 

16.9 

18.8 

20.0 

13.4 

15.6 

16.8 

1902 

15.1 

14.0 

15.5 

17.1 

19.7 

18.6 

17.2 

19.7 

18.1 

17.0 

16.5 

18.1 

17.2 

1903 

15.0 

15.8 

16.7 

19.4 

24.6 

15.5 

19.1 

17.1 

20.7 

16.5 

15.0 

15.8 

17.6 

1904 

17.8 

18.3 

14.7 

17.6 

21.2 

19.8 

18.9 

19.3 

18.0 

17.5 

14.2 

15.6 

17.7 

1905 

13.6 

17.8 

18.7 

19.1 

19.9 

19.7 

18.8 

19.6 

18.5 

19.9 

17.2 

14.4 

18.1 

1906  14.9 

19.9 

14.0 

19.7 

21.4 

20.4 

19.2 

20.0 

21.1 

18.4 

13.5 

15.0 

18.1 

1907 

17.0 

18.0 

17.2 

17.1 

17.9 

20.1 

20.1 

21.0 

15.0 

18.9 

13.1 

12.2 

17.3 

1908 

17.6 

16.7 

16.2 

18.8 

18.6 

22.2 

20.6 

19.6 

22.4 

21.5 

13.8 

14.5 

18.5 

Sums 

564.5 

575.6 

545.4 

630.8 

687.6 

676.6 

665.2 

661.6 

640.8 

600.9 

491.3 

500.7 

603.8 

Normal 

16.1 

16.4 

15.6 

18.0 

19.6 

19.3 

19.0 

18.9 

18.3 

17.2 

14.0 

14.3 

,17.3 

88  METEOROLOGY 

value  of  the  daily  range  for  the  various  months,  and  for  the  year  for 
several  years,  and  also  the  normal  values  for  Albany,  N.Y.  It  will 
be  seen  that  the  range  is  greater  in  summer  and  smaller  in  winter,  with 
a  maximum  in  May  and  a  minimum  in  November. 

(2)  Monthly  extremes  of  temperature.  By  monthly  extremes  of  tem- 
perature are  meant  the  highest  and  lowest  temperatures  which  have 
been  observed  during  the  month  in  question.  (3)  Yearly  extremes. 

(4)  Absolute  highest  and  absolute  lowest  for  the  various  months  and  for  the 
year.     By  absolute  highest  and  absolute  lowest  are  meant  the  very 
highest  and  very  lowest  temperatures  which  have  ever  been  observed. 

(5)  Variability.     By  variability  of  temperature  is  meant  the  difference 
between  successive  daily  averages.     The  average  value  of  the  varia- 
bility for  the  various  months  and  the  year  may  be  determined,  and 
also  normal  values.     The  accompanying  table  gives  the  normal  values 
for  the  various  months  and  the  year  for  several  stations  in  the  United 
States.     The  average  variability  for  the  various  months  and  for  the 
year  and  the   normal  values  are   also  given  for  Albany,  N.Y.     The 
maximum  of  variability  occurs  in  January  and  the  minimum  in  August, 
one  being  more  than  twice  the  other.     (6)  Freezing  days.     By  a  freezing 
day  is  meant  a  day  on  which  the  temperature  falls  to  32°  F.  or  below 
at  some  time  during  the  day.     (7)  Ice  days.     By  an  ice  day  is  meant 
a  day  on  which  the  temperature  remains  below  32°  throughout  the 
whole  day.     (8)  Days  above  90°.    (9)  Days  above  100°.     (10)  Zero  days. 
By  a  zero  day  is  meant  a  day  when  the  temperature  falls  to  zero  or 
below  at  some  time  during  the  day.     (11)   Temperature  on  special  days. 
The  various  normals  of  temperature  for  such  special  days  as  July  4, 
December  25,  etc.,  may  be  computed.     The  table  on  page  91   gives 
for  Albany,  N.Y.,  the  number  of  days  above  90°,  the  number  of  days 
above  100°,  and  the  number  of  zero  days  for  a  number  of  years. 

At  the  regular  stations  of  the  U.  S.  Weather  Bureau  the  following  tem- 
perature records  are  kept  constantly  filled  in  and  computed  to  date : 
monthly  mean  and  departure  from  the  normal ;  monthly  mean  maximum 
and  minimum  (to  tenths) ;  absolute  maximum  and  date  (each  month) ; 
absolute  minimum  and  date  (each  month) ;  greatest  daily  range,  mean 
daily  range ;  absolute  monthly  range,  mean  variability  (var.  to  tenths) ; 
lowest  maximum,  highest  minimum ;  number  of  days  with  maximum  32° 
or  below,  90°  above ;  number  of  days  with  minimum  32°  or  below, 
zero  or  below ;  daily  mean  temperature  (whole  degrees) ;  daily 
maximum  temperature  (whole  degrees) ;  daily  minimum  temperature 
(whole  degrees) ;  mean  hourly  temperature  (to  tenths). 


OBSERVATION  AND   DISTRIBUTION  OF   TEMPERATURE      89 


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90 


METEOROLOGY 


AVERAGE  AND  NORMAL  VALUES  OF  VARIABILITY  OF  TEMPERATURE 
AT  ALBANY,  N.Y. 

(Temp.  F.) 


YEAR 

JAN. 

FEB. 

MAR. 

APR. 

MAY 

JUNE 

JULY 

AUG. 

SEPT. 

OCT. 

Nov. 

DEC. 

AN- 
NUAL 

1874 
1875 
1876 

1877 
1878 
1879 
1880 
1881 
1882 
1883 
1884 
1885 
1886 
1887 
1888 
1889 
1890 
1891 
1892 
1893 
1894 
1895 
1896 
1897 
1898 
1899 
1900 
1901 
1902 
1903 
1904 
1905 
1906 
1907 
1908 

6.6 
6.8 
7.1 
7.4 
7.6 
8.6 

6.2 
8.2 
6.5 
5.5 
6.3 
7.0 

7.5 

5.8 
6.6 
5.2 
5.9 
5.0 

5.2 
3.9 
3.1 
3.9 

2.8 
4.2 

4.7 
3.5 
5.3 
3.5 
4.1 
5.1 

5.3 

4.5 

3.1 
4.8 
3.1 
4.2 

3.6 
3.5 
3.5 
2.7 
3.1 
3.5 

3.4 
3.6 
3.7 
1.7 
2.4 
3.9 

4.0 
4.7 
4.4 
4.0 
4.9 
3.9 

3.0 
4.6 
4.9 
3.4 

4.8 
5.2 

5.0 
5.3 
3.8 
5.4 
3.6 
7.3 

9.3 

6.8 
6.8 
4.9 

4.5 
7.8 

5.3 
5.0 
4.9 

44 
44 
5  5 

7.2 

8.1 

6.5 

5.6 

5.3 

3.3 

3.1 

4.2 

3.5 

4.5 

4.4 

5.2 

5.1 

6.6 

6.9 

3.3 

3.5 

5.0 

3.3 

2.9 

2.6 

3.8 

7.1 

5.6 

6.3 
5.3 
6.5 
8.1 
6.5 
6.2 
4.6 

4.7 
4.5 
4.8 
5.0 
5.4 
4.7 
4.7 

7.2 
6.5 

7.7 
8.4 
6.8 
8.5 

7.5 
6.7 
7.0 

8.4 
7.0 
6.3 

5.2 
6.8 
4.3 
5.3 
5.2 
6.1 

4.1 
3.6 
3.2 
4.7 
4.2 
5.0 

4.5 
4.5 
4.3 
3.9 
4.0 
3.7 

]  3.3 
3.3 
3.5 
4.9 
3.3 
3.6 

2.5 
3.5 
2.7 

4.2 
2.9 

2.8 

2.8 
2.3 
3.1 
4.2 
2.6 
2.6 

3.8 
4.3 
4.9 
4.5 
4.3 
4.5 

3.8 
4.5 
6.5 
5.4 
4.5 
4.5 

4.8 
5.3 
5.1 
4.0 

i  5.8 
4.6 

7.5 

5.7 

8.1 

8.1 

6.5 
4.4 

4.6 
4.2 

4.2 

4.8 

4.0 
3.6 

3.3 
4.1 

3.7 

2.9 

4.3 

&  <& 

3.8 
3.5 

5.3 
3.7 

6.9 
7.0 

5.2 
4.6 

9.3 
7.0 
10.5 

5.8 
6.4 

7.8 
7.3 
7.0 

8.2 
8.5 

6.2 

5.8 
3.9 
6.8 
5.5 

5.0 
5.6 
4.3 
5.5 
3.2 

4.5 
5.4 
4.1 
4.6 
4.5 

4.6 
4.9 
4.2 
4.7 

3.8 

4.4 
2.8 
4.2 
3.5 
4.0 

3.5 
3.7 
2.6 
3.5 
4.2 

4.8 
4.7 
3.5 
3.5 
4.9 

2  7 
4.5 
4.5 
5.0 
3.3 

5.6 
6.4 
4.7 

4.8 
5.6 

8.6 
6.0 
4.5 
9.6 
5.6 

5.6 
5.3 

4.8 
5.5 
5.0 

7.5 
6.7 

6.4 

7.6 

4.5 

6.8 

5.1 
4.5 

4.9 
4.7 

3.2 
3.7 

3.0 
3.4 

3.0 
2.5 

7.0 
4.6 

4.6 
3.8 

5.8 
7.4 

6.0 
6.0 

5.1 
5.1 

7.4 
6.4 
7.1 
7.1 
7.4 
7.5 
6.8 
8.7 
7.4 
6.7 
7.4 
7.3 

6.8 
5.2 
4.7 
6.4 
5.1 
4.8 
4.9 
7.2 
6.8 
8.6 
9.1 
7.3 

4.6 
3.9 
4.6 
7.1 
6.3 
5.6 
5.7 
4.8 
4.3 
5.0 
6.2 
5.9 

6.4 
4.4 
3.6 
4.0 
2.9 
3.8 
3.9 
4.6 
4.5 
3.3 
4.4 
5.2 

4.4 
3.4 
3.9 
6.6 
3.7 
5.2 
3.9 
3.5 
4.6 
5.5 
5.7 
4.1 

4.6 
3.4 
5.0 
3.7 
3.3 
3.9 
3.2 
4.4 
3.7 
3.7 
3.4 
4.7 

3.1 
3.8 
4.3 
4.1 
3.4 
3.7 
4.0 
3.7 
3.7 
1.9 
3.7 
3.5 

2.8 
3.5 
2.9 
4.1 
2.6 
2.3 
2.6 
3.7 
3.4 
3.6 
3.9 
3.7 

5.0 
4.4 
5.5 
4.6 
5.0 
3.8 
4.5 
5.7 
3.9 
5.9 
4.9 
4.2 

5.5 
4.8 
5.2 
4.8 
4.4 
5.2 
3.8 
5.7 
4.5 
5.4 
5.3 
5.3 

6.3 
4.0 
3.4 
4.3 
3.3 
5.7 
4.0 
5.1 
6.0 
4.4 
3.3 
4.0 

6.1 
6.0 
5.8 
5.2 
8.0 
7.9 
5.5 
7.4 
6.2 
8.7 
4.9 
6.4 

5.2 
44 
4.7 
5.2 
4.6 
5.0 
14 
5.4 
4.9 
5.2 
5.2 
5.1 

Sums 
Normal 

256.6 
7.3 

243.5 
7.0 

193.1 
5.5 

150.0 
4.3 

157.6 
4.5 

137.2 
3.9 

120.1 
3.4 

110.8 

3.2 

157.5 
4.5 

162.3 
4.6 

173.1 
4.9 

227.1 
6.5 

173.9 
5.0 

80.  It  has  often  been  found  desirable  to  compute  a  normal  for  a  sta- 
The  compu-  ^^on  a^  which  observations  have  been  taken  for  but  a  few 
tation  of  a  years,  and  the  best  method  of  procedure  is  the  following : 
from1  insuffi-  Choose  some  near-by  station  which  has  a  well-determined 
dent  obser-  normal  and  at  which  observations  have  been  made  during  the 
same  interval.  Assume  that  the  normal  at  the  station  in 
question  will  bear  the  same  relation  to  the  normal  at  the  chosen  station 


OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE      91 


THE  NUMBER  OF  ZERO   DAYS,  DAYS  ABOVE   90°,  AND   DAYS  ABOVE   100°, 

FOR  ALBANY,  N.Y. 


1 

H 
PQ 

§ 
§ 

i 

H 
> 
O 

« 
O 

8 

H 

1 

§ 
| 

S 

w 

PQ 

§ 
o 
1 

CS3 

• 

> 

fl 

M 

0 

1 

H 
1 

<3 

« 
o 

I 

1 

QQ 

§ 
O 

« 
• 

tsa 

5 

§ 

<3 
§ 
1 

I 

§ 

1874  

13 

2 

1886  

9 

8 

1898  

7 

14 

1 

1875  

34 

2 

1887  

8 

13 

1899  

10 

15 

1876  

12 

16 

1888  

16 

8 

1900  

6 

28 

1877  

7 

3 

1889  

1 

1 

1901  

6 

17 

1878  

11 

5 

1890  

5 

7 

1902  

6 

3 

1879  

15 

3 

1891  

3 

9 

1903  

11 

6 

1880  

4 

11 

1892  

6 

15 

1904  

22 

6 



1881  

8 

8 

1893  

7 

12 

1905  

7 

7 



1882  

3 

9 

1894  

7 

16 

1906....:. 

12 

5 

1883 

3 

9 

1895 

10 

1^ 

1907 

16 

8 

1884  

7 

5 

1896  

12 

19 

1908  

8 

9 

1885  

18 

5 

1897  

4 

7 

1909...... 

5 

8 





1910  

11 

5 

&s  the  average  for  the  few  years  at  the  station  in  question  bears  to  the 
average  for  the  same  period  at  the  chosen  station.  The  normal  deter- 
mined in  this  way  is  usually  much  more  reliable  and  correct  than  one 
determined  from  too  short  a  period  of  observations.1 

81.    Differences  of  Temperature  with  Altitude.  —  The  thermometer 
shelters  at  which  the  various  temperature  observations  have  been  made 
all  have  definite  locations  ;  in  a  city  usually  on  the  roof  of  a  The  differ_ 
high  building,  in  the  country  usually  a  few  feet  above  the  sod  ences  in 
or  on  the  north  side  of  some  building.     The  natural  question  j^a^wj?" 
at  once  arises  as  to  whether  the  observations  would  have  been  mometer 
different  if  the  elevation  of  the  shelter  above  the  ground  had  piadTat 
been  different.     In  other  words,  what  are  the  temperature  different 
differences  in  the  small  height  of,  say,  100  feet  above  the  alti 
earth's  surface.     In   the   layer  of   air  within   five  feet  of   the   earth's 
surface  marked  differences    in    temperature  will    be    found.     During 
the  day,  when  convection  is    operative  and  when  wind  velocities  are 
large,  the  difference  will  be  a  comparatively  small  one,  not  more  than 
a  degree  at  most.     The  air  in  immediate  contact  with  the  ground  is,  of 
course,  the  warmer.     At  night  the  temperature  differences  are  more 

i  Monthly  Weather  Review,  April,  1910. 


92  METEOROLOGY 

marked  and  will  perhaps  average  as  high  as  2  or  3°  Fahrenheit,  with 
maximum  values  of  even  5  or  6°.  The  layer  of  air  in  immediate  con- 
tact with  the  ground  is,  of  course,  the  colder.  The  layer  of  air  from  five 
to  one  hundred  feet  is  so  thoroughly  mixed  by  the  wind  at  night,  as  well 
as  during  the  day,  that  a  very  small  temperature  difference  will  be  found, 
probably  not  more  than  a  degree  at  most,  unless  the  air  is  held  by  natural 
or  artificial  barriers.  Above  100  feet  the  regular  vertical  temperature 
gradient  may  be  expected.  As  a  general  conclusion,  then,  a  thermom- 
eter shelter  should  not  be  placed  within  five  feet  of  the  ground  nor  in  a 
location  where  the  air  would  be  held  stagnant  by  means  of  artificial  or 
natural  barriers.  The  temperatures  observed  at  heights  of  from  five  to 
one  hundred  feet  will  probably  be  nearly  the  same. 

82.  Temperature  differences  over  a  limited  area.  —  The  question  here 
arises  as  to  whether  the  observations  of  temperature  would  be  different 
if  the  thermometer  shelter  were  placed  at  different  points 
ture  differ-  within  a  small  area  immediately  surrounding  the  point  in 
{lues^^on-  ^  smau<  or  limited  area  may  be  roughly  defined 
as  a  square  mile  of  surface  in  the  form  of  a  circle  or  square. 
During  the  daytime,  on  account  of  convection  and  the  higher  values 
of  wind  velocity,  no  appreciable  difference  in  temperature  over  such 
a  limited  area  will  be  found  unless  it  is  of  unusual  topography  or 
the  air  is  held  stagnant  by  natural  or  artificial  barriers.  On  some  par- 
ticularly favorable  days,  namely,  those  with  plenty  of  sunshine  and  a  low 
wind  velocity,  the  lower  points,  particularly  those  in  narrow  valleys,  may 
be  a  few  tenths  of  a  degree  Fahrenheit  warmer  than  the  upper  parts  of 
the  area.  At  night  the  layer  of  air  next  to  the  ground  grows  cold  and 
denser,  and  drains  like  water  into  the  valleys  and  places  of  small  eleva- 
tion. If  the  wind  is  unable  to  remove  these  pockets  of  cold  air,  a 
marked  variation  in  temperature  over  a  limited  area  will  be  found. 
For  every  limited  area  there  will  be  a  critical  value  of  wind  velocity, 
which  for  most  areas  is  probably  not  far  from  three  miles  per  hour.  As 
long  as  the  wind  velocity  remains  larger  than  three  miles  these  pockets 
of  air  will  be  removed  and  mixed  with  the  air  at  other  points,  and  no 
variation  in  temperature  will  be  found.  As  soon  as  the  wind  velocity 
sinks  below  this  critical  value,  a  variation  will  begin  to  be  manifest,  and  it 
is  the  valley  station  and  those  of  low  elevation  which  are  ordinarily  the 
coldest.  Since  the  question  of  the  variation  in  temperature  depends 
upon  the  interplay  between  the  drainage  of  colder  air  and  the  ability  of 
the  wind  to  remove  those  pockets  of  cold  air,  the  variation  will  depend 
not  only  upon  the  elevation,  but  also  upon  the  openness  of  the  valleys, 


OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE      93 

their  direction,  the  roughness  of  the  surface,  and  the  direction  from 
which  the  wind  comes.  The  average  difference  of  temperature  between 
the  warmest  and  coldest  places  in  the  limited  area  will  probably  average 
about  4°  throughout  the  year,  and  will  show  at  times  differences  as 
much  as  from  10°  to  15°.  The  location  of  the  shelter  in  a  limited  area 
will  thus  make  a  great  difference  with  the  temperature  observations  ;  and 
if  records  are  to  be  of  value,  the  limited  area  surrounding  each  station 
should  be  critically  investigated  for  several  years. 

The  chief  factor  which  determines  the  variation  in  temperature  over  a 
limited  area,  particularly  at  night,  is  without  doubt  the  interplay  between 
the  drainage  of  colder  air  into  the  valleys,  and  the  ability  of  the  wind  to 
remove  these  pockets  of  cold  air.  A  second  factor  is  the  nature  of  the 
surface.  That  this  plays  an  important  part  has  been  well  shown  by 
the  research  work  of  Professor  Henry  J.  Cox,  of  the  U.  S.  Weather 
Bureau  and  others  in  connection  with  the  minimum  temperatures  and 
frosts  observed  over  the  cranberry  marshes  of  Wisconsin.1  It  has  been 
found  that  cultivation,  drainage,  and  sanding  are  very  efficacious  in  pre- 
venting destructive  frosts.  The  minimum  temperatures  observed  have 
sometimes  been  10°  F.  and  even  more,  higher  than  over  near-by  un- 
treated marshes.  Here  there  is  essentially  no  difference  in  elevation  and 
the  wind  velocity  can  be  assumed  to  have  been  the  same.  The  differences 
in  temperature  were  brought  about  entirely  by  the  nature  of  the  surface. 
In  causing  temperature  differences  the  surface  acts  in  two  ways.  Differ- 
ent surfaces  heat  unequally  during  the  day  and  at  night  they  cool  off  at  / 
very  different  rates.  Since  on  frosty  nights  the  air  is  always  particularly 
quiet,  its  tempeiature  is  determined  almost  entirely  by  the  temperature 
of  the  surface  upon  which  it  rests. 

THE  DISTRIBUTION  OF  TEMPERATURE  OVER  THE  EARTH     * 

83.  Construction  of  Isothermal  Charts.  —  Observations  of  temperature 
have  been  made  at  many  stations  in  all  parts  of  the  world,  and  some  of  the 
records  are  long  ones.  From  these  observations  the  normal 


monthly  and  annual  temperatures  may  be  computed.     The  tion  of  the 
stations  are,  however,  at  different  elevations  above  mean  sea  mah>0for°tiie 
level,  and  in  order  to  make  the  observations  comparable  with  land  sta- 
one  another  it  is  necessary  to  reduce  them  all  to  mean  sea  level.  t 
Since  the  average  vertical  temperature  gradient  is  1°  F.  for  300  feet,  the 
reduction  could  well  be  made  by  using  this  factor  and  adding  1°  F.  for 
1  See  Bulletin  T,  U.  S.  Weather  Bureau. 


94  METEOROLOGY 

each  300  feet  of  elevation.  As  a  matter  of  fact  the  reduction  factors 
used  by  different  investigators  in  preparing  charts  are  very  different. 
Buchan  used  1°  F.,  for  270  feet,  while  others  have  used  values  all  the 
way  to  1°  F.  for  500  feet. 

The  normal  temperatures  of  the  air  over  the  ocean  have  been  com- 
puted from  temperature  observations  made  by  vessels  while  at  sea.  At 
The  method  Present  nearly  all  vessels  take  meteorological  observations, 
of  determin-  and  these  are  reported  to  the  weather  bureaus  of  the  various 
temperature  countries  as  soon  as  port  is  reached.  These  observations  are 
over  the  grouped  by  months  and  by  squares,  each  square  being  gener- 
ally 5°  on  a  side.  For  some  squares  of  the  Atlantic  Ocean 
thousands  of  observations  are  made  during  a  single  month.  Although 
these  observations  are  not  always  simultaneous,  yet,  since  the  daily 
variation  in  temperature  over  the  ocean  is  so  small,  the  normals  are 
nearly  as  correct  as  those  for  the  land  stations. 

These  normal  temperatures  may  be  charted  on  a  map  and  lines  drawn 

through  those  places  which  have  the  same  temperature.      These  lines 

are  called  ordinarily  isothermal  lines  or  simply  isotherms.1 

struction  of    The  chart  is  often  spoken  of  as  an  isothermal  chart.     The 

isothermal      grs^  isotherms  for  the  world  were  drawn  by  Humbolt  in 

1817.     These  isotherms  were  improved  by  Kamtz  in  1831 

and  Mohlmann  in  1841  as  new  data  were  gathered.     Dove  in  1852  was 

the  first  to  chart  the  normal  monthly  temperatures. 

84.  Isothermal  lines  for  the  year.  —  The  normal  yearly  temperatures 
have  been  used  in  the  construction  of  chart  I,2  and  this  chart  thus  repre- 
isotherms  sents  the  isotherms  for  the  year.  There  are  several  charac- 
for  the  teristics  of  these  isothermal  lines  which  deserve  careful  con- 

sideration and  explanation. 

(1)  Highest  temperature  at  the  equator  and  lowest  temperatures  at  the 
poles.  The  first  and  most  obvious  fact  in  connection  with  the  isothermal 
Hi  best  ^mes  ^or  ^ne  year  *s  ^e  exigence  of  a  hot  belt  near  the  equator 
temperature  and  low  temperatures  at  the  two  poles.  The  northern  part  of 
^oVamUow-  South  America,  most  of  Africa,  India,  and  a  portion  of  Aus- 
est  at  the  tralin  arc  surrounded  by  a  line  marked  80°  F.  This  means 
poles'  that  all  points  within  this  closed  curve  have  a  normal  annual 

temperature  of  80°  F.  or  more.  This  is  the  hot  belt,  and  a  line  passing 
through  its  center  is  often  spoken  of  as  the  heat  equator  or  the  ther- 
mal equator.  The  temperature  at  the  north  pole  is  0°  F.,  while  the 

1  From  the  Greek :  foot  =  equal ;  &tpivt\  ~  heat. 
1  See  end  of  book  for  the  50  charts. 


OBSERVATION       p  DISTRIBUTION  OP  TEMPERATURE     95 

• 
isothermal  lines  near  the  south  pole  have  been  omitted   on  account 

of  insufficient  data.     The  explanation  of  the  existence  of 

this  hot  equatorial  belt  and  the  low  polar  temperatures  is   for  the  tem- 

the  well-known  fact  that  the  equator  receives  jnuch  more  Pfrature 

insolation  from  the  sun  than  the  polar  regions.     The  ratio 

between  the  equator  and  pole  is  347  to  143,  and  this  is  sufficient  to 

account  forvthe  temperature  differences. 

(2)   The  deflection  of  isothermal  lines  from  parallels  of  latitude.     All 
places  with  the  same  latitude  receive  the  same  amount  of  insolation  from 
the  sun.     It  would  thus  be  expected  that  all  places  on  the 
same  parallel  of   latitude  would   have  the  same  tempera-  tionofiso- 
ture,  and  that  the  isothermal  lines  would  be  parallel  to  the  thermal 
parallels  of  latitude.     This  is,  however,  by  no  means  the  case, 
and  the  chief  cause  of  the  deflection  is  the  existence  of  ocean 


It  is  beyond  the  scope  of  this  book  to  discuss  fully  the  causes  for 
the  existence  and  the  direction  of  ocean  currents.      The  factors  which 
cause  ocean  currents  and  determine  their  direction  are  :  tem- 
perature differences  between  different  parts  of  the  earth  ;  Whicif  deter- 
the  permanent  wind  system  of  the  earth  ;    the  evaporation  mine  the 
which  is  greater  at  some  parts  of  the  ocean  than  at  others  ;  and^kec- 
the  inflow,  which  is  also  greater  at  certain  places  than  at  tionof 
others  ;  varying  degrees  of  saltiness  and  thus  density  ;  the  ™^  c 
earth's  rotation  on  its  axis  ;  the  configuration  of  the  coast  line. 

Chart  II  presents  the  general  scheme  of  the  ocean  Currents.  In  the 
Atlantic  Ocean  the  Gulf  Stream,  consisting  of  warm  water  which  has 
made  the  circuit  of  the  Gulf  of  Mexico  and  emerged  between 

The  cen- 

Florida  and  Cuba,  together  with  a  considerable  quantity  of  erai  scheme 
water  which  has  passed  northward  outside  of  the  West  Indies,  of  ocean 
sets  diagonally  across  the  Atlantic  Ocean  towards  England 
and  Scandinavia.     The  return  current  flows  southward  along  the  coast  of 
Europe  and  along  the  northern  part  of  Africa  and  back  again  along  the 
equator.     Another  return  current  flows  southward  along  Greenland  and 
Newfoundland.     The  northern  Pacific  Ocean  possesses  a  similar  set  of 
ocean  currents.   In  the  South  Atlantic  and  South  Indian  and  South  Pacific 
oceans  there  is  an  oval  circulation  turning  in  a  counterclockwise  direction. 
On  the  European  side  of  the  Atlantic  Ocean  the  isothermal  lines  over 
England   and  Scandinavia  are  carried  far  to  the  north  by  The  effect 
the  Gulf  Stream  and  even  bend  backward  on  themselves.   Of  ocean 


The  cool  return  current  along  the  coast  of  Spain  and  northern  c 
Africa  carries  the  isothermal  lines  towards  the  equator.     The 


96  METEOROLOGY 

result  is  a  fan-shaped  spreading  out  of  the  isothermal  lines  over  Europe. 
On  the  North  American  side  of  the  Atlantic  the  cold  return  current  from 
Greenland  carries  the  isothermal  lines  southward,  while  the  warm  water 
coming  up  from  the  Gulf  carries  them  northward,  and  the  result  is  a 
crowding  together  of  the  isotherms.  The  same  difference  in  temperature 
will  be  found  in  one  half  the  distance  on  the  North  American  side  of  the 
Atlantic  as  on  the  European.  Numerous  other  illustrations  of  the  deflec- 
tion of  isotherms  by  ocean  currents  can  be  noticed  by  comparing  the 
scheme  of  ocean  currents  with  the  isothermal  lines  for  the  year.  Al- 
though the  ocean  currents  are  the  chief  cause  of  deflection,  they  are  not 
the  only  causes,  a-sjlivpirsifry  n'f  aiir-fara,  whether  land  or  water,  vegetation^ 
covered  or  barren,  and  the  genera,!  winr|  system,  also  play  a  part. 

(3)  Regularity  in  the  southern  hemisphere.     It  will  be  noticed  that  the- 
isothermal  lines  are  much  more  regular  in  the  southern  hemisphere  than 

.  in  the  northern.  The  reason  for  this  is  that  the  southern 
in  the  south-  hemisphere  is  largely  a  water  hemisphere,  while  the  northern 
ernhemi-  hemisphere  has  a  diversified  surface  consisting  of  both  land 
and  water.  A  water  surface  tends  to  even  out  temperature 
irregularities  and  also  to  equalize  the  temperature  between  equator  and 
pole. 

(4)  The   heat   equator  north  of  the  geographical  equator.     It  will  be 
noticed  that  the  central  line  of  the  hot  belt  lies,  on  the  whole,  north  of  the 
The  heat        geographical  equator.     The  reason  for  this  is  not  far  to  seek, 
equator          Since  the  southern  hemisphere  is  largely  a  water  hemisphere 
tiuTgeo^         the  equatorial  portion  has  been  cooled  arid  the  polar  regions 
graphical        have  been  warmed  by  the  exchange  of  water  between  the 

equator  and  poles.  In  the  case  of  the  northern  hemisphere 
this  is  not  so  easily  possible,  since  it  Is  largely  a  lancl^surface.  For 
this  reason  the  equatorial  belt  of  high  temperature  lies  north  of  the 
geographical  equator. 

(5)  The  hot  belt  not  of  the  same  width  and  temperature  throughout.     It 
will  be  noticed  that  the  hot  belt  inclosed  by  the  isothermal  line  80°  is 

widest  over  South  America  and  Africa.     It  narrows  markedly 

The  hot  belt    . 

not  of  the       m  crossing  the  Atlantic  Ocean  and  disappears  entirely  over 

same  width     the  pacinc  Ocean.     The  reason  for  this  is  again  the  tendency 

of  the  ocean  to  equalize  equatorial  and  polar  temperatures. 

85.  Chart  III  represents  the  isothermal  lines  for  the  year  for  the 
United  States. 

86.  Isotherms  for  January  and  July.  —  In  the  construction  of  Charts 
IV  and  V  the  normal  January  and  July  temperatures  have  been  used,  and 


OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE     97 

these  charts  thus  present  the  isothermal  lines  for  January  and  July. 
The  first  three  characteristics  noted  in  connection  with  the  isothermal 
lines  for  the  year  are  the  same  for  the  isothermal  lines  for 
January  and  July.     These  three  characteristics  are  :  the  hot  thermTfor 
belt  at  the  equator  and  the  low  temperatures  at  the  poles  ;  the  Januafy  and 
deflection  of  isothermal  lines  from  parallels  of  latitude ;  the 
regularity  in  the  southern  hemisphere.      In  addition  there  are  two 
new  characteristics  which  deserve  attention  and  explanation. 

(1)  The  hot  belt  and  all  isothermal  lines  migrate  north  and  south  in  the 
course  of  the  year.     On  the  January  chart  it  will  be  seen  that  the  highest 
temperatures  oecur  in  South  Africa  and  in  Australia,  while  on 

the  July  chart  it  will  be  seen  tfrat  the  highest  temperatures  tion  of  iso- 
occur  over  the  southern  part  of  North  America,  Northern  *hermal 
Africa,  and  Australia.  There  has  thus  been  a  decided  migra- 
tion of  the  hot  belt  between  January  and  July.  If  any  other  isothermal 
line  is  considered  it  will  be  found  that  it  too  has«iigrated.  O*n  the  Pacific 
the  hot  belt  migrates  from  15  to  20°  of  latitude  and  on  the  Atlantic  it 
migrates  still  less,  andtonly  on  the  western  portion  of  this  ocean  does  it 
cross  the  geographical  equator  to  the  southern  hemisphere.  On  the  con- 
tinents it  shifts  over  a  somewhat  greater  distance.  In  Africa  it  moves 
from  about  23°  north  to  20°  south  latitude.  In  America  the  migration 
is  from  about  35°  nbrth  to  15°  south.  The  average /however,  is  less  than 
47°,  which  is  the  amount  the  sun  migrates  in  the  course  of  the  year. 
Three  things  may  be  noted  in  connection  with'  this  migration  :  (a)  the 
migration  is  less  than  the  migration  of  the  sun ;  (6)  the  hot  belt  lags 
behind  the  sun  in  its  migration ;  (c)  the  migration  is  greatest  on  land 
and  least  on  the  ocean. 

(2)  The   highest  and  lowest  temperatures  on  land.     On  the  January 
chart  the  highest  temperatures  are  in  the  northern  part  of  Africa  and  in 
Australia,  while  the  lowest  temperature  is  in  north  central 

Siberia      On  the  July  chart  it  will  be  seen  that  the  highest  and  iowest 
temperature  -  are  in  central  North  America,  North  Africa,  and  tempera- 
Arabia.     The  truth  of  the  statement  that  the  highest  and  on'Tam' 
lowest  temperatures  occur  on  land  is  thus  demonstrated. 
The  reason  for  thl    is  not  far  to  seek.     The,  land,  as  compared  with  the 
ocean,  is  always  radical  in  its  behavior  as  regards  temperature  changes. 
During  the  day  and  during  the»summer  it  heats  to  a  high  temperature, 
during  the  night  and  during  the  winter  it  cools  correspondingly  low. 
Thus  the  highest  and  lowest  temperatures  are  always  to  be  expected  on 
land. 


98 


METEOROLOGY 


87.  Charts  VI  and  VII  represent  the  July  and  January  isotherms  for 
the  United  States. 

88.  Poleward  temperature  gradient.  —  The  diminution  in  tempera- 
ture in  going  from  equator  to  pole  is  often  spoken  of  as  the  poleward 

temperature  gradient.  By  examining  the  isothermal  charts 
for  the  year,  and  for  January  and  July,  the  three  following 
generalizations  may  be  formed :  (1)  the  poleward  tempera- 
ture gradient  is  larger  in  winter  than  in  summer;  (2)  the 
poleward  temperature  gradient  is  larger  in  the  northern 
hemisphere  than  in  the  southern  hemisphere;  (3)  the  poleward 
temperature  gradient  is  larger  on  land  than  over  the  ocean.  It  will 


The  char- 
acteristics 
of  the  pole- 
ward tem- 
perature 
gradient. 


120°        160g        160°       120° 


0"        40°        SO"       120" 


:^ 


120*  160 


ISANOMALOUS 
TEMPERATURE  LINES 
FOR    JANUARY ( 

K)°  0°  40°  80°  120" 


FIG.  41.  —  Isanoixialous  Temperature  Lines  for  January  (Temp.  F.).     (After  BATCHELDER.) 

be  seen  later  that  to  the  first  of  these,  namely,  the  larger  value  of 
/  poleward  temperature  gradient  in  winter,  is  due  the  increased  wind 
s  velocity  and  the  greater  violence  of  storms  during  the  wint  ?r. 

It  is  also  an  interesting  fact  that  the  poleward  temperature  gradient 
is  about  800  times  smaller  than  the  vertical  temperature  gradient.  That 
is,  it  would  be  necessary  to  travel  poleward  800  miles  to  get  the  same 
diminution  in  temperature  which  would  be  gained  by  ascendi  ig  one  mile. 
Thermal  89.  Thermal  anomalies.  —  The  average  temperature  of  a 

anomalies,      given  parallel  of  latitude  may  be  found  by  knowing  the  actual 
temperature  at  equally  distant  intervals  along  this  parallel  and  finding 


OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE     99 


their  average.  The  difference  between  the  temperature  of  a  place  and 
the  average  for  its  parallel  of  latitude  is  called  t,he  thermal  anomaly. 
These  thermal  anomalies  for  January  and  July  are  pictured  in  Figs. 
41  and  42.  On  the  January  chart  it  will  be  seen  that  the  north  Atlantic 


ISANOMALOUS 

TEMPERATURE  LINES 

FOR    JULY 


FIG.  42. —  Isanomalous  Temperature  Lines  for  July  (Temp.  F.).     (After  BATCHELDER.) 

is  40°  above  the  average  of  this  latitude,  the  north  Pacific  is  20°  above 
the  average,  while  the  central  part  of  Asia  and  the  central  part  of  North 
America  are  30°  below  the  average  for  their  latitude.  If  the  July  chart  is 
examined,  it  will  be  seen  that  the  center  of  North  America  and  of  Asia 
are  above  their  latitudes 
in  temperature,  while 
the  Pacific  and  the 
Atlantic  Oceans  average 
below  their  latitudes. 
These  statements  may 
be  summarized  as  a 
general  law:  in  summer 
the  continents  are  above 
the  average  in  tempera- 
ture, while  the  oceans  are  below  ;  during  the  winter  the  continents 
are  below  the  average  in  temperature,  while  the  oceans  are  above. 

^  Figure  43,  which  represents  the  isothermal  lines  for  Spain  and  Por- 
tugal for  January  and  July,  illustrates  this  general  law  for  a  small  area. 


JANUARY  JtJLJ 

FIG.  43.  —  Isothermal  Lines  for  Spain  and  Portugal  for 
January  and  July  (Temp.  F.). 


100 


METEOROLOGY 


90.  Annual  range  of  temperature.  —  Figure  44  represents  the  annual 
range  of  temperature  for  the  earth's  surface.  Many  different  kinds  of 
annual  range  may  be  computed.  The  kind  represented  here 
is  the  difference  between  the  January  and  July  normals.  The 
greatest  value  of  range  is  120°  F.  in  north  central  Siberia. 
The  northern  part  of  North  America  comes  next  with  80°  F., 
while  South  America,  South  Africa,  and  Australia  have  30° 
each.  It  will  be  noticed  that  the  greatest  value  of  range  always  occurs 
on  land,  and  that  it  is  roughly  proportional  to  the  amount  of 


The  char- 
acteristics 
of  the 
annual 
range  of 
temperature. 


120 160 160 120 80 40 


LINES  OF 

EQUAL   ANNUAL   RANGE 
OF   TEMPERATURE 


FIG.  44. —  Annual  Range  of  Temperature  (Temp.  F.). 
(After  CONNOLLT  —  from  DAVIS'S  Elementary  Meteorology.) 

land  surrounding  the  place  in  question.  This  is  but  another  illustra- 
tion of  the  well-known  principle  that  a  land  surface  is  radical  in  its 
temperature  behavior  as  compared  with  an  ocean  surface.  A  land 
surface  becomes  excessively  warm  in  summer  and  correspondingly  cold 
during  the  winter,  while  a  water  surface  is  more  conserva- 
tive in  its  behavior. 

91.   Extremes  of  temperature.  —  The  lowest  temperature 
ever    observed    on    the    earth's    surface    is  —90.4°    F.    at 
Werchojansk    (or   Verkhoyansk)    in  north   central    Siberia. 
This  temperature  was  observed  on  January  15,  1885.     The  highest 


The  highest 
and  lowest 
tempera- 
tures ever 
observed  in 
-the  world. 
I 


OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE    401 

temperature  ever  observed  was  127.4°  F.  at  Ouargla  in  Algeria.  It 
was  observed  July  17,  1879.  North  central  Siberia  and  the  northern 
portion  of  Africa  are  thus  the  coldest  and  hottest  places  in  the  world 
respectively. 

The  lowest  temperature  ever  observed  in  the  United  States  is  —  65°  F. 
at  Poplar  River  in  Montana.     It  was  observed  January  1,  1885.     The 
highest  temperature  ever  observed  in  the  United  States  is 
119°  F.,  and  this  was  observed  at  Phoenix,  Arizona.     The  and  lowest 
isothermal  lines  for  the  United  States,  for  January  and  July,  temPera- 
indicate  the  portions  of  the  country  which  are  hottest  and  served 


in 


coldest  respectively.  jj6  ^snited 

In  making  these  statements  only  those  temperatures 
which  have  been  observed  in  regular  series  at  weather  bureau  sta- 
tions have  been  taken  into  account.  A  temperature  of  122°  F.  is 
reported  for  Death  Valley,  Cal.,  during  the  summer  of  1891,  and  a 
temperature  of  130°  F.  is  said  to  have  been  observed  at  Mammoth  Tank, 
Cal.,  on  August  17,  1885.  Since  the  thermometers  may  not  have  been 
properly  sheltered,  these  temperatures  are  not  usually  considered  as 
authentic.  Temperatures  as  high  as  154°  F.  have  been  reported  from 
parts  of  the  Sahara,  and  a  temperature  as  low  as  —96°  F.  is  reported 
from  the  Arctic  regions  of  North  America. 

At  every  place  abnormally  high  and  abnormally  low  temperatures 
have  occurred  if  long  periods  of  time  are  considered.  Unusually  cold 
winters  and  unusually  hot  summers  have  also  been  described.  At  many 
European  cities  where  the  records  are  long  ones,  these  abnormalities 
make  very  interesting  reading.  Space  does  not  permit,  however,  a  full 
treatment  of  this  subject. 

92.  Other   temperature   charts.  —  All    the   temperature   data  men- 
tioned in  section  79  may  be  charted,  provided  the  data  are  available  for 
many  stations  in  all  parts  of  the  world  or  in  a  given  country.   Other  tem_ 
Figures  45,  46,  and  47  represent  the  highest  temperatures  ever  perature 
observed  in  the  United  States,  the  lowest  temperatures  ev^r  c 
observed  in  the  United  States,  and  the  variability  of  temperature  for 
January  in  the  United  States. 

93.  Polar  temperatures.  —  Charts  VIII  and  IX  represent  the  normal 
temperatures  for  the  north  polar  regions 1  for  January  and  for  North 
July.     It  will  be  seen  that  during  July  the  north  pole  is  the  polar  tem- 
coldest  part  of  the  northern  hemisphere.     During  January  J 

north  central  Siberia  is  the  coldest  part  of  the  northern  hemisphere.    Both 

1  For  isotherms  for  the  north  polar  regions  see  Meteorologische  Zeitschrift,  1906,  p.  111. 


102     ^    - 


METEOROLOGY 


of  these  facts  are  somewhat  anomalous,  since  for  a  short  time  during  the 
summer  the  North  Pole  receives  more  insolation  than  any  other  place  in 


the  northern  hemisphere,  and  during  the  winter  it  receives  less  than  any 
other  place  in  the  northern  hemisphere.     These  seeming  anomalies  can, 


OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE     103 

however,  be  easily  explained.     During  the  summer  the  North  Pole  is  the 

coldest  part  of  the  northern  hemisphere,  in  spite  of  its  value 

of  insolation,  for  three  reasons :  first,    these  large  values  of  £h,e  No^h  x 

»         ,      »          .,  «  i  oi6  coldest 

insolation  last  for  a  very  short  time  ;  secondly,  the  polar  in  summer. 


104 


METEOROLOGY 


regions  are  covered  with  snow  and  ice,  which  reflect  about  30  to  40  per 
cent  of  the  insolation,  which  is  thus  lost  as  far  as  heating  is  concerned ; 


thirdly,  the  temperature  cannot  be  raised  markedly  above  32°  F.  until 
all  the  snow  and  ice  is  melted,  and  this  never  occurs. 


OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE     105 

There  are  two  reasons  why  north  central  Siberia  surpasses  the  polar 
regions  for  cold  during  the  winter ;  first,  the  exchange  of  water  between 
the  equator  and  pole  warms  the  polar  regions.     This  is  not 
possible  in  the  case  of  a  land  surface.    Secondly,  north  central  trai  Siberia 
Siberia  is  a  land  surface,  while  the  polar  regions  is  largely  a  coldest  »* 
water  surface,  or  a  water  surface  covered  with  ice  and  snow. 
Land  is  always  radical  in  its  temperature  behavior,  and  thus  cools  to  a 
greater  extent  than  a  water  surface. 


THE  TEMPERATURE  OF  LAND  AND  WATER 

94.  Ocean  temperatures. —  The  normal  annual  temperature  of  the 
surface  of  the  ocean  varies  from  between  80°  and  90°  F.  at  the  equator  to 
about  28°  F.  in  the  polar  regions.     The  temperature  of  the  The  tem_ 
bottom  of  the  ocean  varies  from  36°  in  the  equatorial  regions  perature  of 
to  about  28°  in  the  polar  regions.     Thus,  at  the  equator  the  ^a^J^ 
temperature  change  between  the  surface  and  bottom  is  from  of  the 
between  80°  and  90°  to  28° ;  in  the  polar  region  the  tempera-  ocean* 
ture  is  28°  throughout.     The  change  in  temperature  between  day  and 
night  is  extremely  small,  not  amounting  to  more  than  a  degree  or  two 
at  most.     The  temperature  change  between  winter  and  summer  is  also 
small  at  the  equator  and  in  the  polar  regions,  but  greater  in  middle 
latitudes.     At  New  York  the  temperature  change  between 
summer  and  winter  is  from  about  70°  to  30°  F.,  and  this  is 
perhaps  the  greatest  yearly  change  anywhere  in  the  Atlantic  the  tempera- 
Ocean.     On  account  of  the  ocean  currents  the  difference  in 
temperature  between  places  but  a  few  hundred  miles  apart 

may  be  considerable.     Salt  water  freezes  at  27°  F.     Thus  all  harbors 
north  of  50°  north  latitude  are  frozen  shut  in  winter,  and  ice  forms  all 
over  the  polar  seas,  seldom,  however,  to  a  depth  of  more  than  5  or  6  feet. 
Special  thermometers  must  be  used  for  determining  the  temperature 
of  the  ocean  water,  particularly  at  considerable  depths.     The 
thermometer  must  not  be  influenced  by  pressure  and  must  ist 
indicate  the  temperature  at  the  required  depth,  regardless  of  special  ther- 
the  temperature  of  the  layers  of  water  through  which  the  ^ed^ 
thermometer  must  be  raised  in  drawing  it  to  the  surface. 
For  a  more  detailed   treatment  of  this  subject  the  reader  must  be 
referred  to  special  works  on  ocean  temperatures. 

95.  Lake  temperatures.  —  In  the  case  of  a  deep  lake  in  the  middle 
latitudes,  the  temperature  of  the  surf  ace  water  in  summer  will  be  between 


106  METEOROLOGY 

60°  and  80°,  while  at  considerable  depths  the  temperature  will  be  39°  or 
above.  With  the  coming  of  winter  the  surface  water  cools  and  thus 

becomes  heavier  and  sinks  to  the  bottom.  This  process  con- 
perature  of  tinues  until  the  temperature  of  the  lake  becomes  39°  through- 
a  lake  from  ou^  39°  jp  js  ^he  temperature  of  water  at  its  maximum 
torn  at  dif-  density.  As  the  temperature  falls  below  this,  the  water  again 
ferent  times  expands  and  becomes  lighter.  Thus,  as  the  surface  water 

cools  below  39°,  it  is  now  lighter  than  the  water  below  and 
remains  at  the  top.  It  cools  finally  to  32°  and  ice  forms  and  the  thick- 
ness of  the  ice  grows  greater  and  greater  during  the  winter.  Thus,  in 
winter  the  temperature  of  the  surface  of  a  lake  next  the  ice  will  be  32°  F., 
while  the  bottom  will  have  a  temperature  of  39°.  With  the  coming  of 
spring  the  ice  melts  and  the  surface  layers  become  warmed  until  the  tem- 
perature again  becomes  39°  throughout.  From  this  point  on  the  warmer 
layers  remain  at  the  top  until  finally  in  late  summer  the  surface  may  have 
heated  up  to  60°  or  even  80°  F.  •  The  temperature  of  the  bottom,  if  a  lake 
is  deep,  will  remain  39° ;  if  shallow,  the  lake  may  have  become  heated 
throughout,  and  the  bottom  temperature  may  be  somewhat  above  39°. 

96.  River  temperatures.  —  If  a  river  is  deep   and   slow  flowing,  its 
River  tem-      temperature  behavior  will  be  the  same  as  that  of  a  lake.     In 
peratures.      winter  the  temperature  of  the  water  underneath  the  ice 
will  be  32°,  and  the  temperature  of  the  bottom  39°  or  slightly  lower. 

If  the  river  is  shallow  and  rapidly  flowing,  the  water  will  be  so 
thoroughly  mixed  that  the  temperature  will  be  the  same  throughout. 

97.  Temperature  below  the  surface  of  the  land.  —  The  ground  is  a 
very  poor  conductor  of  heat,  and  for  this  reason  the  daily  variation  in 
The  annual     temperature  does  not  penetrate  to  a  greater  depth  than  two  or 
variation        three  feet,  and  requires  many  hours  to  reach  even  that  depth, 
penetrate       The  annual  variation  does  not  penetrate  to  a  depth  of  more 
more  than      than  fifty  feet  and  requires  nearly  six  months  to  reach  that 

depth.  Thus,  at  a  depth  of  thirty  or  forty  feet  in  the  ground 
the  highest  temperatures  will  be  experienced  in  winter  and  the  lowest 
temperatures  in  summer. 

Below  fifty  feet  comes  the  layer  of  invariable  temperature.     This  will 
have  a  thickness  from  perhaps  fifty  feet  to  several  hundred  feet,  and  this 

temperature  is  always  the  normal  annual  temperature  for  the 
of  hivari-  place  in  question.  Caves  are  often  located  in  this  layer  of 
able  temper-  invariable  temperature,  and  the  temperature  of  the  air  in  such 

caves  is  always  the  normal  yearly  temperature  for  the  place. 
Some  cellars  are  also  sufficiently  deep  to  be  located  in  this  layer.  They 


OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE     107 

will  have  a  constant  temperature  throughout  the  year,  and  its  value 
will  be  the  normal  annual  temperature  for  the  place.  If  the  normal 
annual  temperature  is  below  32°,  there  will  be  a  layer  which  remains 
constantly  frozen  throughout  the  year. 

Below  the  layer  of  invariable  temperature,  the  temperature  increases  at 
the  rate  of  1°  for  every  52  feet  (although  this  varies  from  40  to  100  feet). 
These  observations  have  been  obtained  from  tunnels  and 
from  deep  mines.     These  extend,  however,  not  more  than  a  tureen-*1 
mile  below  the  surface  of  the  earth,  which  is  a  mere  scratch  crease  with 
compared    with    the    radius    of    the    earth.     It    would    be 
impossible   from   astronomical  and   geological   considerations   for  the 
temperature  to  increase  at  this  rate  all  the  way  down  to  the  earth's 
center. 

The  temperature  of  spring  water  gives  an  idea  of  the  depth  of  the  layer 
through  which  the  water  percolates  and  finally  emerges  as  a  spring.     If 
the  water  comes  through  the  layer  of  invariable  temperature, 
the  temperature  of  spring  water  would  remain  the  same  perature  of 
throughout  the  year  and  would  have  the  normal  annual  tern-  sPrins 
perature  for  the  place.     If  the  layer  through  which  the  water 
passes  is  below  this,  the  temperature  would  remain  the  same  throughout 
the  year,  but  have  a  higher  value  than  the  normal  yearly  temperature  of 
the  place.     If  the  spring  is  a  shallow  one,  there  would  be  a  change  in  tem- 
perature between  summer  and  winter,  but  the  average  would  again  be 
the  normal  yearly  temperature  for  the  place  in  question. 

QUESTIONS 

(1)  Define  thermometry.  (2)  Name  the  three  systems  of  thermometry. 
(3)  How  are  the  fixed  points  numbered  in  each  system?  (4)  State  the  interre- 
lation between  the  three  scales.  (5)  What  is  the  best  method  of  making  a 
mental  computation  of  the  Centigrade  temperature  corresponding  to  a  Fahren- 
heit? (6)  When  and  where  was  the  thermometer  invented?  (7)  Describe 
the  early  form  of  the  thermometer.  (8)  Describe  the  origin  of  the  Fahrenheit, 
Centigrade,  and  Reaumur  thermometers.  (9)  Describe  the  appearance  and 
state  the  working  principle  of  a  thermometer.  (10)  Describe  the  various 
steps  in  the  construction  of  a  thermometer.  (11)  Name  the  essentials  in  a 
good  thermometer.  (12)  What  are  the  advantages  and  disadvantages  of  hav- 
ing a  large  bulb?  (13)  What  is  the  ordinary  form  of  the  bulb  and  the  reason 
for  it  ?  (14)  Why  should  the  graduation  be  placed  on  the  stem  of  a  thermometer 
itself?  (15)  Name  the  inaccuracies  in  determining  a  temperature,  (16) 
What  is  meant  by  the  error  of  parallax?  (17)  What  is  the  effect  of  atmos- 
pheric pressure  on  a  thermometer?  (18)  What  is  the  cost  and  accuracy  of  a 
thermometer?  (19)  Why  is  it  hard  to  determine  the  temperature  of  a  gas? 
(20)  How  should  a  thermometer  be  placed  to  give  approximately  the  real  air 
temperature?  (21)  What  are  the  three  methods  of  determining  the  real  air 


108  METEOROLOGY 

temperature?  (22)  Describe  the  thermometer  shelter  of  the  U.  S.  Weather 
Bureau.  (23)  Describe  the  other  types  of  thermometer  shelter.  (24)  De- 
scribe the  sling  thermometer.  (25)  Describe  in  full  the  ventilated  thermometer. 
(26)  What  are  the  two  kinds  of  thermographs  in  ordinary  use?  (27)  Describe 
the  Draper  thermograph.  (28)  Describe  the  Richard  Freres  thermograph. 
(29)  What  is  the  working  principle  in  each  case  ?  (30)  What  is  the  purpose  of 
maximum  and  minimum  thermometers?  (31)  Describe  the  maximum  and 
minimum  thermometers  of  the  regular  Weather  Bureau  form.  (32)  Describe 
the  Six  maximum  and  minimum  thermometer.  (33)  How  are  maximum  and 
minimum  thermometers  set?  (34)  Describe  the  black  bulb  thermometer. 
(35)  What  is  the  purpose  of  the  black  bulb  thermometer?  (36)  Name  some 
special  purposes  for  which  thermometers  of  various  forms  have  been  adopted. 
(37)  State  the  three  kinds  of  U.  S.  Weather  Bureau  stations.  (38)  What  tem- 
perature observations  are  made  at  each?  (39)  What  instruments  are  used? 
(40)  Where  are  these  instruments  located?  (41)  Where  is  the  shelter  located? 
(42)  Distinguish  between  average,  mean,  and  normal.  (43)  How  are  normal 
hourly  temperatures  computed?  (44)  How  is  the  average  daily  temperature 
computed?  (45)  How  are  the  average  and  normal  yearly  temperatures  com- 
puted ?  (46)  What  is  meant  by  station  normals  of  temperature  ?  (47)  How  may 
these  be  represented  graphically?  (48)  What  are  thermo-isopleths.  (49)  How 
is  the  graph  which  represents  the  daily  variation  found  ?  (50)  Explain  the  time 
of  occurrence  of  the  highest  temperature.  (51)  Describe  the  annual  variation  in 
temperature.  (52)  Describe  the  irregular  variation  in  temperature.  (53) 
What  temperature  data  beside  normals  may  be  computed?  (54)  Define  varia- 
bility of  temperature.  (55)  How  are  normal  temperatures  computed  for 
stations  where  the  record  is  a  short  one?  (56)  State  the  differences  in  tem- 
perature in  a  thermometer  shelter  placed  at  different  altitudes.  (57)  Describe 
the  temperature  differences  over  a  limited  area  during  the  day.  (58)  Describe 
the  temperature  differences  over  a  limited  area  at  night.  (59)  What  observa- 
tions are  used  for  the  construction  of  isothermal  charts?  (60)  How  are  iso- 
thermal charts  constructed?  (61)  State  the  characteristics  of  the  isothermal 
lines  for  the  year.  (62)  What  are  the  causes  of  ocean  currents  and  their  direc- 
tion? (63)  Illustrate  the  effect  of  ocean  currents  on  isothermal  lines.  (64) 
Explain  the  characteristics  of  the  annual  isothermal  lines.  (65)  State  the  char- 
acteristics of  the  isotherms  for  January  and  July.  (66)  Describe  the  migration 
of  the  hot  belt.  (67)  What  is  meant  by  poleward  temperature  gradient  ?  (68) 
What  are  its  characteristics?  (69)  Define  thermal*  anomaly.  (70)  What  are 
its  characteristics?  (71)  What  are  the  characteristics  of  the  annual  range  of 
temperature  ?  (72)  In  what  regions  of  the  world  have  the  highest  and  lowest 
temperatures  been  observed?  (73)  In  what  parts  of  the  United  States  have  the 
highest  and  lowest  temperatures  been  observed?  (74)  Name  other  tempera- 
ture charts  which  can  be  constructed.  (75)  Describe  polar  temperatures  during 
the  summer  and  during  the  winter.  (76)  Why  is  the  north  pole  the  coldest 
place  in  the  northern  hemisphere  during  the  summer  ?  (77)  Why  is  north  central 
Siberia  colder  than  the  north  pole  in  winter  ?  (78)  Describe  the  temperature 
of  the  ocean.  (79)  State  the  diurnal  variation  of  this  temperature.  (80)  De- 
scribe the  annual  variation  of  this  temperature.  (81)  Does  the  ocean  water 
freeze?  (82)  Describe  the  temperature  changes  which  take  place  in  the  water 
of  a  lake  between  summer  and  winter.  (83)  Describe  the  condition  of  a  river 
as  regards  temperature  during  the  summer  and  in  winter.  (84)  To  what  depths 
does  the  daily  variation  of  temperature  penetrate?  (85)  To  what  depths  does 
the  annual  variation  of  temperature  penetrate?  (86)  What  is  meant  by  the 
layer  of  invariable  temperature?  (87)  What  evidences  are  there  of  its  exist- 


OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE     109 

ence?  (88)  What  is  the  temperature  of  the  ground  below  the  invariable 
layer  of  temperature?  (89)  To  what  depths  have  observations  been  made? 
(90)  Upon  what  does  the  temperature  of  the  water  of  a  spring  depend? 

TOPICS   FOR   INVESTIGATION 

(1)  The  early  history  of  the  thermometer. 

(2)  The  details  of  the  methods  used  in  constructing  thermometers  in  large 
numbers. 

(3)  The  inaccuracies  in  determining   a  temperature   and   the  methods  of 
eliminating  them.     ' 

(4)  Thermometer  shelters  —  their  construction,  use,  and  accuracy. 

(5)  Thermometers  for  determining  the  temperature  of  the  earth. 

(6)  Thermometers  for  determining  the  temperature  of  deep  water. 

(7)  The  methods  of  computing  the  average  daily  temperature  and  their 
accuracy. 

(8)  Temperature  differences  over  a  limited  area. 

(9)  Ocean  currents. 

(10)  Abnormalities  of  temperature  and  seasons  at  various  places. 

(11)  Ocean  temperatures  and  the  thermometers  used  in  determining  them. 

(12)  Lake  temperatures. 

(13)  The  te    perature  below  the  earth's  surface. 

PRACTICAL   EXERCISES 

(1)  Draw  the  three  thermometer  scales  side,  by  side  and  of  such  a  length  that 
the  fixed  points  fall  together.     Number   the  points  of  division  so   that  corre- 
sponding temperatures  to  the  nearest  degree  can  be  read  off. 

(2)  If  physical  apparatus  is  available,  study  critically  one  or  two  good  ther- 
mometers.    Determine  their  fixed  points ;    test  the  uniformity  of  the  bore  ; 
determine  the  sluggishness ;  determine  the  effect  of  sudden  changes  in  tempera- 
ture, etc. 

(3)  Compare  the  indications  of  thermometers  of  several  different  forms  in 
the  open,  thermometers  in  a  thermometer  shelter,  a  sling  thermometer,  and  a 
ventilated  thermometer  under  the  most  varied  conditions. 

(4)  Determine  the  transmission  coefficient  of  the  atmosphere  on  several  dif- 
ferent days  by  means  of  the  black  bulb  thermometer  in  vacuo. 

(5)  Determine  the  inaccuracies  and  behavior  of  a  thermograph  by  checking 
its  indications  by  means  of  a  thermometer. 

(6)  Compute  the  station  normals  of  temperature  for  some  station  and  represent 
them  graphically. 

(7)  Determine  how  well  the  different  methods  of  computing  an  average  daily 
temperature  agree. 

(8)  Plot  the  daily  and  annual  variation  in  temperature  for  several  stations 
in  different  parts  of  the  world  and  in  each  case  explain  the  characteristics  of  the 
graph. 

(9)  Work  up  all  or  some  of  the  temperature  data  mentioned  in  section  79 
for  several  stations.     Those  stations  may  be  chosen  in  which  the  student  has  a 
particular  interest. 

(10)  Contrast  two  near-by  stations.     One  should  be  chosen  in  a  large  city 
and  the  other  in  the  country  near  by. 

(11)  Compare  the  observations  made  at  the  base  and  top  of  some  tower. 

i 


110  METEOROLOGY 

(12)  Investigate  the  limited  area  surrounding  a  given  station  for  temperature 
differences. 

(13)  If    the    observations    can    be    obtained,  construct    temperature  charts 
showing  for  a  certain  country  or  for  the  world  some  of  the  temperature  data 
mentioned  in  section  79. 

(14)  Determine  the  temperature  behavior  of  certain  springs  during  the  year 
and  then  determine  the  depth  of  the  layer  through  which  the  water  must  come. 

(In  the  case  of  nearly  half  of  the  above  problems,  if  the  results  were  carefully 
worked  out,  they  would  be  worthy  of  publication  in  some  meteorological  maga- 
zine.) 

REFERENCES 

For  the  history,  description,  illustration,  and  use  of  apparatus  for  determining 
temperature,  see : 

ABBE,  Meteorological  Apparatus  and  Methods,  Washington,  1888.  Pages  11  to 

107  treat  of  thermometers  and  thermometry. 
BOLTON,  HENRY  C..  Evolution  of  the  Thermometer. 
The  apparatus  catalogues  of  such  firms  as : 

Henry  J.  Green,  1191  Bedford  Ave.,  Brooklyn,  N.  Y. 

Julien  P.  Friez,  Belfort  Observatory,  Baltimore,  Md. 

Queen  &  Co.,  8th  and  Arch  Sts.,  Philadelphia,  Pa. 

Negretti  &  Zambra,  38  Holborn  Viaduct,  London. 

James  J.  Hicks,  8-10  Hatton  Garden,  London,  E.  C.,  England. 

C.  F.  Casella  &  Co.,  11-15  Rochester  Row,  Victoria  St.,  London,  S.  W. 

R.  Fuess,  Diintherstrasse  8,  Steglitz  bei  Berlin,  Germany. 

Wilh.  Lambrecht,  Gottingen,  Germany. 

Max  Kohl,  Chemnitz,  Germany. 

For  the  exposure  and  care  of  thermometers,  see : 

HAZEN,  HENRY  A.,  Thermometer  Exposure,  Professional  Papers  of  the  Signal 

Service,  No.  XVIII,  1885. 

Instructions  for  cooperative  observers,  U.  S.  Weather  Bureau. 
See  also  the  various  guides  for  observers  mentioned  in  Appendix  IX  in  group 

(2)B. 

For  meteorological  observations,  usually  somewhat  summarized,  consult : 

(1)  The  serial  publications  of  the  U.  S.  Weather  Bureau. 
(A}  Daily  Weather  Map. 

(B)  National   Weather    Bulletin,   weekly   during    the    summer,   monthly 

during  the  winter. 

(C)  Climatological  Reports.      These   were   issued   monthly  at   44  section 

centers  until  July,  1909.     Since  then  they  have  been  combined  with 
Monthly  Weather  Review. 

(Z))  Weather    Bulletins,  weekly  during  the  summer  at  44  section   centers. 
(Discontinued  in  1908.) 

(E)  Snow  and  Ice  Bulletins,  weekly  during  the  winter. 

(F)  Monthly  Weather  Review  with  annual  summary  and  index. 

(G)  Mount  Weather  Bulletin. 

(H )  Annual  Report  of  the  Chief  of  the  Weather  Bureau. 

(2)  The  periodical  publications  of  the  weather  bureaus  of  the  various  countries. 

For    a   list    of    these    see    BARTHOLOMEW'S    Physical    Atlas,    Vol.    Ill 
(Atlas  of  Meteorology). 

(3)  The  serial  publications  of  many  private  stations  and  observatories.     For 

example :    Meteorological    Observations    of    the   Massachusetts   Agri- 


OBSERVATION  AND  DISTRIBUTION  OF  TEMPERATURE      111 

cultural  Experiment  Station,  published  monthly  since  January,  1889 ; 
Observations  at  Blue  Hill  Observatory  by  Professor  A.  L.  Rotch,  pub- 
lished since  1886  in  the  Annals  of  the  Harvard  College  Observatory. 
(4)  Scientific  magazines  and  special  publications.  The  best  way  to  locate 
these  is  to  consult  some  digest  of  meteorological  literature;  as,  Fort- 
schritte  der  Physik  (part  three). 

For  temperature  normals  for  various  places,  see : 

BUCHAN,  ALEXANDER,  Report  on  Atmospheric  Circulation. 

HANN,  Lehrbuch  der.  Meteorologie. 

HANN,  Handbuch  der  Klimatologie. 

VAN  BEBBER,  Handbuch  der  Meteorologie. 

Report  of  the  Chief  of  the  Weather  Bureau,  particularly  for  1891-1892,  1896- 

1897,  1897-1898,  1900-1901,  1901-1902. 
Temperature  and  Relative  Humidity  Data,  Bulletin  O,  U.  S.  Weather  Bureau, 

WILLIAM  B.  STOCKMAN. 
Climatology  of  the  United  States,  Bulletin  Q,  U.  S.  Weather  Bureau,  ALFRED 

JUDSON  HENRY. 
The  Daily  Normal  Temperature  and  the  Daily  Normal  Precipitation  in  the 

United  States,  Bulletin  R,  U.  S.  Weather  Bureau,  FRANK  H.  BIGELOW. 
Report  on  the  Temperatures  and  Vapor  Tensions  in  the  United  States,  Bulletin  S, 

U.  S.  Weather  Bureau,  FRANK  H.  BIGELOW. 
Summary  of  the  Climatological  Data  for  the  United  States  by  Sections  (106 

are  to  be  issued). 

For  isothermal  and  climatological  charts,  see : 

BARTHOLOMEW,  Physical  Atlas,  Vol.  Ill  (Atlas  of  Meteorology),  Prepared  by 
Bartholomew  and  Herbertson  and  edited  by  Alexander  Buchan,  1899. 

BUCHAN,  ALEXANDER,  Report  on  Atmospheric  Circulation  (Report  on  the  scien- 
tific results  of  the  voyage  of  H.  M.  S.  Challenger}. 

HANN,  Atlas  der  Meteorologie,  1887,  A  section  of  the  Berghaus  Atlas,  but  can 
be  bought  separately. 

HILDEBRANDSSON,  H.  H.,  ET  TEissERENC  DE  BORT,  Les  bases  de  la  meteorologie 
dynamique  (2  vols.  have  appeared),  Paris,  1900-1907. 

Summary  of  International  Meteorological  Observations,  Bulletin  A  of  the 
U.  S.  Weather  Bureau. 

ELIOT,  SIR  JOHN,  Climatological  Atlas  of  India,  Edinburgh,  1906. 

Russia,  Atlas  climatologique  de  I' empire  de  Russie,  St.  Petersburg,  1900. 

BLODGET,  LORIN,  Climatology  of  the    United  States,  Philadelphia,  1857. 

Isothermal  Lines  for  the  United  States,  1871-1880,  by  A.  W.  GREELY. 

Professional  Papers  of  the  Signal  Service,  No.  II,  1881. 

GREELY,  GEN.  A.  W.,  American  Weather,  New  York,  1888. 

Report  of  the  Chief  of  the  Weather  Bureau,  particularly  for  1896-1897, 
1897-1898,  1900-1901,  1901-1902. 

Climatic  Charts  of  the  United  States,  U.  S.  Weather  Bureau,  Washington, 
D.  C.,  1904  (W.  B.  301). 

Climatology  of  the  United  States,  Bulletin  Q,  by  A.  J.  HENRY,  1906  (W.  B.  361). 

For  temperature  differences  over  a  limited  area,  see : 
Monthly  Weather  Review: 

July,  1905,  XXXIII,  p.  305. 

August,  1906,  XXXIV,  p.  370. 

August,  1908,  XXXVI,  p.  250. 


CHAPTER  IV 

THE   PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE 
A.     THE  OBSERVATION  AND  DISTRIBUTION  OF  PRESSURE 

THE  DETERMINATION  OF  ATMOSPHERIC  PRESSURE 

Atmospheric  pressure,  98. 

Mercurial  barometer :  history,  construction,  corrections,  99-101. 

The  aneroid  barometer,  102. 

Barographs,  103. 

Other  so-called  barometers,  104. 

THE  RESULTS  OF  OBSERVATION 

The  Observations,  105. 

Normal  hourly,  daily,  monthly,  and  yearly  pressure,  106. 
Diurnal,  annual,  and  irregular  variation,  107-109. 
Barometric  data,  no. 

THE  VARIATION  WITH  ALTITUDE 

Reduction  to  sea  level,  in. 

Barometric  determination  of  altitude,  112,  113. 

Variation  with  altitude,  114. 

THE  DISTRIBUTION  OF  PRESSURE  OVER  THE  EARTH 

Construction  of  isobaric  charts,  115. 

Isobars  for  the  year,  116. 

Vertical  section  along  a  meridian,  117. 

Isobaric  surfaces,  118. 

Isobars  for  January  and  July,  119. 

Other  pressure  charts,  120. 


B.     THE   OBSERVATION  AND  DISTRIBUTION   OF  THE  WINDS 

THE  DETERMINATION  OF  THE  DIRECTION,  FORCE,  AND  VELOCITY  OF 
THE  WIND 

Wind  direction,  force,  and  velocity,  121. 
Wind  vane,  122. 
Anemoscope,  123. 
Velocity  estimations,  124. 
Anemometers,  125-127. 

112 


PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE     113 
THE  LOCATION  OF  OBSERVATORIES 

Effect  of  surroundings,  128. 

Hill  and  mountain  observatories,  129. 

THE  RESULTS  OF  OBSERVATION 

The  observations,  130. 

Prevailing  wind  direction;   wind  roses,  131. 

Normal  hourly,  daily,  monthly,  and  yearly  velocity,  132. 

Diurnal,  annual,  and  irregular  variation,  133-135. 

Wind  data,  136. 

Prevailing  winds  of  the  world,  137. 

Other  wind  charts,  138. 


C.     THE    CONVECTIONAL   THEORY   AND   ITS    COMPARISON  WITH 
OBSERVED   FACTS 

THE  CONVECTIONAL  THEORY 

General  convectional  motion,  139. 

Arrangement  of  isobaric  surfaces  in  a  general  convectional  circulation,  140. 

Conditions  of  steady  motion,  141. 

Barometric  gradients,  142. 

Relation  of  wind  direction  to  pressure  gradient,  143. 

Effects  of  the  earth's  rotation  on  wind  direction  and  pressure,  144-147. 

Buys  Ballot's  law,  148. 

THE   COMPARISON   OF   THE   CONSEQUENCES   OF    THE    CONVECTIONAL 
THEORY  WITH  THE  OBSERVATIONS  OF  PRESSURE  AND  WIND,  149 


D.     A   GENERAL   CLASSIFICATION   OF  THE  WINDS 

THE  CLASSIFICATION  OF  THE  WINDS,  150 
PLANETARY  WINDS 

Typical  system,  151-155. 
Trade  winds,  156. 
Doldrums,  157. 
Horse  latitudes,  158. 
Prevailing  westerly  winds,  159. 
Upper  currents,  160. 

TERRESTRIAL  WINDS 

Definition,  161. 

Annual  migration  of  the  winds,  162. 

Subequatorial  and  subtropical  wind  belts,  163. 

CONTINENTAL  WINDS 

Definition,  164. 
Monsoons,  165. 
Other  land  effects,  166. 


LAND  AND  SEA  BREEZES,  167 


114  METEOROLOGY 

MOUNTAIN  AND  VALLEY  BREEZES,  168 

ECLIPSE,  LANDSLIDE,  TIDAL,  AND  VOLCANIC  WINDS 

Eclipse  winds,  169. 

Landslide  and  avalanche  winds,  170. 

Tidal  winds,  171. 

V6lcanic  winds,  172. 

CYCLONIC  STORMS,  173 

WINDS  OF  OTHER  PLANETS,  174,  175 

A.    THE  OBSERVATION  AND  DISTRIBUTION  OF  PRESSURE 

THE  DETERMINATION  OF  ATMOSPHERIC  PRESSURE 

98.  Atmospheric  pressure.  —  The  second  meteorological  element  to  be 
considered  is  the  pressure  of  the  atmosphere.  We  are  probably  less  con- 
scious of  atmospheric  pressure  and  its  changes  than  of  any  of 
^e  °^ner  meteorological  elements.  It  is  true  that  great 
tant  mete-  changes  in  atmospheric  pressure  do  produce  marked  physio- 
dement"1  logical  effects,  but  we  are  absolutely  unconscious  of  the  ordi- 
nary changes  in  atmospheric  pressure  from  day  to  day.  In 
meteorological  work  and  weather  forecasting,  however,  the  pressure,  with 
the  possible  exception  of  temperature,  is  the  most  important  of  the 
elements. 

The  atmosphere  has  mass  and  is  acted  upon  by  gravity,  and  thus 
possesses  weight  and  exerts  a  downward  pressure.  The  pressure  of  the 
The  atmos-  atmosphere  is  simply  the  weight  of  the  column  of  air  above 
pheric  pres-  the  station  in  question,  extending  to  the  limits  of  the  atmos- 
wdghtof6  phere.  Atmospheric  pressure  thus  diminishes  with  elevation 
the  atmos-  above  the  earth's  surface  because  there  is  a  less  quantity  of 
air  to  exert  a  downward  pressure.  Since  the  atmosphere 
is  a  gas,  the  pressure,  as  in  the  case  of  all  fluids,  is  exerted  in  every 
direction.  If  there  were  no  temperature  differences  and  thus  no  winds, 
the  pressure  would  be  the  same  at  all  points  on  a  level  surface,  as 
for  example,*  at  all  points  on  the  hydrosphere.  This  is  not 

Atmosphenc  . 

pressure  is      the  case,  however,  and  the  pressure  is  different  at  dinerent 
not  con-         points  at  the  same  level  and  is  also  constantly  changing  at 
the  same  station.     It  is  desirable,  therefore,  to  have  instru- 
ments for  determining  the  pressure  of  the  atmosphere. 

The  instrument  for  determining  atmospheric  pressure  is  called  a  ba- 


PRESSURE   AND   CIRCULATION  OF  THE   ATMOSPHERE     115 


rometer;1  and  there  are  two  kinds  of  barometers,  those  employing  a  fluid 
and  those  without  fluid.      Since  mercury  is  the  fluid  ordinarily  used, 
such  barometers  are  usually  called  mercurial  barometers,   The  two 
Barometers  which  do  not  use  a  fluid  are  called  aneroid  2  ba-  kinds  of 
rometers.      The  pressure  of  the  atmosphere  might  be  ex-  l  srs* 

pressed  in  poundals  per  square  foot,  or  in  dynes  per  square  centimeter. 
As  a  matter  of  fact,  however,  it  is  expressed  in  terms  of  The  pres- 
the  length  of  an  equivalent  or  balancing  mercury  column,   sure  is  ex- 
A  pressure  of  thirty  inches  thus  means  that  the  pressure  of  terms'  of"1 
the  atmosphere  is  the  same  as  the  pressure  exerted  by  a  inches  of 
column  of  mercury  thirty  inches  long. 

99.    Mercurial     barometer:     history,     construction,     corrections.  — 
The  history  of  the  mercurial  barometer3  dates  from  a  series  of  experi- 
ments made  by  Torricelli  in  1643.     It  was  a  remark  of  Galileo  Torriceiii's 
Galilei  of  Pisa,  the  father  of  experimental  science,  when  it  was  experiment 
called  to  his  attention  that  water  would  not  rise  in  a  pump  " 
more  than  eighteen  cubits  above  the  level    of    a  well,   that   nature 
probably  did  not  abhor  a  vacuum  above  that  height,  which  attracted 
the  attention  of  Torricelli,  who  was  then  his  pupil 
and  later  his  successor  in  the  chair  of  Philosophy 
and  Mathematics  at  Florence.     Not  being  satisfied 
with  this  explanation,  he  instituted  a  series  of  experi- 
ments which  led  to  the  invention  of  the  barometer 
in  1643.     His  most  famous  experiment,  as  pictured 
in  Fig.  48,  consisted  in  filling  a  glass  tube  The 


more  than  thirty  inches  long  with  mer-  tion  of  the 
cury,  covering  the  open  end,  and  then 
inverting  it  over  a  vessel  containing  mercury.     When 
the  open  end  was  uncovered,  the  mercury  immedi- 
ately fell  to  a  height  of  about  thirty  inches  regard- 
less of  the  length  of  the  glass  tube.      Torriceiii's 
explanation  was  that  it  was  the   pressure  of  the 
atmosphere  which  was  supporting  the  column  of 
mercury.      This  explanation  received  full  confirma- 

*  . 

tion  a  few  years  later,  in  1648,  when  Pascal  per- 

suaded his  brother-in-law  Perrier  to  ascend  the  Puy  de  Dome  near 

Clermont  with  a  Torricellian  barometer.     The  diminution  in  length  of 

1  j3dpos  =  weight;  ptrpov  =  measure.  2  d  =  without;  vepbs  =  fluid. 

3  For  an  historical  account  of  the  barometer,  see  Quarterly  Journal  of  the  Royal  Meteor- 
ological Society,  No.  59,  July,  1886,  p.  131  ;  or  Meteorologische  Zeitschrift,  1894,  p.  445. 


FIG.  48.  —  Torriceiii's 
Experiment. 


116 


METEOROLOGY 


the  mercury  column  during  the  ascent  supplied  final  proof  that  it 
was  the  pressure  of  the  atmosphere  which  was  supporting 
the  column  of  mercury. 

100.   A  mercurial  barometer  of  the  Fortin  form  as  used 
at  the  present  time  consists  essentially  of  a  glass  tube  more 
than  thirty-four  inches  long,  filled  with  mercury, 

Description  . 

of  a  mercu-  and  inverted  over  a  vessel  containing  mercury 
rial  barom-  ancj  cane(j  the  cistern.  The  mercury  used  in 

the  tube  must  be  pure,  and  it  must  have  been 
previously  boiled  in  order  to  extract  all  air  and  moisture. 
An  air  trap  is  often  inserted  in  the  tube  to  prevent  the 
ascent  of  any  air  or  moisture  into  the  Torricellian  vacuum 
above  the  mercury  column.  The  whole  is  inclosed  in  a 
brass  case  for  protection,  in  order  to  make  it  somewhat 
portable,  and  to  enable  it  to  be  suspended  vertically.  The 
brass  case  is  cut  away  at  the  top,  exposing  the  glass  tube 
containing  mercury,  and  is  provided  with  a  scale  and 
vernier  for  reading  the  height  of  the  mercury  in  the  tube. 
A  barometer  is  usually  suspended  from  the  top  and  held 
vertically  by  means  of  a  ring  with  three  set  screws  at  the 
bottom.  The  back  board  is  provided  with  two  translucent 
windows  at  the  bottom  and  top  to  illuminate  the  cistern 
and  top  of  the  mercury  column,  particularly  at  night, 
when  a  light  is  placed  back  of  them.  Figures  49  and  50 
represent  the  barometer  as  a  whole  and  a  sectional  view. 
The  cistern  is  made  up  of  a  glass  cylinder  F,  which  al- 
lows the  surface  of  the  mercury  q  to  be  seen,  and  a  top 
plate  G,  through  the  neck  of  which  the  barometer  tube  t 

passes,  and  to  which  it  is  fastened  by  a  piece 
fcription  of6"  of  kid  leather,  making  a  strong  but  flexible 
the  cistern  joint.  To  this  plate,  also  is  attached  a  small 

ivory  point  h,  the  extremity  of  which  marks  the 

commencement  or  zero  of  the  scale  above.  The 
lower  part,  containing  the  mercury,  in  which  the  end  of 
the  barometer  tube  t  is  plunged,  is  formed  of  two  parts  i, 
j,  held  together  by  four  screws  and  two  divided  rings.  To 
the  lower  piece  j  is  fastened  the  flexible  bag  N,  made  of 
kid  leather,  furnished  in  the  middle  with  a  socket  k,  which 
FIG.  49.  rests  on  the  end  of  the  adjusting  screw  0.  Those  parts, 
Barometer.  with  the  -glass  cylinder  F,  are  clamped  to  the  flange  B  by 


of  a  barom- 
eter. 


PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE     117 


means  of  four  long  screws  P  and  the  ring  R ;  on  the  ring  R  screws  the 
cap  S,  which  covers  the  lower  parts  of  the 
cistern,  and  supports  at  the  end  the  adjust- 
ing screw  0.  G,  i,  j,  and  k  are  of  boxwood  ; 
the  other  parts  of  brass  or  German  silver. 
The  screw  0  serves  to  adjust  the  mercury 
to  the  ivory  point,  and  also,  by  raising  the 
bag,  so  as  to  completely  fill  the  cistern 
and  the  tube  with  mercury,  to  put  the  in- 
strument in  condition  for  transportation. 

A  thermometer  is  attached  to  the  middle 
of  the  barometer  to  indicate  the  tempera- 
ture of  the  instrument.  A  good  barometer 
costs  from  $30  to  $200. 

1 01.  There  are  two  steps  in  reading  a 
barometer.  In  the  first  place  the  screw  at 
the  bottom  of  the  instrument  must  be 
turned  until  the  surface  of  the 

.  The  two 

mercury  in  the  cistern  has  been  steps  in 
brought  to  the  end  of  the  scale  or  Beading  a 

.  .         barometer. 

to  the  end  of  the  ivory  point 
which  serves  as  the  end  of  the  scale.  This 
can  be  done  very  exactly  because  the  ivory 
point  is  reflected  in  the  mercury  in  the 
cistern.  The  surface  is  raised  until  the 
point  and  its  image  appear  just  to  touch. 
The  vernier  is  then  set  tangential  to  the 
top  of  the  mercury  column  and  the  scale 
reading  observed.  There  are  three  correc- 
tions to  be  applied  to  this  reading  of  the 
length  of  the  mercury  column:  (1)  the 
meniscus  correction;  (2)  the  tern- 

ine  tnree 
perature  correction ;    (3)  the  grav-  corrections 

ity    correction.       Since    mercury 
does  not  wet  glass,  capillary  ac- 
tion will  depress  the  mercury  column  and 
give  it  a  rounded  top,  called  the  meniscus, 
as  shown  in  Fig.  51.     In  reading  a  barom- 
e         the    zero    of    the    vernier    is    placed 

FIG.  50.  —  Cross-section  of   the 

tangential    to   the    upper    surface    of    the .  em  of  a  Barometer. 


to  be 
applied. 


118  METEOROLOGY 

meniscus.  A  correction  must  thus  be  applied  in  order  to  determine 
what  the  reading  would  be  if  capillary  depression  did  not  exist,  and 
The  menis-  ^ne  mercury  column  were  cut  square  across  in- 
cus correc-  stead  of  rounded.  This  correction  is  usually 
applied  by  the  maker  by  moving  the  scale  the 
proper  amount.  If  this  has  not  been  done,  the  correction 
is  usually  combined  with  the  temperature  correction  and 
furnished  by  the  maker  of  the  instrument  as  a  table  of 
corrections. 

The  pressure  inside  a  warm  room  is  the  same  as  out  of 
doors,  whatever  the  temperature  may  be  there.  Ordinary 
The  tem-  buildings  are  not  sufficiently  air-tight  to  permit 
perature  differences  of  pressure  between  the  inside  and 
correction.  ^g  outside  to  exist  for  more  than  a  few 

FIG.  51.  —  The  T»      , 

Meniscus.  moments.  If  a  barometer  were  taken  from  a  warm  room 
into  the  cold  outside  air,  both  the  mercury  and  scale 
would  contract  and  the  reading  would  become  different,  although  the 
pressure  would  be  the  same.  It  is  necessary,  therefore,  to  reduce  all 
readings  of  a  barometer  to  a  standard  temperature.  32°  F.  or  0°  C. 
are  considered  standard  temperatures,  and  the  accompanying  table 
gives  the  corrections  to  be  applied  to  a  mercury  barometer  with  a 
brass  scale  for  various  temperatures  and  pressures. 

Temp.  F.      0°       10°      20°      30°      40°      50°      60°      70°      80°      90°       100° 

Pressure 

in 
Inches 

26  +0.068  +0.020         -0.027         -0.074        -0.121  -0.167 

+  0.044    -  0.003    -  0.050    -  0.097   -  0. 144 

27  +0.070  +0.021          -0.028          -0.077        -0.125  -0.174 

+  0.046         -0.003          -0.052         -0.101        -0.150 

28  +0.073  +0.022          -0.029          -0.080        -0.130  -0.180 

+  0.047          -0.003          -0.054          -0.105        -0.155 

29  +0.076  +0.023          -0.030          -0.082        -0.135  -0.187 

+  0.049          -0.004          -0.056          -0.109        -0.161 

30  +0.078  +0.024          -0.031          -0.085        -0.139          -0.193 

+  0.051          -0.004          -0.058          -0.112        -0.166 


31       +0.081  +0.024          -0.032          -0.088        -0.144          -0.200 

+  0.053         -0.004         -0.060         -0.116        -0.172 


PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE     119 

The  value  of  gravity  is  not  the  same  at  all  points  on  the  same  level  sur- 
face.    Thus  a  column  of  mercury  of  the  same  length  would  The  gravity 
not  give  the  same  pressure  at  all  points  on  a  level  surface.    It  correction, 
is  necessary,  therefore,  to  reduce  the  readings  to  a  standard  value  of 
gravity.      The  value  of  gravity  at  45°  north  latitude  is  considered 
standard,  and  the  accompanying  table  gives  the  corrections  to  be  ap- 
plied for  various  latitudes. 

Latitude         90°      80°      70°      60°      50°      40°  ,    30°      20°      10°      0° 

+  0.07  +0.04  -0.01  -0.06          -0.08  inches 

Correction  +  0.08  +  0.06  +  0.01  -  0.04  -  0.07 

A  good  mercury  barometer  will  indicate  the  pressure  accurately  to  the 
hundredth  of  an  inch,  and  will  give  a  fair  approximation  to  the  thou- 
sandth.    There  are,  however,  several  sources  of  error  which  Accuracy 
depend  upon  the  accuracy  of  construction.     Some  of  these  and  sources 
are  the  accuracy  of  the  scale,  the  correct  adjusting  of  the  ivory  c 
pointer,  the  purity  of  the  mercury,  the  excellence  of  the  vacuum  above 
the  mercury  column,  etc. 

There  are  various  forms  of  mercury  barometers.     A  modified  form  is 
sometimes  used  on  shipboard ;  and  other  special  forms,  such  as  the  siphon 
barometer,  and  others,  have  been  devised.     For  a  full  treat-  Modified 
ment  of  these  the  reader  must  be  referred,  however,  to  special  forms  of  the 
treatises  on  this  subject.1 

102.    The  aneroid  barometer.  —  The  aneroid  barometer,  as  the  name 
implies,  is  a  fluidless  barometer,  and  is  thus  much  more  portable  than  the 
mercury  barometer.     It  was  invented  by  Vidi  in  1848.     It 
consists  essentially  of  a  so-called  vacuum  box  about  an  inch  Of  an  aner- 
and  a  half  in  diameter  and  one  quarter  inch  thick,  made  of  oid  barom- 

eter. 

German  silver  with  corrugated  top  and  bottom.  The  air  has 
been  exhausted  and  it  is  hermetically  sealed.  It  is  kept  from  collapsing 
by  a  strong  leaf  spring  which  extends  over  the  vacuum  box.  If  the  pres- 
sure increases,  the  box  is  pressed  together  against  the  action  of  the  spring; 
and  conversely,  if  the  pressure  decreases,  the  elasticity  of  the  spring  causes 
the  box  to  expand  slightly.  These  small  motions  are  magnified  by  a  sys- 
tem of  levers  and  communicated  to  a  pointer  which  moves  over  a  dial 
which  is  graduated  to  inches  to  correspond  to  a  mefrcury  barometer. 
Figures  52  and  53  represent  an  aneroid  barometer  and  its  internal  con- 
struction. 

1  Both  the  English  and  metric  barometric  scales  are  used.     Sometimes  both  are  put  on 
the  same  barometer.     In  Appendix  III  a  graphical  comparison  of  the  two  scales  is  given. 


120 


METEOROLOGY 


FIG.  52.  —  An  Aneroid  Barometer. 


FIG.  53.  -T- The  Internal  Construction  •  '    .      \M<>roid  Ham:' 


PRESSURE  AND   CIRCULATION  OF  THE  ATMOSPHERE     121 


The  meniscus  and  gravity  corrections  do  not  exist  in  connection  with 
the  aneroid  barometer.     The  instrument  is,  however,  slightly 
affected  by  temperature.     In  order  to  compensate  for  tern-  ?°wthe 
perature  changes  a  small  amount  of  air  is  often  left  by  the  tions  are 
maker  in  the  vacuum  box,  and  such  an  instrument  is  usually 
marked  with  the  word  "  compensated  "  on  its  face. 

An    aneroid    barometer    is    at    best    an  inaccurate    instrument    as 
compared  with  the   mercurial  barometer.     The   great   advantage  lies 
in^tsjortability.     If  it   is   not  jarred  unduly  and  is  fre-  The  accu_ 
quently  compared  with  a  mercury  standard,  its  indications  racy  of  an 
can   be   trusted   to   a   tenth  of   an   inch  and  may  give  a  a 
fair  approximation    to    the  hundredth.     The  cost  of    a  good  aneroid 
barometer  varies  from  $5   to   $40.     The  words   fair,   storm,   change, 
rain,  very  dry,  and  the  like,  often  found  on  the  face  of  an  instrument, 
are  meaningless. 


FIG.  54.  —  The  Richard  Freres  Barograph. 

103.   Barographs.  —  For  many  purposes  it  is  desirable  to  have  a  con- 
tinuous record  of  barometric  pressure  and  the  instrument  for  keeping 
this  continuous  record  is  called  a  barograph.     The  Richard  Descri  tion 
Freres  form  of  barograph  (Fig.  54)  is  the  one  ordinarily  used  Of  the  Rich- 
by  the  U.  S.  Weather  Bureau  and  the  one  commercially  on 
the  market.     It  consists  of  a  battery  of  from  six  to  ten  of  the 
vacuum  boxes  of  the  aneroid  barometer  placed  one  above  the  other.     The 


122 


METEOROLOGY 


reason  for  the  large  number  of  vacuum  boxes  is  to  lessen  the  effect  of  the 
irregularities  in  any  one,  and  to  make  the  instrument  more  sensitive. 
The  motion  of  these  vacuum  boxes  is  communicated  by  a  system  of 
levers  to  an  arm  which  carries  the  V-shaped  trough  which  contains  the 
non-freezing  glycerine  ink.  As  the  pressure  changes  this  pen  moves  up 
and  down.  The  details  in  the  construction  of  the  pen  and  recording 
mechanism  have  been  fully  described  in  connection  with  the  thermo- 
graph in  section  68. 

There  are  other  more  accurate  forms  of  barographs  in  use  for  the  pur- 
pose of  research  and  in  special  observatories.  The  reader  must  again  be 
referred  to  special  treatises  on  the  subject  for  the  full  description  of  these. 
104.  Other  so-called  barometers.  In  addition  to  the  barometers 
just  described,  there  are  two  instruments,  usually  called  barometers, 
±\  Description  which  deserve  a  passing  mention, 
of  the  mouth  One  is  hardly  more  than  a  scientific 
iter*  toy  and  the  other  merely  masquer- 
ades under  the  name  of  barometer.  The  one, 
to  translate  its  German  name,  is  called  a 
"  mouth-barometer "  and,  as  represented  in 
Fig.  55,  consists  of  a  bulb  usually  filled  with 
a  colored  liquid  which  does  not  readily  evap- 
orate. The  bulb  is  attached  to  a  graduated 
stem  with  a  moderately  small  bore.  The  fluid 
does  not  entirely  fill  the  bulb,  but  an  air  space 
is  left  and  the  stem  protrudes  into  the  bulb  in 
such  a  way  that  the  air  which  constitutes  this 
bubble  cannot  enter  the  stem  in  any  position  of 
the  instrument.  The  size  of  the  air  bubble 
and  thus  the  height  of  the  fluid  in  the  stem 
depends  upon  the  temperature  and  pressure.  If  the  temperature  could 
be  kept  constant,  the  height  of  the  fluid  in  the  stem  would  depend 
upon  the  pressure  alone  and  the  instrument  would  thus  be  a  barom- 
eter. Now  the  temperature  of  the  human  body  is  remarkably  con- 
stant, and  if  the  bulb  of  the  instrument  is  held  in  the  mouth,  it  can 
be  assumed  that  the  temperature  is  always  the  same.  Such  an  instru- 
ment is  cheap  and  very  portable,  and,  when  calibrated  in  terms  of  a 
mercurial  barometer,  it  will  give  results  comparable  with  those  ob- 
tained with  an  aneroid  barometer. 

The  instrument  which  merely  masquerades  under  the  name  of  barom- 
eter is  usually  designated  by  its  makers  as  "  the  cottage  barometer  " 


FIG.  55.  —  The  Mouth- 
Barometer. 


:\ 


PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE     123 

or  "  the  signal  service  barometer  "  or  sometimes  as  "  the  chemical 
weather  glass."  It  is  a  sealed  glass  tube  about  six  or  eight  inches 
long  and  half  an  inch  in  diameter  filled  with  a  clear  liquid 
which  has  in  it  a  flocculent,  sometimes  partly  crystalline,  tionof^he^ 
substance.  The  amount  of  this  substance,  its  appearance,  composition 
and  position  in  the  tube  are  supposed  to  indicate  the  com- 
ing  weather;  and  it  is,  of  course,  advertised  as  an  infallible  chemical 
guide  to  the  coming  weather.  It  is  usually  mounted  in  the 
same  case  with  a  thermometer  and  sold  for  50  cents  or  much 
less.  The  actual  composition  and  action  of  the  instrument  is  this : 
The  clear  fluid  is  nearly  always  alcohol  and  the  substance  in  it  consists 
of  equal  parts  of  nitrate  of  potash,  camphor,  and  ammonium  chlorid. 
More  of  this  mixture  has  been  added  than  the  alcohol  can  dissolve  and 
the  excess  appears  in  the  tube  as  the  solid  substance  in  the  clear  liquid. 
It  is  a  thick  tube  and  is  hermetically  sealed  so  that  it  is  not  affected 
in  the  least  by  changes  in  pressure  and  thus  is  no  barometer  at 
all.  It  is  affected  simply  by  temperature  changes.  As  the  tem- 
perature rises,  more  of  the  substance  goes  into  solution.  As  the  tem- 
perature falls,  more  of  the .  substance  must  come  out  of  solution 
and  appear  as  solid.  Now  the  rapidity  of  temperature  changes,  un- 
equal temperatures  on  different  sides  of  the  tube,  the  direction  and  the 
amount  of  the  light  falling  upon  it,  may  make  a  difference  in  the  form, 
amount,  and  position  of  the  substance  which  comes  out  of  solution  as  the 
temperature  drops,  and  this  probably  accounts  for  the  varied  appear- 
ances of  the  instrument.  Now,  temperature  changes  alone  are  no  in- 
dicator of  the  kind  of  weather  that  is  coming  or  of  the  characteristics  of 
a  coming  storm.  Thus  this  instrument  is  neither  a  barometer  nor  an  in- 
dicator of  the  coming  weather. 

THE  RESULTS  OF  OBSERVATION 

105.    The    observations.  —  At   the   regular    stations   of    the   U.  S. 
Weather  Bureau  the  atmospheric  pressure  is  determined  at  8  A.M.  and 
8  P.M.  by  means  of  a  mercurial  barometer,  and  a  continuous 
barograph  record  is  also  kept.     The  instruments  used  for 
taking  these  observations  are  a  good  mercurial  barometer  taken  at  the 
and  a  Richard  Freres  barograph.     These  instruments  are  BtfreatMsta- 
lo-jsted  ordinarily  in  the  office  part  of  the  Weather  Bureau  tfons  and 
Station  and  not  in  the  thermometer  shelter  or  in  the  open.   mentsS<used. 
They  are  more  conveniently  and  safely  located  within  the 
building,  and  the  atmospheric  pressure  is  the  same  within  the  build- 


124  METEOROLOGY 

ing  as  out  in  the  open.  No  building  is  sufficiently  air-tight  to  per- 
mit differences  of  pressure  to  exist  for  more  than  a  few  moments  except 
perhaps  in  the  case  of  a  tornado.  No  pressure  observations  are  required 
of  the  cooperative  and  special  stations. 

1 06.  Normal  hourly,  daily,  monthly,  and  yearly  pressure.  —  These 
normals  of  pressure  are  computed  in  exactly  the  same  way  as  the  corre- 

.   spending  temperature  normals.     In  order  to  obtain  a  good 

The  method  111  i 

of  comput-  normal  hourly  pressure,  a  barograph  record  for  at  least  twenty 
ing  pressure  years  is  necessary.  If,  then,  for  any  given  date  the  pressures 
at  any  given  hour  for  the  last  twenty  years  are  averaged,  the 
result  would  be  the  normal  pressure  at  that  hour  for  the  given  day.  In 
this  way  the  normal  pressure  for  every  hour  of  every  day  in  the  year 
might  be  determined.  As  a  matter  of  fact,  the  days  in  a  month  are  usu- 
ally grouped  together,  and  thus  the  normal  hourly  pressures  for  a  Janu- 
ary day  or  a  February  day,  etc.,  are  determined.  If  the  average  pressure 
for  a  day  is  to  be  determined,  the  barograph  record  is  almost  always  used. 
One  half  of  the  pressure  observed  at  8  A.M.  and  8  P.M.,  one  third  of  the 
pressure  observed  at  7  A.M.,  2  P.M.,  and  9  P.M.,  would  give  a  rough  approx- 
imation to  the  average  daily  pressure.  Normals  of  pressure  are  not  or- 
dinarily computed  for  any  station  where  a  barograph  record  has  not  been 
kept  for  several  years. 

107.  Diurnal,    annual,    and    irregular    variation.  —  If    the    normal 
hourly  pressures  are  plotted  to  scale,  the  resulting  graph  represents  the 

.  .  diurnal  variation  in  pressure.  If  these  normals  are  not 
of  the  daily  available,  a  fairly  good  idea  of  the  characteristics  of  the 
variation  in  diurnal  variation  can  be  formed  by  considering  the  change 
in  pressure  on  some  day  when  the  other  meteorological 
elements  have  been  as  normal  as  possible.  Figure  56  illustrates 
a  series  of  days  during  which  the  remaining  elements  remained  un- 
usually normal.  The  general  characteristics  of  the  daily  variation  are 
these :  the  chief  maximum  usually  occurs  at  about  10  in  the  morning, 
the  chief  minimum  at  4  in  the  afternoon,  a  secondary  maximum  at  10  in 
the  evening,  and  a  secondary  minimum  at  4  in  the  morning.  The  pres- 
sure is  thus  subject  to  a  double  oscillation  in  the  course  of  a  day.  The 
amplitude  or  amount  of  the  daily  variation  is  always  small,  never 
amounting  to  more  than  0.2  of  an  inch,  and  in  many  places  being  of  ten  less 
than  a  tenth  of  this.  It  varies  somewhat  with  the  time  of  year,  being 
usually  greater  in  summer  and  somewhat  less  in  winter.  It  varies  also 
with  latitude,  its  greatest  values  being  found  in 'the  equatorial  regions, 
while  the  amount  grows  steadily  less  with  the  higher  latitudes.  It  is 


PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE      125 


also  somewhat  less  on  cloudy 
days  than  on  days  with  plenty 
of  sunshine.  It  is 

also  somewhat  varies^i&nt 
greater  for  interior  the  season, 
stations  than  for  ^on^tcT 
coast  or  island 
stations.  For  interior  stations 
the  secondary  maximum  and 
minimum  become  less  promi- 
nent, while  for  coast  and 
island  stations  the  two  are 
of  equal  prominence,  or  the 
night  maximum  and  minimum 
may  even  become  the  most 
important.  Elevation  also 
plays  an  important  part  in 
the  time  of  occurrence  of  the 
maximum  and  minimum. 

Approximate  values  of  the 
amplitude  or  amount  of  the 
daily  variation  for  various 
places  are  here  _ 

*\  The  amount 

given  :         Calcutta,    and  charac- 

India,      lat.      24°,  teristics  of 

1    the  varia- 

0.116;  Greenwich,  tion  at  dif- 
lat.  52°,  0.020; 
Dublin,  0.020;  St. 
Petersburg,  0.012 ;  Fort  Con- 
ger, lat.  83°,  0.010;  Yuma, 
Arizona,  0.129;  San  Antonio, 
0.117;  Denver,  0.079;  Al- 
bany, 0.074 ;  St.  Louis,  0.068  ; 
Philadr  Ma,  0.061;  San 
Franci  0.052 ;  Bismarck, 
0.038;  k  %  0.014.  Figure 
57  represents  graphically  the 
daily  variation  at  Mexico 
City,  San  Francisco,  St. 
Louis,  New  York,  and  Sitka, 

/  (? 


ffent 

places. 


126 


METEOROLOGY 


Alaska.     By  contrasting  the  values  here 


Sitka  Alaska 


(Lat.57°3) 


RE  Due :  OT 


30.020,   23456789  1°'J00jL  2345    67 
30.16' 
80.14 

80.12 

30.10 
80.08 
80.06 
80.04 
30.02 
30.00 
29.98 
29.96 
29.94 

o.oe' 

0.04 
0.02 
0.00 
0.02 
0.04 
0.06 


10Moo.l 


m 

3   4  5  e 


(Lal',19-26; 


loTT' 


56789   101112 


FIG.  57.  —  The  Diurnal  Variation  in  Pressure  at 
Sitka,  New  York,  St.  Louis,  San  Francisco,  New 
Orleans,  and  Mexico  City. 


by  moving 
waves  of 
higher  and 
lower 
pressure. 


given  for  various  places,  the 
truth  of  the  foregoing  state- 
ments as  to  the  amount  of 
the  oscillation  may  be 
tested. 

Since  the  local  time  of 
occurrence  of  the  maxima 
and  minima  is  approxi- 
mately the  same  for  all 
places,  the  diurnal  changes 
in  barometric  pressure  may 
be  thought  of  as 

The  change     produced      by 
in  pressure 

considered  waves  of  higher 
as  caused  ancj  iower  pres- 
sure which  move 
westward  from 
the  Atlantic 
Ocean,  cross  the 
continent,  and  pass  off  into 
the  Pacific  Ocean.  The 
location  and  height  of  these 
waves  have  been  computed 
by  Dr.  Oliver  O.  Fassig  for 
each  hour  of  the  day  for 
the  western  hemisphere. 
The  results  were  found  by 
using  the  daily  variation  in 
pressure,  which  had  been 
determined  from  observa- 
tions at  many  stations  in 
both  North  and  South 
America,  and  drawing  lines 
through  those  places  which 
at  the  hour  in  question 
showed  the  same  departure 
from  the  normal  for  the 
day.  These  charts  will  be 
found  in  Bulletin  No.  31  of 
the  U.  S.  Weather  Bureau 


PRESSURE  AND   CIRCULATION  OF  THE  ATMOSPHERE     127 


or  in  the  Monthly  Weather  Review  for  November,  1901.      Two  of 
them,  for  10  A.M.  and  4  P.M.,  the  times  of  greatest  and  least  pressure, 
are  reproduced  as  Figs.  58  and  59.     The  numbers  here  represent  depar- 
ture from  the  normal  for  the 
day  expressed  as  thousandths 
of  an  inch. 

1 08.  The  cause  of  this  diurnal 
variation  in  pressure  is  not  a 
tide  in  the  atmosphere  caused 
by  either  the  sun  or  the  moon, 
for  if  it  were  both  the  solar 
and  lunar  influence  would  be 
noticed  and  the  corresponding 
periods  detected. 

Temperature  and  the  forma- 
tion of  dew  without 
doubt  play  a  large  TemPera- 


FIG.  58.  —  Diurnal  Barometric  Wave  at  10  A.M., 
75th  Meridian  Time. 


(FASSIG,  U.  S.  Weather  Bureau). 


part  in  causing  this  and  dew  as 

diurnal       variation, 

but    the  exact  way 

in  which  these  two  operate  to  produce  the  result   cannot  be   satis- 

factorily stated.  Due  to  the  temperature  change  alone,  the  maximum 

would  be  expected  at  the  time  of 
least  temperature  or  a  little  later, 
that  is,  at  sunrise  or  a  few  hours 
after.  vThe  minimum  would  be 
expected  at  the  time  of  highest 
temperature,  that  is,  from  two 
to  four  in  the  afternoon,  or 
somewhat  later.  Due  to  the 
formation  of  dew  the  largest 
amount  of  moisture  is  present 
in  the  atmosphere  in  the  late 
afternoon  and  the  least  amount 
at  the  time  of  sunrise.  The 
interaction  of  these  two  influen- 

ces    of    temperature    and     dew, 
,    -          n 

however,  cannot  account  for  all 
the  characteristics  which  have 
De-en    '  ••t-rveu  in  oormoptmn  with  the  daily  variation. 


FIG.  59.  —  Diurnal  Barometric  Wave  at  4  P.M. 
75th  Meridian  Time. 
,  U.  S.  Weather  Bureau.) 


128  METEOROLOGY 

By  means  of  harmonic  analysis  1  this  daily  oscillation  or  variation  in 
pressure  may  be  separated  into  two  components,  one  with  a  daily  or 
twenty-four  hour  period,  and  another  with  a  half-daily  or 
tfoxftato  two  twelve-hour  period.  If  this  is  done  for  many  stations,  choos- 
components  ing  those  in  equatorial  regions  as  well  as  those  in  higher  lat- 
ent*1 eriods  tudes,  those  in  the  interior  of  the  continents  as  well  as  on  the 
seashore  or  on  islands,  those  located  in  valleys  as  well  as  on 
mountain  sides  and  on  mountain  tops,  it  will  be  found  that  the  twelve- 
hour  periodic  oscillation  is  remarkably  regular  and  shows  essentially 
the  same  characteristics  everywhere,  while  the  twenty-four  hour  oscilla- 
tion proves  to  be  due  almost  entirely  to  local  causes  and  is  very  different 
at  different  stations.  Some  have  even  gone  so  far  as  to  ascribe  a  cosmic 
cause  to  this  twelve-hour  oscillation,  that  is,  to  ascribe  it  to  some  influence 
outside  of  the  earth  itself.  It  has  also  been  thought  that  this  twelve- 
hour  oscillation  may  be  simply  a  free  oscillation  of  the 
corresponds  atmosphere  as  a  whole,  considering  it  as  an  elastic  body- 
to  nothing  This  twelve-hour  periodic  oscillation  is  nearly  as  large  as  the 
twenty-four  hour  oscillation,  and  whenever  this  is  the  case 
and  no  reason  in  nature  for  a  twelve-hour  period  can  be  found,  it  is  ques- 
tionable whether  this  separation  into  two  components  can  be  considered 
as  corresponding  to  anything  in  nature,  or  may  not  be  merely  a  device  of 
the  mathematician. 

109.  The  annual  variation  in  pressure  may  be  found  by  plotting  to 
scale  the  normal  monthly  pressures.  If  this  is  done,  it  will  be  noted  for 
the  interior  of  continents  that  the  pressure  is  somewhat  higher 
variation^  /knan  ^ne  vearly  normal)  in  winter  and  less  in  summer ;  for  the 
pressure  oceans  the  opposite  is  true,  the  pressure  being  somewhat 
cause*8  higher  in  summer  and  lower  in  winter.  The  cause  for  this 

is  not  far  to  seek.  During  the  winter,  as  was  seen  in  con- 
nection with  temperature  anomalies,  the  continents  are  colder  than  the 
surrounding  oceans.  That  means  that  the  air  is  colder,  denser,  and 
heavier,  and  thus  an  increase  in  pressure  during  the  winter  is  to  be 
expected.  During  the  summer  the  contrary  is  true.  The  continents 
are  warmer  than  the  surrounding  oceans,  the  air  is  expanded  and 
light,  and  the  atmospheric  pressure  is  correspondingly  lower.  This 
variation,  again,  does  not  amount  ordinarily  to  more  than  a  few  tenths 
of  an  inch. 

The  irregular  variations  in  pressure,  particularly  in  the  temperate 
zones,  are  far  larger  than  any  of  the  periodic  variations.     Variations  in 

1  For  an  illustration  see  Monthly  Weather  Review,  November,  1906. 


PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE     129 

pressure  of  an  inch  or  two  follow  each  other  in  rapid  succession,  and  at 
irregular  intervals. 

no.  Barometric  data. — In  addition  to  the  normals  described  above, 
but  few  results  are  computed  for  the  various  stations  from  the  Ooserva- 
tions  of  pressure.  Among  those  sometimes  computed,  the 
following  may  be  mentioned:  (1)  The  daily  range.  From  data  which 
the  barograph  record  the  highest  and  the  lowest  pressure  for  may  be 
the  day  may  be  determined.  The  difference  between  these  f^nfthe  ob- 
gives  the  daily  range.  From  these  values  of  range,  normal  servations  of 
values  for  the  day,  for  the  month,  and  for  the  year  may  be  P 
computed  in  the  regular  way.  (2)  Absolute  range  for  the  months 
and  the  year.  By  the  absolute  range  in  pressure  for  the  months  and 
for  the  year  is  meant  the  difference  between  the  very  highest  and 
very  lowest  pressures  observed  during  the  period  of  time  in  question. 
(3)  Frequency  of  irregular  variations.  The  interval  of  time  between 
successive,  marked,  irregular  variations  in  pressure  may  be  deter- 
mined from  the  records  of  the  barograph,  and  the  normal  frequency 
of  these  irregular  fluctuations  can  thus  be  determined  for  the  vari- 
ous months  and  for  the  year  as  a  whole.  (4)  Magnitude  of  irregular 
fluctuations.  The  magnitude  of  each  irregular  fluctuation  can  be  deter- 
mined. If  these  are  averaged  in  the  regular  way,  the  normal  amount  of 
•the  irregular  fluctuations  for  the  various  months  and  for  the  year  as  a 
whole  may  be  determined. 

None  of  these  results  is  of  any  great  interest  or  of  far-reaching  practi- 
cal importance. 

At   the  regular  stations   of   the  U.  S.  Weather  Bureau  there   are 
three  tables  for  pressure  which  are  kept  up  to  date.     These  contain  : 

(1)  Highest  and  lowest  in  inches  and  hundred ths  (reduced  to  sea  level) 
(the  data  are  given  for  each  month  and  the  year  as  a  whole) . 

(2)  Mean  station  and  absolute  monthly  range  (inches  and  hundredths) 
(the  data  are  again  given  for  each  month  and  the  year  as  a  whole). 

(3)  Mean  hourly  pressure  (inches  and  hundredths). 

THE  VARIATION  WITH  ALTITUDE 

in.    Reduction  to  sea  level.  — Since  the  pressure  of  the  atmosphere 
decreases  with  elevation,  in  order  to  compare  pressures  observed  at  differ- 
ent ^cations,  it  is  necessary  to  take  account  of  the  elevation.  The  old 
rpl  s  was  formerly  done  by  determining  tha  difference  be-  way- 
t,,*  ecu*  the  observed  pressure  and  the  normal  yearly  pressure  for  the  sta- 


130  METEOROLOGY 

tion  in  question.  These  differences  could  then 'be  compare^:  and  thus 
the  conclusion  reached  as  to  which  place  had  the  higher  or  lower  pressure. 

At  the  present  time  all  pressure  observations  are  reduced  to  the 
same  level;  that  is," to  what  the  pressure  would  be  if  the  observation 
had  been  made  at  sea  level.  In  order  to  make  this  correc- 
way  iTto  **  *  ^lon)  ^e  weight  of  a  column  of  air  reaching  from  the  station 
add  the  in  question  to  sea  level  must  be  added  to  the  observed 
the*  column  Pressure-  Now  the  weight  of  this  column  of  air  is  not  a 
of  air  reach-  constant,  but  varies  with  the  pressure,  with  the  tempera- 
sea  leveL*0  ^ureJ  and  slightly  with  the  moisture  in  it.  If  the  pressure  is 
high,  then  the  air  is  dense  and  heavy.  If  the  temperature 
is  high,  it  is  correspondingly  expanded  and  light.  Moist  air  is  lighter 
than  dry  air,  when  other  conditions  are  the  same.  In  order  to  deter- 
mine the  value  of  this'  correction,  elaborate  tables  are  ordinarily  used. 
The  best  set  of  tables  is  probably  the  Smithsonian  Meteorological  Tables, 
published  at  Washington  and  carrying  the  number  1032  in  the  Smithso- 
nian Miscellaneous  Collections.  In  this  volume  are  contained  the  neces- 
sary tables  for  reducing  barometric  pressure  to  sea  level,  together  with 
the  explanation  and  derivation  of  the  formulas  used.  In  Appendix  IV 
a  short  table  is  given,  but  this  is  intended  not  to  serve  for  the  reduction 
of  well-taken  observations,  but  simply  to  give  a  rough  idea  of  the  amount 
of  the  correction  in  a  few  instances. 

In  a  certain  sense  this  reduction  to  sea  level  is  fictitious.  The  mass  of 
a  column  of  air  reaching  down  to  sea  level  is  added.  The  mass  of  this 
column  of  air  is  quite  different  from  what  it  would  be  if  there  were  no 
mountain  or  plateau  and  the  station  were  actually  located  at  sea  level. 
For  this  reason,  the  older  method  by  means  of  differences  from  normal 
might  be  better,  particularly  in  the  construction  of  weather  maps. 

112.  Barometric  determination  of  altitude.  —  Since  barometric  pres- 
sure depends  upon  elevation,  it  is  possible,  by  observing  the  barometric 
pressure  at  two  different,  near-by  stations  to  determine  their 
taneous  ob-  difference  °f  elevation.  In  order  to  make  a  precise  determina- 
servations  tion,  simultaneous  observations  of  pressure  and  temperature 
and  tenTer  are  necessarv-  The  moisture  at  the  two  stations  is  also  some- 
ature  at  the  times  determined.  The  practical  details  in  securing  these 
two  sta-  simultaneous  observations  will  at  once  suggest  themselves. 

tions  must  .  ° 

be  obtained.       If  two  observers  and  a  sufficient  number  of  instruments  are 

available,  it  may  be  easily  arranged  to  take  the  observations 

at  some  definitely  appointed  hour.      For  the  pressure  observations 

at  the  higher  elevation,   particularly  if  the  station  is  located  on  a 


PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE     131 


mountain  top,  the  aneroid  barometer  is  ordinarily  used.  This  is  not  as 
accurate  as  a  mercurial  barometer,  but  it  is  much  more  portable  and 
convenient  to  carry.  The  temperature  is  ordinarily  deter- 
mined by  means  of  a  sling  thermometer.  If  a  ventilated 
thermometer  is  available,  it  will  give  far  better  results.  It  curing  the 
is  ordinarily  too  troublesome  to  construct  or  to  carry  up  a  ^ons^*" 
thermometer  shelter  for  protecting  an  ordinary  thermometer. 
If  two  observers  and  the  proper  apparatus  are  not  available,  fair  results 
can  be  obtained  by  taking  observations  at  the  base  station,  both  before 
and  after  the  ascent,  and  then  determining  by  interpolation  the  proper 
value  of  pressure,  temperature,  and  possibly  moisture,  at  the  time  when 
the  observations  were  taken  at  the  summit.  If  the  aneroid  barometer  is 
used,  it  should  be  compared  both  before  and  after  the  ascent  with  a  mer- 
curial standard,  in  order  to  check  its  accuracy  and  to  be  sure  that  it 
has  not  become  deranged  by  the  jars  of  transportation. 

113..  Let  BQ  and  T0  indicate  the  pressure  and  temperature  at  the  base 
station,  that  is,  at  the  station  whose  elevation  is  known,  and  let  Bl  and 
Tl  indicate  the  pressure  and  temperature  at  the  summit  or  at  the  station 
for  which  the  elevation  is  to  be  determined. 

The  first  and  simplest  method  of  computing  the  difference  in  eleva- 
tion is  to  take  no  account  of  the  temperature  and  moisture,  and  allow 
90  feet  for  each  tenth  of  an  inch  of  pressure  difference.     It 
has  been  found  that  the  general  average  for  all  conditions  There  are 
of  pressure  and  of  temperature  and  of  moisture  is  about  90  methods  of 
feet  to  a  tenth  of  an  inch.     Thus,  if  the  difference  in  pres-  computing 

.  .  .  «,          a  difference 

sure  between  two  stations  is  two  inches,  it  means  a  differ-  in  elevation 
ence   in  elevation  of  approximately  1800  feet.     Whenever  from  the  ob- 

servanons 

aneroid  barometers   are  provided  with  a  separate  scale  for  made. 
indicating  elevations,  it  is  always  divided  so  that  a  tenth 
of  an  inch  corresponds  to  90  feet. 


TEMP.  F. 


Pressure 

0° 

10° 

20° 

30° 

40°. 

50° 

60° 

70° 

80° 

90° 

100° 

22  inches 

111 

114 

116 

119 

122 

124 

127 

130 

132 

135 

138 

24  . 

101 

104 

106 

109 

111 

114 

116 

119 

121 

124 

126 

26  , 

94 

96 

98 

101 

103 

105 

107 

110 

112 

114 

116 

28  . 

87, 

89 

91 

93 

95 

98 

100 

102 

104 

106 

108 

29  . 

84 

86 

88 

90 

92 

94 

96 

98, 

100 

102  v 

104 

29.5 

83 

85 

87 

89 

91 

93 

95 

97 

99 

101 

aos 

30.0 

81 

83 

85 

87 

89 

91 

93 

95 

97 

99 

1P1 

30.5 

80 

82 

84 

86 

88 

90 

92 

94 

96 

98 

100 

31.0 

78 

xn 

83 

84 

86 

88 

90 

92 

94 

96 

98 

METEOROLOGY 


A  better  method  of  computing  the  difference  in  elevation  is  to  take 
from  the  accompanying  table  the  actual  number  of  feet  which  correspond 
to  a  tenth  of  an  inch  difference  in  pressure  for  the  average  pressure  and 
the  average  temperature  at  the  two  stations. 

The  third  way  of  computing  the  difference  in  elevation,  which  is  per- 
haps a  little  more  accurate  than  the  use  of  the  table  given  above  is  by 
means  of  the  following  formula :  — 


Difference  in  elevation 


55,761  +  117 


-  60)  } 

/  J 


This  formula,  as  will  be  seen,  contains  nothing  but  the  observations  of 
pressure  and  temperature  at  the  two  stations. 

If  the  most  exact  possible  use  of  the  observations  is  to  be  made  in 
determining  the  difference  in  elevation,  the  elaborate  Smithsonian 
Meteorological  Tables  referred  to  above  must  be  used.  The  formula  is 
there  derived,  the  necessary  tables  are  given  and  their  use  explained. 

114.  Variation  with  altitude. — The  accompanying  table  shows  ap- 
proximately, for  average  conditions,  the  barometric  pressure  which  cor- 
The  baro-  responds  to  various  elevations.  These  values,  however,  cor- 
metric  pres-  respond  to  average  conditions,  and  are  not  exact  enough  for 
coirerponds  ^ne  determination  of  elevation,  particularly  as  the  baro- 
to  various  metric  pressure  at  any  given  station  is  constantly  changing. 

altitudes.  • 


BAROMETRIC  PRESSURE 

ALTITUDE 

BAROMETRIC  PRESSURE 

ALTITUDE 

30  inches 

Ofeet 

21  inches 

9,300  feet 

29  inches 

910  feet 

20  inches 

10,600  feet 

28  inches 

1,950  feet 

18  inches 

13,200  feet 

27  inches 

2,820  feet 

16  inches 

16,000  feet 

26  inches 

3,800  feet 

15  inches 

3.6  miles 

25  inches 

4,800  feet 

1\  inches 

6.8  miles 

24  inches 

5,900  feet 

3|  inches 

10.2  miles 

23  inches 

7,000  feet 

T^y  inches 

24.1  miles 

22  inches 

8,200  feet 

THE  DISTRIBUTION  OF  PRESSURE  OVER  THE  EARTH 

115.    Construction    of    isobaric    charts.  —  Isobaric    charts    are    con- 
structed by  making  use  of  the  normals  of  pressure  which 
of  construct-  have  been  determined  for  various  stations  .in  all  parts  of 
ing  an  iso-      the  world.      These  normals  of  pressure  must  first  be  reduced 
to  sea  level,  and  they  are  then  charted  on  a  map,  and  lines 


PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE      133 

are  drawn  connecting    those    stations  which    have    the    same  value. 
These  lines  are  called  isobaric  lines,  or  simply  isobars.1 

1 1 6.  Isobars  for  the  year.  —  Chart  X  shows  the  isobars  for  the  world 
for  the  year.     It  is  the  normal  annual  pressures  which  have  been  here 
used  for  the  construction  of  the  chart.     The  following  charac-  The  three 
teristics  are  at  once  evident :    (1)   The  equatorial  belt  of  low  character- 
pressure  and  the  belts  of  high  pressure  at  35°  N.  and  30°  S.  b^forthe" 
latitude.     It  will  be  noticed  that  along  the  equator  there  is  a  year  for  the 
belt  of  low  pressure,  the  least'^pfessUre  being  29.80  inches.  world' 
This  belt  is  irregular,  of  varying  width,  and  not  of  the  same  pres- 
sure throughout,  and  it  lies  somewhat  on  the  north  side  of  the  equa- 
tor.    The  belts  of  high  pressure  are  also  not  of  the  same  pressure  through- 
out. .  The  one  at  35°  N'.  has  its  areas  of  highest  pressure  over  the  Pacific 
Ocean,  the  Atlantic  Ocean,  and  central  Siberia.     In  passing  over  Siberia 
it  lies  far  north  of  .the  equator.     The  southern  belt  of  high  pressure  has 
its  peaks  of  pressure  over  Ihe  Pacific  Ocean,  the  South  Atlantic  Ocean,  and 
the  Indian  Ocean.     From  these  two  belts  of  high  pressure,  the  pressure 
diminishes  rapidly  toward  the  poles ;   in  the  southern  hemisphere  quite 
regularly;  in  the  northern,  however,  with  less  regularity.     (2)    Regu- 
larity in  the  southern  hemisphere.     It  will  be  noticed  that  the  low  pressure 
near  the  north  pole  consists  of  two  depressions,  one  in  the  North  Pacific 
near  Alaska,  and  the  other  in  the  North  Atlantic  near  Iceland,  each  with 
a  central  pressure  of  29.70  inches.     In  the  southern  hemisphere,  how- 
ever, the  pressure  drops  much  more  uniformly  and  rapidly  from  the  belt 
of  high  pressure  toward  the  pole.     (3)  Lower  in  southern  hemisphere 
than  in-the  northern  hemisphere.     It  will  also  be  seen  that  the  diminution 
in  pressure  is  much  greater  in  the  southern  hemisphere  than  it  is  in  the 
northern. 

117.  Vertical  section  along  a  meridian.  —  A  study  of  the  distribution 
of  pressure  in  a  vertical  section  along  a  meridian,  as  shown  in  Fig.  60, 
will  prove  instructive.     The  north  and  south  poles  of  the  How  the  30_ 
earth  and  the  equator  are  indicated  by  N.,  S.,  and  E.,  respec-  inch  line  is 
tively.     At  the  equator,  in  .order  to  reach  a  barometric  pres- 
sure of  30  inches,  it  is  necessary  to  go  a  certain  distance  below  sea  level, 
arid  the  same  is  true  at  the  north  and  south  poles.     At  35°  N.  and  30°  S.. 
it  is  necessary  to  ascend  a  certain  distance  above  the  eartti's  surface  in 
order  to  find  the  30-inch  line.     The  heavy  line  marked  30  in  the  diagram 
thus  indicates  in  the  vertical  set  aon  those  places  which  would  have  a 

•pressure  of  30  inches.     In  order  to  locate  the  29-inch  line,  it  is  necessary 

1  From  the  Greek '.  t<ros  —  equal ;  pdpos  =  pressure. 


134 


METEOROLOGY 


15JN 


SEA  LEVEL 
27  »N. 

.30  IN. 

FIG.  60.  —  The    Distribution    of    Pressure    in    a    Vertical 
Section  along  a  Meridian. 


to  ascend  a  certain  distance  above  the  30-inch  line,  on  the  average  about 
900  feet.  At  the  equator,  the  air  is  warm  and  moist,  while  at  the  pole 

How  the  it    is     CO^ 

other  lines  and  COm- 
are  located.  ,  •  , 

paratively 

dry.  As  a  result,  it 
is  necessary  to  ascend 
a  larger  number  of 
feet  at  the  equator  to 
obtain  the  same  dimi- 
nution in  pressure 
than  at  the  pole.  A 
glance  at  the  table  in 
section  113  will  show 
approximately  the 

amount.  The  figures  would  be  about  810  feet  at  the  pole  and  990 
feet  at  the  equator.  As  a  result,  the  isobaric  lines  will  show  a  greater 
and  greater  upward  bulging  at  the  equator  and  drooping  at  the 
poles.  By  the  time  the  27-inch  line  has  been  reached,  the  equatorial 
depression  in  the  30-inch  line  will  have  practically  disappeared,  and, 
with  increasing  elevation,  the  upward  bending  or  convexity  of  the 
isobaric  lines  becomes  more  and  more  pronounced,  as  is  shown  in  the 
figure. 

1 1 8.  Isobaric  surfaces.  —  If  there  were  no  temperature  differences, 
the  pressure  at  sea  level  would  be  everywhere  the  same.  As  it  is, 
Tempera-  the  temperature  diji^uces  cause  the  air  to  be  light  and 
encTesoause  exPan^ec^  at  one^Bice  and  dense  and  contracted  in 
difference  in  another.  As  a  result,  movements  of  air  take  place,  increas- 
pressure.  mg  the  pressure  at  one  point  and  lessening  it  at  another. 

By  an  isobaric  surface  is  meant  a  surface  at  every  point  of  which  the 
pressure  is  the  same.  If  there  were  no  temperature  differences,  the 
Definition  of  surface  of  the  hydrosphere  would  be  an  isobaric  surface, 
an  isobaric  and  all  other  isobaric  surfaces  would  be  concentric  with 
it  and  at  a  certain  distance  above  it.  The  29-inch 
surface,  for  example,  would  be  concentric  with  the  hydrosphere  and 
The  normal  approximately  900  feet  above  it  everywhere.  Temperature 
form.  differences,  however,  warp  and  twist  these  isobaric  surfaces 

so  that  they  are  no  longer  parallel  to  the  hydrosphere.  Whenever  they 
are  so  much  warped  as  to  intersect  the  hydrosphere,  their  intersection 
forms  isobaric  lines  or  isobars.  In  the  centv  of  the  area  of  high  pressure, 


PRESSURE  AND   CIRCULATION  OF   THE   ATMOSPHERE     135 


FIG.  61.  —  Areas  of  High  and  Low  Pressure. 


shown  in  Fig.  61,  it  is  necessary  to  ascend  a  certain  distance  in  order 
to  find  the  29.9-inch  surface,  and  a  slightly  greater  ascent  is  necessary 
in  order  to  reach  the  29.8-inch  surface.     Over  such  an  area  of  high 
pressure,     then,     the 
isobar  i  c  „ 

Their  form 
Surfaces    over  areas 

are  warped  ofhighor 

lowpressure. 
upward 

and  have  the  form  of 
an  inverted  bowl. 
Conversely,  over  an 
area  of  low  pressure 
the  isobaric  surfaces 
are  warped  downward 
and  have  the  appear- 
ance of  a  saucer.  If  the  isobars  for  the  year  are  interpreted  as  being 
the  intersections  of  isobaric  surfaces  with  the  hydrosphere,  it  will  be 
seen  that  over  the  equator  and  at  the  poles  they  are  warped  down- 
ward, whereas  at  35°  N.  and  30°  S.  latitude  they  are  warped  upward. 
From  the  isobaric  lines  the  isobaric  surfaces  can  always  be  constructed. 
119.  Isobars  for  January  and  July.  —  Chart&J^I  and  XII  show  the 
isobars  for  the  world  for  January  and  July.  It  is  the  normal  pressure  £or 
January  and  for  July,  corrected  for  elevation,  which  have  been 

Ine  ooser- 

used  in  the  construction  of  these  charts.     The  same  three  vations  used 
characteristics  which  were  noted  above  in  connection  with  the  f°r  ^ 

charts. 

lines  for  the  year  are  again  evident.     (1)   The  equatorial  belt 
of  low  pressure  and  high  pressure  belts  at  35°  N.  and  30°  S.     (2)  Regular- 
ity in  the  southern  hemisphere.     (3)  Lower  in  the  southern  than  in  the 
northern  hemisphere.     In  addition,   two  new  characteristics  Three  old 
are  to  be  noted.     (1)   The  belts  of  high  and  low  pressure  no  and  two 
longer  remain  continuous  belts,  but  break  up  into  peaks  and  acteristics 
depressions.     Thus,  the  equatorial  belt  of  low  pressure  shows  are  to  be 
depressions  over  South  America,  South  Africa,  and  Austra- 
lia, on  the  January  chart,  and  one  very  pronounced  depression  over 
India  on  the  July  ch?rt.     This  depression  over  India  is  so  pronounced 
that  it  persists  in  the  annual  averages  and  appears  on  the  chart  of  the 
isobaric  lines  for  rhe  year.     On  the  January  chart  the  belt  of  high  pres- 
sure in  the  northern  hemisphere  shows  two  immense  peaks  of  pressure  over 
North  America  and  Siberia.     In  each  case  these  is  a  lip  of  high  pressure 
extending  to\\  ••••«.  and  forming  really  a  small  area  of  high  pres- 


136  METEOROLOGY 

sure  over  the  ocean  to  the  west  of  the  continent.  On  the  July  chart 
the  highs  over  the  continents  have  disappeared,  and  two  peaks  of 
pressure  over  the  Pacific  and  Atlantic  oceans  have  built  up 
break  up*  where  these  small  areas  were  located.  The  southern  belt 
into  peaks  of  high  pressure  shows  three  peaks  of  pressure  _over  the 
sionsdePreS~  South  Atlantic,  South  Pacific,  and  Indian  oceans,  and  these 
peaks  are  the  same  on  both  the  January  and  July  charts. 
In  addition,  a  small  peak  of  high  pressure,  appears  over  South  Africa 
on  the  July  chart.  In  the  north  polar  regions  a  marked  depression 
appears  near  Iceland  on  both  the  January  and  July  charts.  A  decided 
low  appears  on  the  January  chart  near  Alaska,  but  this  does  not  exist 
on  the  July  chart.  There  are  thus  eight  highs  and  six  lows  to  be  con- 
sidered. It  will  be  noticed  in  some  cases  that  the  highs  are  located  on 
the  land  in  winter  and  over  the  oceans  in  summer.  The  reason  for 
these  highs  and  lows  will  be  considered  in  a  later  section. 

(2)   The  belts  of  pressure  migrate  somewhat  with  the  sun.     The  migra- 
tion, however,  is  not  as  great  as   the  migration  of  the  sun,  47°,  or  of 
Thepres-       ^ne  temperature  belts,  which  have  been  considered  before, 
sure  belts       The  pressure     belts     also     lag     behind   the    sun    in    their 
migration. 

120.  Other  pressure  charts.  —  In  addition  to  the  three  charts  which 
have  been  given  and  which  represent  the  isobars  for  the  year,  for  Jan- 
other  pres-     uary,  and  for  July  for  the  whole  world,  the  isobars  for  the 
sure  charts.    various  months  and  for  the  year  for  various  countries,  or 
for  the  world  as  a  whole,  may  be  represented.     In  addition,  all  of  the 
barometric  data  mentioned  in  section  110,  such  as  the  absolute  range 
for  the  months  and  for  the  year,  the  frequency  of  irregular  variations, 
the  magnitude  of  the  irregular  variations,  etc.,  may  be  charted. 

B.   THE    OBSERVATION  AND    DISTRIBUTION    OF   THE   WINDS 

THE  DETERMINATION  OF  THE  DIRECTION,  FORCE,  AND  VELOCITY  OF  THE 

WIND 

121.  Wind  direction,  force,  and  velocity. — Air  in  motion  near  the 
earth's  surface  and  nearly  parallel  to*  it,  is  called  wind.  ler  mo- 
Wind             tions  of  masses  of  air  should  be  spoken  of  as  air     irrents, 

although  this  distinction  is  not  always  recogniz-  In  con- 
things  to  be  nection  with  wind  there  are  three  things  to  be  cki^rmined 

or  measured;  namely,  the  direction,  the  velocity,  ;nd  the 
force  or  pressure. 


PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE     137 

The  wind  is  named  from  the  direction  from  which  it  comes ;  thus  if 
air  moves  from  the  north  toward  the  south,  it  is  spoken  of  as  a  northjvind. 
The  direction  from  which  the  wind  comes  is  called  windward, 
and  the  direction  toward  which  it  goes  is  called  leeward,   ^f^ard 
Whenever  the  wind  direction  changes  steadily  in  a  clockwise  veering  and' 
direction,  as,  for  example,  from  southeast 'through  the  south 
and  southwest  to  west,  it  is  said  to  veer ;  when  the  direction 
changes  steadily  in  a  counterclockwise  direction,  as,  for  example,  from  east 
through  the  northeast  and  north  to  northwest,  it  is  said  to  back.     In 
noting  wind  direction,  eight  points  of  the  compass  are  used ; 
namely,  the  four  cardinal  points,  north,  south,  eafet,  and  west ;   directions 
and  the  four  intermediary  points,  northwest,  southwest,  south-  •?«  rec°g- 
east,  and  northeast. 

^  The  velocity  of  thejadad-atuLthe  force  or  pressure  of  the  wind  are  so 
related  that  when  one  has  been  observed  the  other  can  at  once  be  deter- 
mined. The  relation  between  them  is  such  that  the  pressure,  The  rela_ 
in  pounds  per  square  foot,  is  equal  to  a  constant  multiplied  tion  be- 
by  the  square  of  the  velocity  in  miles  perjiour.  Experimental 
determinations  of  the  value  of  the  constant  have  given  results  the  pressure 
which  vary  from  .004  to  .007.  If  an  average  value  of  the  of  the  wmd* 
constant  is  used,  the  relation  may  be  expressed  by  the  simple  formula, 
P  =  .005  F2.  The  accompanying  table  computed  from  the  above  formula 
gives  the  pressure  in  pounds  per  square  foot  for-  several  wind  velocities. 


VELOCITY  IN  MILES 
PER  HOUR 

PRESSURE  IN  LBS. 
PER  SQ.  FOOT 

VELOCITY  IN  MILES 
PER  HOUR 

PRESSURE  IN  LBS. 
PER  SQ.  FOOT 

0 

0 

50 

12.50 

5 

0.12 

60 

18.00 

10 

0.50 

70 

24.50 

15 

1.12 

80 

32.00 

20 

2.00 

100 

50.00 

25 

3.12 

125 

78.12 

30 

4.50 

150 

112.50 

40 

8.00 

: 

The  value  of  the  constant  depends  to  a  slight  extent  on  temperature, 
pressure,  and  moisture.  \  When  the  temperature  is  high,  or  the  pressure 
is  low,  or  the  moisture  content  is  large,  the  air  is  ieds  dense,  and  thus  the 
pressure  exerted  by  the  moving  air  would  be  smaller.  Conversely,  if  the 
temperature  is  low,  or  the  pressure  is  high,  or  the  moisture  content  is 
small,  the  air  is  denser,  and  thus  the  pressure  exerted  would  be  greater. 


138  METEOROLOGY 

122.  Wind  vane.  —  The  direction  of  the  wind  is  determined  by  means 
of  a  wind  vane,  and  wind  vanes  are  too  well  known  to  need  any  further 
Wind  vane,     description  here.     A  wind  vane  is  often  spoken  of  popularly 
not  weather    as  a  weather  vane,  but  this  is  a  misnomer,  since  the  wind  vane 

indicates  wind  direction  only,  and  not  the  present  or  future 
condition  of  the  weather.  The  Weather  Bureau  form  of  wind  vane  has 
a  tail  made  up  of  two  thin  pieces  of  wood,  making  an  angle  of  about  22° 

with  each  other.    There  are  two  advantages  of  this  form,  since 

TheWeath-  .  .  ° 

er  Bureau       the  wind  vane  is  more  sensitive  in  a  light  wind  and  steadier 

form  of  jn  a  gusty  wind.  The  wind  vane  is  sometimes  attached  to  a 
vertical  rod  'which  extends  down  into  a  building  below  and  is 
there  attached,  perhaps  by  means  of  a  series  of  cog  wheels,  to  a  pointer 
which  moves  over  a  dial  so  that  the  wind  direction  can  be  read  within 
the  building. 

There  are  instruments  for  getting  the  direction  of  air  currents  as  well 

as  of  the  wind.     These  consist  ordinarily  of  a  large  wind  vane  which  in- 

The  direc-      dicates  the  horizontal  direction.    To  this  is  attached  a  second 

tion  of  air       wind  vane  pivoted  about  a  horizontal  axis  so  as  to  be  free 

to  indicate  the  vertical  component  of  the  air  motion. 

123.  Anemoscope.  —  The  purpose  of  an  anemoscope  is  to  give  a  con- 
tinuous record  of  wind  direction.     The  vertical  rod  to  which  the  wind 
The  c  lin-       vane  is  attached  sometimes  reaches  down  into  a  room  below, 
der  form  of     and  a  cylinder  is  attached  to  the  lower  end  of  the  rod.     This 

icope'  cylinder  turns  with  the  rod  and  is  covered  with  a  piece  of 
paper  for  obtaining  the  record.  A  pen  presses  lightly  against  this  paper 
and  is  carried  downward  at  a  slow  and  regular  rate  by  clock-work,  so 
as  to  descend  through  the  length  of  the  cylinder  in  a  day  or,  if  so  con- 
structed, in  a  week.  With  every  change  in  wind  direction  the  cylinder 
turns  underneath  the  pen,  and  thus  a  continuous  record  of  wind  direction 
is  made. 

At  regular  Weather  Bureau  stations  the  wind  direction  is  recorded 
automatically  by  attaching  a  contact  maker  to  the  wind  vane  and  its 
direction  is  then  recorded  electrically  in  the  office.     The 
er  Bureau       contact  maker,  as  shown  diagrammatically  in  Fig.  62,  con- 
form of          gjg^s  of  four  blocks  lettered  N,  S,  E,  and  W,  from  which 
wires  extend  to  four  electro  /magnets  connected  with  the 
recording  drum  in  the  office  below.     An  arm  is  attached  to  the  vertical 
rod  of  the  wind  vane,  and  at  the  end  of  this  arm  is  a  contact  shoe  which 
runs  over  the  four  contact  blocks.     The  circuit  is  closed  automatically 
by  a  clock  every  minute.     If  the  wind  direction  is  north,  the  contact  shoe 


PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE     139 


will  be  located  over  the  block  marked  N,  and  the  corresponding  circuit 
will  be  closed,  and  the  armature  in  the  office  below  will  print  N,  or  make 
a  dot  in  the  proper  place  on  the  revolving  drum.  The  contact  shoe  is  of 
such  form  and  size  that  if  the  wind  direction  is  northeast,  it  covers  a  por- 
tion of  both  the  N  and  E  blocks,  and  thus 
when  the  circuit  is  closed,  both  N  and  E  are 
printed  on  the  record  or  two  appropriately 
placed  dots  are  made.  In  this  way,  an  auto- 
matic record  of  the  wind  direction  every 
minute  is  kept  at  Weather  Bureau  stations. 
In  the  actual  instrument  the  contact  blocks 
are  not  placed  horizontally  or  out  in  the  open. 
The  whole  is  made  compact,  usually  of  cylin- 
drical form,  and  is  carefully  protected.  The 
principle,  however,  is  as  described. 

124.    Velocity  estimations. — Before  instru- 
ments were   in  use  for  the  measurement   of 
wind  velocity,  its  value  was  deter-  The  Beau. 
mined  by    the    estimation    of    its  fort  wind 
effects.     THe  first  attempt  to  make  scale* 
definite  these  estimations  was  made  by  Admiral 
Beaufort  of  the  English  Navy  in  1805.     He 
devised  the  so-called  twelve-point  wind  scale, 

in  which  twelve  names  for  different  winds  were  introduced,  and  these 
were  defined  in  terms  of  the  amount  of  sail  which  a  vessel  could  carry 
under    the    different  conditions.      Velocities    corresponding    to    those 
various  winds  have  since  been  determined  experimentally.     From  1805 
on,  many  wind  scales  were  proposed  in  which  the  number  of  winds  named 
and  defined  varied  all  the  way  from  twelve  to  four.     An  xhe  ten- 
attempt  was  made  recently  by  international  agreement  to  point  wind 
adopt  a  ten-point  wind  scale.     The  accompanying  table  gives 
the  names  of  the  winds,  the  average  velocity  in  miles  per  hour  and  in 
meters  per  second,  for  the  Beaufort  twelve-point  and  this  ten-point  wind 
scale.    In  order  to  change  from  the  metric  to  the  English  system,  it  should 
be  remembered  that  the  velocity  in  miles  per  hour  multiplied  by  . 
equals  the  velocity  in  meters  per  second ;  or,  conversely,  the  velocity  in 
mete; ,-  per  second  multiplied  by  2.237  equals  the  velocity  in  miles  per 
hour. 


FIG.  62. — The  Electrically 
Recording  Wind  Vane  of 
the  Weather  Bureau. 


140 


METEOROLOGY 
THE  TEN-POINT  WIND  SCALE 


SCALE  NUMBER 

NAME  OP  WIND 

MILES  PER  HOUR 

METERS  PER  SECOND 

0 

Calm 

0 

0 

1 

Very  light  breeze 

0     to    4.5 

0     to    2.0 

2 

Gentle  breeze 

4.6  to    9.0 

2.1  to    4.0 

3 

Fresh  breeze 

9,1  to  13.5 

4.1  to    6.0 

4 

Strong  wind 

.  13L6  to  22.5 

6.1  to  10.1 

5 

High  wind 

22.6  to  31.  5 

10.1  to  14.1 

6 

Gale 

31.6  to  40.5 

14.2  to  18.1 

7 

Strong  gale 

40.6  to  49.5 

18.2  to  22.1 

8 

Violent  gale 

49.6  to  67.5 

22.2  to  30.2 

9 

Hurricane 

67.6  to  85.5 

30.3  to  38.2 

in 

•**•         j.         •      I           j     T_,___ 

qc  A 

oq  q 

J.U 

IVLOSU  vioiem  Hur- 
ricane 

oO.O 

oo.o  — 

THE  BEAUFORT  TWELVE-POINT  WIND  SCALE 


SCALE 
NUMBER 

NAME  OP  WIND 

MILES  PER  HOUR 

SCALE 
NUMBER 

NAME  OF  WIND 

MILES  PER  HOUR 

0 

Calm 

0 

7 

Moderate  gale 

40 

1 

Light  air 

3 

8 

Fresh  gale 

48 

2 

Light  breeze 

13 

9 

Strong  gale 

56 

3 

Gentle  breeze 

18 

10 

Whole  gale 

65 

4 

Moderate  breeze 

23 

11 

Storm 

75 

5 

Fresh  breeze 

28 

12 

Hurricane 

90 

6 

Strong  breeze 

34 

At  the  present  time  wind  scales  are  of  little  advantage,  since  instru- 
ments for  determining  wind  velocity  have  become  so  common. 
In  estimating  wind  velocities,  however,  it  is  of  advantage  to 
know  about  what  effects  are  produced  by  a  wind  of  a  certain 
velocity.  The  following  table  may  be  of  use  in  guiding  one's 
estimation  of  wind  velocity: 


The  effects 
produced  by 
winds  of 
different 
velocities. 


MILES  PER  HOUR 

0 

0  to     4 


No  perceptible  movement  of  anything. 
This  just  moves    the  leaves  of  a  tree  without  moving 
or  swaying  the  branches. 
4  to  12          This  moves  the  branches  of  a  tree,  and  blows  up  dry 

leaves  and  paper  from  the  ground. 
12  to  22          This  sways  the  branches  of  trees,  blows  up  dust  from 

the  ground,  and  drives  leaves  and  paper  rapidly  before  it. 
22  to  32          This  sways  whole  trees,  blows  twigs  and  small  branches 
along  the  ground,  raises  clouds  of  dust,  and  hinders  walk- 
ing somewhat. 


PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE     141 


MILES  PER  HOUR 

32  to  72  This  breaks  small  branches,  loosens  bricks  on  chimneys, 
etc.,  litters  the  ground  with  twigs  and  branches  from  trees, 
and  hinders  walking  decidedly. 

72  on  This  brings  about  more  or  less  complete  destruction  of 

everything  in  its  path. 

The  adjectives  used  in  describing  wind  in  order  are:  light,  feeble, 
gentle,,  mild,  fresh,  strong,  high,  heavy,  violent.     The  names  „ 

Terms  used* 

used  for  various  winds  are  often  calm,  breeze,  wjnd,  storm, 

gale,  hurricane.     A  weak  adjective  should  not,  of  course,  be  used  with 

a  strong  wind  or  a  strong  adjective  with  a  weak  wind. 

125.   Anemometers.  —  The  anemometer,1  as  the  name  implies,  is  an 
instrument    for    determining    the    velocity    of    the   wind.     The  first 
av        A^eter  was  devised  in  1667,  and 
since  that  time  anemometers  of  such 
varied    form    and    in    such  There  are 
large    numbers    have    been  many  kinds 

of  anemom- 

devised  that  it  is  well-nigh  eters,  but 
impossible  to  classify  them,  ordinary 

,          .,       ones  may  be 

to  say  nothing  of  descnb-  divided  into 
ing  them  here.  Those  in  three  groups, 
ordinary  use  may  be  divided  into  three 
groups;  namely,  deflection  anemome- 
ters, pressure  anemometers,  and  rota- 
tion anemometers. 

The  simplest  deflection  anemometer 
consists  of  a  square  board  or  sheet  of 
metal  hinged  along  its  upper  A  simple 
edge  and  always  turned  to  deflection 
face  the  wind  by  means  of  *tneermom~ 
a  wind  vane.     As  the  wind 
velocity  increases,  the  deflection  ^from 
the  vertical  position  increases  and  gives 
a  measure  of  the  pressure,   and  thus 
the  velocity  of  the  wind.     A  device  for. 
recording  the  max'm  ^r,  wind  velocity 
can  easily  be  as  instrument. 

A  simple  iometer  consists  of  a  small  board  directed 

against  the  w  m  of  a  wind  vane  and  held  in  position  by  a 

wind  ;   pir pov  =  measure. 


FIG.  63. — A  Simple  Deflection  Ane- 
mometer. 

(ABBE,  U.  S.  Weather  Bureau.) 


142 


METEOROLOGY 


Two  com- 
mon pres- 
sure ane- 
mometers. 


FIG.  64. 


spring  at  its  back.     As  the  wind  velocity  in- 
creases, this  board  is  pressed  back  more  and 
more  against  the  spring,  or  possibly 
a  weight,   and  by  the    amount   of 
motion   the  wind   velocity  can   be 
determined.     Another  form  of  pres- 
sure anemometer  consists  of  a  U-tube  contain- 
ing some  light  fluid  which  is  again  directed  to 
the  wind  by  means  of  a  wind  vane.      The  pres- 
sure of  the  wind  causes  the  fluid  to  sink  in  one 
arm  and  rise  in  the  other,  and  this  difference 
in  height,  which  can  be  determined  by  means 
of  a  scale,  will  give  a  measure- 
ment   of    the    wind   velocity. 
These   anemometers   are   pic- 
tured in  Figs.  63,  64,  and  65. 

126.   The  anemometer  used 
by  the  U.  S.  Weather  Bureau 
is  the  well-known  Robinson  cup 
anemometer,  and  it  is  a  rotation  instrument. 
As  pictured  in  Fig. 

A  Simple  Pressure  66,    it    Consists    6S- 

sentially     of     two 


Anemometer. 


(From  J.  W.  MOORE'S  Meteorology, 

Practical  and  Applied.)         horizontal  arms  at 
right  angles  to  each 

other,  which  carry  hemispherical  cups  at  the 
ends  of  the  arms.  The  pressure  of  the  wind 
Robinson's  *s  grea^er  on  the  concave  side  of 
cup  ane-  the  cup  than  on  the  convex  -side, 
and  thus  the  cups  rotate  with  the 
convex  side  forward.  The  cross  arms  which 
carry  the  cups  are  attached  to  a  vertical  rod 
which  is  connected  with  cog  wheels  for  re- 
cording the  number  of  revolutions  of  the 
cups.  It  is  also  usually  arranged  so  that 
contact  is  made  at  the  end  of  each  mile  of 
wind  that  passes.  It  has  been  found  experi- 
mentally that  the  velocity  of  the  motion  of 
the  centers  of  the  cups  must  be  multiplied 
by  three  in  order  to  obtain  the  wind  velocity, 


FIG.  65.  —  Lind's  Pressure 
Anemometer. 

(From  J.  W.  MOORE'S  Meteorology, 
Practical  and  Applied.) 


PRESSURE  AND   CIRCULATION  OF  THE  ATMOSPHERE      143 


The  rela- 
tion of  the 
velocity  of 
the  cups  to 
the  wind 
velocity. 


and  this  factor  is  used  in  the  construction  of  the  instruments.  This  is 
unfortunately,  however,  not  a  constant,  but  it  varies  with  the  size  of 
the  cups,  with  the 
length  of 
the  hori- 
z  o  n  t  a  1 
arms,  and 
with  the 

friction  in  the  instru- 
ment. It  has  been 
found  to  vary  all  the 
way  from  2.2  to  3.1. 
This  constant  is  de- 
termined experiment- 
ally by  attaching  the 
anemometer  to  a  long 
arm  and  whirling  it 
at  an  even  rate  of 
speed.  There  are 
several  sources  of 
error  in  this  form  of 
anemometer;  one  of 
the  large  ones  is  the 
fact  that  due  to  in- 
ertia, the  instrument  is  apt  to  run  past  lulls  in  the  wind,  and  to  gain 
speed  slowly  if  the  velocity  of  the  wind  suddenly  increases. 

The  constant  used  by  the  U.  S.  Weather  Bureau  for  the  anemometer  of 
the  size  employed  is  three.  All  recorded  velocities  are  thus  based  on  this 
assumption.  It  has  been  found  that  the  recorded  velocities  are  nearly 
correct  for  wind  velocities  below  15  miles  an  hour,  but  that  corrections , 
must  be  applied  for  higher  velocities.  In  the  accompanying  table  the 
corrected  velocities  are  given  for  all  recorded  velocities  from  zero  to  90. 


FIG.  66.  —  Robinson's  Cup  Anemometer. 


+0 

4-1 

+2 

+3 

+4 

+5 

+6 

+7 

+8 

+9 

0 

5.1 

6.0 

6.9 

7.8 

8.7 

10 

9.6 

10.4 

11.3 

12.1 

12.9 

13.8 

14.6 

15.4 

16.2 

17.0 

20 

17.8 

18.6 

19.4 

20.2 

21.0 

21.8 

22.6 

23.4 

24.2 

24.9 

30 

25.7 

26.5 

27.3 

28.0 

28.8 

29.6 

30.3 

31.1 

31.8 

32.6 

40 

33.3 

34.1 

34.8 

35.6 

36.3 

37.1 

37.8 

38.5 

39.3 

40.0 

50 

40.8 

41.5 

42.2 

43.0 

43.7 

44.4 

45.1 

45.9 

46.6 

47.3 

60 

48.0 

48.7 

49.4 

50.2 

50.9 

51.6 

52.3 

53.0 

53.8 

54.5 

70 

55.2 

55.9 

56.6 

57.3 

58.0 

58.7 

59.4 

60.1 

60.8 

61.5 

80 

62.2 

62.9 

63.6 

64.3 

65.0 

65.7 

66.4 

67.1 

67.8 

68.5 

•90 

•69.2 

144 


METEOROLOGY 


127.  In  addition  to  the  four  anemometers  described  above,  there  are 
many  forms  of  instruments  belonging  to  the  deflection,  pressure,  and  rota- 
Other  forms  ti°n  tyPe'  and  there  are  also  several  other  types  of  anemome- 
ofanemom-  ters.  The  rate  of  evaporation  of  a  fluid  has  been  used  to 
determine  wind  velocity.  Sand  and  mercury  have  been 
allowed  to  fall  vertically  and  to  be  caught  on  a  tray  with  a  series  of  com- 
partments. The  compartment  receiving  the  largest  quantity  of  the  fall- 
ing substance  would  give  a  measure  of  the  wind  velocity.  Arrangements 

have  also  been  used  in  which 
the  wind  causes  a  musical 
sound,  and  the  velocity  has 
been  determined  by  means 
of  the  pitch  of  the  sound.1 

For     determining     small 
wind  velocities  or  the  veloc- 
ity of  motion    of   air    cur- 
rents,     so-called 

Anemom-  i 

etersfor  pocket  anemom- 
determining  eters  like  the 

small  wind  »    ,          i 

velocities.  one  Pictured  in 
Fig.  67  are  some- 
times used.  Here  a  fan 
with  a  series  of  plates  takes 
the  place  of  the  cups.  An 
instrument  of  this  kind  is 
best  calibrated,  and  the 
wind  velocity  determined  by 
walking  at  a  known  rate  of 
speed  both  toward  and  away 
from  the  wind.  Great  care  must  be,  of  course,  taken  not  to  influence 
the  instrument  by  the  presence  of  the  observer's  body.  Very  light 
wind  velocities  may  also  be  determined  by  watching  the  velocity  of 
motion  of  light  substances  in  the  air.  For  this  purpose  thistledown 
or  very  light  balls  of  cotton  may  be  used,  or  an  artificial  cloud 
may  be  formed  by  combining  the  fumes  from  hydrochloric  acid  and 
ammonia. 

1  See  DINES,   "Anemometer  Comparisons,"  Quarterly  Journal  of  the  Royal  Meteoro- 
logical Society,  Vol.  XVIII,  1892,  p.  165. 


FIG.  67.  —  Pocket  Anemometer. 
(Cut  furnished  by  KEUFFEL  and  ESSER  Co.,  New  York.) 


PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE     145 


THE  LOCATION  OF  OBSERVATORIES 

128.   Effect  of  surroundings.  —  Wind,  both  as  regards  direction  and 
velocity,  is  probably  more  affected  by  the  immediate  surroundings  of  the 
station  at  which  the  observations  are  made  than  any  other 
one  of  the  meteorological  elements.     There  are  four  things  buildings, 
to  be  especially  considered;  namely,  valleys,  buildings,  nature  nature  of 
of  the  surface,  and  altitude.     Valleys  influence  wind  direction  altitude 
markedly  and  velocity  to  a  slight  extent.     The  wind  ordi-  must  be 

.,      ,  ,  ,        ,  ,    .  ,1          .  considered. 

narily  blows  harder  on  a  mountain  top  than  in  a  near-by 
valley.      A  good  illustration  of  the  effect  of  the  valley  on  wind  direction 
may  be  seen  in  considering  the  prevailing  wind  direction  in  the  Hudson 
River  Valley  and  in  the  Mohawk  Valley  in  New  York  State.      The 
Mohawk  Valley  runs  nearly  east  and  west,  and  the  prevailing  The  influ_ 
wind  direction  is  also  west.     The  Hudson  River  Valley  runs  ence  of  a 
nearly  north  and  south,  and  the  prevailing  wind  direction,  vaUey- 
only  a  few  hundred  miles  from  the  Mohawk  Valley,  is  nearly  north.    Val- 
leys, therefore,  have  a  tendency  to  cause  the  wind  to  blow  along  their 
length.     Buildings  increase  the  wind  velocity  near  them  and  also  make 
the  wind  gusty.     In  fact,  one  result  of  all  unevennesses  in  the  surface  over 
which  air  passes  is  to  cause  gusts.    The  nature  of  the  surface,  also,  has  a 
marked  influence  on  wind  velocity.     On  land  the  wind  veloc- 
ity is  very  much  reduced  near  the  earth's  surface.     This  is  Of  the  na- 
brought  about  not  only  by  friction,  but  also  by  the  inter-  tur®  of  the 
mingling  of  air  masses,  and  by  the  formation  of  eddies  which 
result  from  the  uneven  surface.    Wind  velocity  increases  markedly  with 
altitude.    The  increase  is  very  rapid  in  the  first  hundred  or  two  hundred 
feet,  particularly  over  the  land.     On  the  top  of  the  Eiffel  The  effect 
Tower  at  Paris,  at  an  altitude  of  990  feet,  the  average  wind   of  ^titude. 
velocity  is  3.1  times  the  velocity  at  an  altitude  of  60  feet.     The  accom- 
panying table  gives  the  wind  velocity  at  various  altitudes  as  determined 
by  cloud  observations  at  Blue  Hill,  near  Boston. 


200 
to 
1000 

1000 
to 
3000 

3000 
to 
5000 

5000 
to 
7000 

7000 
to 
9000 

9000 
to 
11,000 

11,000 
to 
13,000 

Mean  velocity  in  me-  f  Summer 
ters  per  second       \  Winter 

7.5 

8.8 

8.2 
14.7 

10.6 
21.6 

19.1 
49.3 

23.5 
54.0 

31.1 

35.2 

The  immediate  surroundings  of  a  station,  then,  influence  markedly 
wind  as  regards  both  direction  and  velocity,  and  a  station  should  be  very 


146  METEOROLOGY 

* 

carefully  chosen  in  order  to  give  values  which  shall  be  characteristic  for 
the  section  in  question. 

129.    Hill  and  mountain  observatories.  —  In  order  to  get  observations 
of  the  meteorological  elements  at  considerable  altitudes  above  the  earth's 
surface,  many  stations  have  been  established  on  hills  and  moun- 
'ta'e  oJvan"    tains.     Such  observations  are  also  freer  from  the  influence  of 
mountain       'the  immediate  surroundings  of  the  station.     Every  hill  and 
torie™         mountain,  however,  influence  to  a  certain  extent  the  meteoro- 
logical elements,  and  the  values  are  not  the  same  as  would  be 
obtained  at  the  same  altitude  in  the  open  air.     For  this  reason  kite  and 
balloon  observations  in  recent  times  have  supplanted  the  observations 
taken  at  hill  and  mountain  stations.     In  this  country  among  the  more 
important  mountain  observatories  maybe  mentioned  the  one  on  Blue  Hill, 
near  Boston,  at  an  altitude  of  675  feet,  which  has  been  main- 
Some  well-    tained  many  years  by  Professor  A.  L.  Rotch,  and  the  research 
mountain       observatory  of  the  U.  S.  Weather  Bureau  at  Mount.  Weather 
observa-        m  Virginia.     In  1871  an  observatory  was  established  on  Mt. 
country.         Washington,  New  Hampshire,  at  an  altitude  of  6279  feet,  and 
in  1877  one  was  established  on  Pike's  Peak,  Colorado,  at  an 
altitude  of  14,134  feet.     These  last  two  observatories  were  given  up  in 
1887  and  1889  respectively.     The  Lick  observatory  on  Mt.  Hamilton, 
California,  at  an  altitude  of  4400  feet,  maintains  a  full  meteorological 
record.     Among  the  many  important  mountain  observatories  in  Europe 
may  be  mentioned  Ben  Nevis.  Scotland,  4407  feet :   Brocken, 

European 

mountain       North  Germany,  3743 ;    Hoch  Obir,  Austria,  7047 ;    Pic  du 

Midi'  France>  9381  J  Puy  de  Dome>  France,  4800 ;  Schnee- 
koppe,  Germany,  5246 ;  Sentis,  Switzerland,  8215 ;  Sonn- 
blick,  Austria,  10,155;  Wendelstein,  Germany,  5669.  The  Eiffel  Tower 
in  Paris  at  an  altitude  of  990  feet  has  given  very  interesting  meteoro- 
logical records.  These  records  are  of  especial  value  because  the  slender 
form  of  the  tower  causes  no  disturbing  influence  in  the  condition  of 
the  air  around  it. 


THE  RESULTS  OF  OBSERVATION 

130.  The  observations.  —  At  all  regular  stations  of  the  U.  S.  Weather 
Bureau  continuous  records  of  the  wind  direction  and  wind  velocity  are 
maintained.  The  instrument  used  for  determining  the  wind  direction 
is  the  wind  vane  with  an  electric  contact  maker.  It  will  be  remem- 
bered that  by  means  of  a  sliding  contact  one  or  two  of  the  four  cir- 


PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE     147 


cuits  N,  S,  E,  and  TF,  are  constantly  kept  closed.     The  current  is  sent 
through  the  instrument  every  minute  by  means  of  a  clock,  and  on  the 
revolving  drum  in  the  office  below  the   wind  direction  is 
thus  recorded  to  eight  points  of  the  compass.     The  instru-   Thf  obser- 
ment  used  for  determining  wind  velocity  is  the  Robinson   wind  at  a 
cup  anemometer.     The  contact  is  made  after  every  mile   r.egular  sta- 

'  tion ;  the 

of  wind  has  gone  by,  and  a  spur  is  made  on  the  revolving  instruments 
drum  to  indicate  that  fact.     Wind  direction,  wind  velocity,   u^e.d  and 
amount  of  rainfall,  and  the  duration  of  sunshine1  are  all   location, 
recorded  on  the  same  revolving    drum,  which   is    usually 
spoken  of  as  a  triple  register  or  meteograph  and  is  located  in  the  office 
part  of  the  Weather  Bureau.     The  wind  vane  and  anemometer  are 
usually  exposed  at  the  top  of  some  high  building  at  a  considerable 
distance  above  the  ground.     The  purpose  of  this  is  to  give  them  as 
free  an  exposure  as  possible  and  thus  prevent  local  influences  from 
disturbing  the  wind  direction  or  velocity. 

At  the  cooperative  stations  of  the  U.  S.  Weather  Bureau  the  wind 
direction  for  the  day  is  the  only  thing  recorded.     If  the  wind   Observa 
has  shifted  during  the  day,  the  middle  of  the  arc  through  tions  at  a 
which  it  has  shifted  during  the  twenty-four  hours  is  deter- 
mined and  the  corresponding  direction  recorded. 

131.    Prevailing  wind  direction;    wind  roses.  —  Since  wind  direction 
is  not   a  mere  number, 
normals  cannot  be  com- 
puted in  the  usual  way. 
For  this  rea- 

j.       i    Prevailing 

son,     instead   wjn(j  ah-ec_ 
of  the    three   tion ;  two 
words    mean, 
average,    and 
normal,  the  single  word 
"  prevailing  "     is    used. 
This    distinction,    how- 
ever, is  not  always  rec- 
ognized.     The    prevail- 
ing wind  direction  may     ^  6g  _  wind  Rose  for  ^  1909>  at  gyracuse>  N  Y 
be     expiressed     in     two 
ways :  either  by  means  of  a  table,  or  graphically  by  means  of  what  is 


JAN.,  1909 


1  One  circuit  is  used  for  the  double  purpose  of  recording  rainfall  and  sunshine.     But 
confusion  results,  because  it  seldom  rains  while  the  sun  is  shining. 


148 


METEOROLOGY 


ALBANY 


JAN.  ,1909, 


called  a  wind  rose.  If  the  prevailing  wind  direction,  be  it  for  a  month 
or  a  year,  or  an  indefinite  time,  is  expressed  by  means  of  a  table,  this 
table  contains  simply  the  number  of  times  each  wind  direction  was  ob- 
served. The  number  of  calms  must  also  be  noted.  The  graphic 

representation  of  the 
table  is  known  as 
the  wind  rose.  The 
four  directions  — 
north,  south,  east, 
and  west  —  are  first 
drawn  from  a  central 

W- ^ — E     point,  and  then  the 

four  intermediary 
directions.  On  these 
eight  lines  distances 
are  laid  off  propor- 
tionately to  the  num- 
ber of  times  each  of 
these  wind  directions 
was  observed.  If 
these  points  are  con- 
nected by  straight 

lines  the  resulting  figure  is  called  a  wind  rose.     The  number  of  calms 

may  be  expressed  by  a  circle  described  about  the  center  with  a  radius 

,  proportionate      to      the 

The  method    * 

of  construct-  number     of    calms.     In 

ing  a  wind      tne     following    table    is 
rose.  . 

given     the     number    of 

times    the    wind    blew  from    each 


FIG.  69.  — Wind  Rose  for  Jan.,  1909,  at  Albany,  N.Y. 


direction  at  Syracuse  in  the  Mohawk 
Valley,  and  Albany  in  the  Hudson 
River  Valley  during  January,  1909. 
Figures  68  and  69  are  the  corre- 
sponding wind  roses.  The  reason 
why  east  and  west  winds  occurred 
at  Syracuse  and  not  at  Albany  is  probably  because  the  Mohawk  Valley 
runs  east  and  west,  while  the  Hudson  Valley  runs  north  and  south. 
In  other  respects  the  wind  roses  are  quite  similar. 

132.    Normal  hourly,  daily,  monthly,  and  yearly  velocity.  —  Since  the 
wind  velocity  is  a  mere  number,  the  various  normals  may  be  computed 


SYRACUSE 

ALBANY 

N 

0 

5 

NE 

1 

1 

E 

4 

0 

SE 

0 

0 

S 

10 

10 

sw 

3 

5 

W 

3 

0 

NW 

10 

10 

Calms 

0 

0 

31 

31 

PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE     149 


in  the  usual  way.     In  determining  a  normal 
hourly    wind    velocity,    however,  it    is    not 
customary    to    determine    it  for 
every  hour  of  every  day  in  the 
year,  but  for  the  various  hours  of  ing  the 
the  months  as  a  whole.     For  ex-  ^^es.ve" 
ample,  it  would  be  the  8  A.M., 
9  A.M.,  10  A.M.,  etc.,  wind  velocities  for  Jan- 
uary which  would  be  determined. 

133.    Diurnal,  annual,  and  irregular  vari- 
ation. —  The  graph    which    represents    the 

diurnal  variation 
in  wind  velocity 
may  be  deter- 
mined by  plot- 
ting to  scale  the 
normal  hourly 
wind  velocities. 
For  New  Eng- 


or 

D 
O 

I 

I 

1. 
*n 

10 

9 
8 

7 

s* 

^ 

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\Jft 

NUARY 

^—v 

^ 

^ 

* 

St 

L^ulf 

^ 

*->  — 

11 

89 

D-191 

^ 

\ 

V 

k 

JUL-Y 

/ 

' 

*-^ 

*>s 

/ 

^— 

17 
16 
15 
14 
13 
12 
11 
10 
9 
8 

I 

-(• 

89 

9- 

91 

o)- 

-N 

EV\ 

rv 

OFK- 

S 

^~* 

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, 

Ais 

uJ 

VR 

f 

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-V 

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^ 

/ 

f~-+ 

"•^ 

S 

*> 

K 

^ 

YE 

AF 

[  ^ 

f 

s4 

5s 

^"N, 

? 

/ 

\ 

/ 

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^w 

JL 

L\ 

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s 

^ 

J? 

(Ar 

em 

orr 

eter  350 

ft. 

abc 

ve 

ore 

un   ) 

>  2  4  e 

8  1012  246   81012 

FIG.  70.  —  The  Daily  Variation 
in  Wind  Velocity  at  New 
York,  St.  Louis,  and  San 
Francisco  for  January  and 
July. 


12  2   4    6   8  1012  2468  1012 
NOON 

FIG.  70  a.  —  The  Daily  Variation 
in  Wind  Velocity  at  New 
York,  St.  Louis,  and  San 
Francisco  for  January  and 
July. 


land   and  the   larger  part   of  the   United 
States  the   higher  values  of  wind  velocity 
occur      during 
the    day    and 
the  lower  veloc- 
ities  at  night.- 
The  maximum 
usually    occurs 
between  twelve 
and  four  in  the 


afternoon,  and 
the  minimum  at  about  the  time  of  sunrise. 
The  reason  for  this  daily  variation  is  to  be 
found    in    convection.     During 
the  day  the  layers  of  air  near 
the  ground  become  heated ;  and  wind  ve- 
as  they  rise,  due  to  convection,  i°sacau*e. 
the  colder  air  must  come  down 
to  take  the  place  of  the  rising  air  and  bring 
with  it  the  higher  wind  velocities  of  the 
upper  atmosphere.      The  diurnal  variation 


122  4    6    8  10  122    46    8  1012 
-      NOON 

FIG.  70  b.  —  The  Daily  Variation 
in  Wind  Velocity  at  New 
York,  St.  Louis,  ,  and  San 
Francisco  for  January  and 
July. 


150  METEOROLOGY 

of  wind  velocity  is  greatest  on  land  and  practically  disappears  over 
the  ocean.  It  is  less  in  winter  than  in  summer  and  less  on  cloudy 
days  than  on  clear  days.  The  reason  for  these  facts  is  evident,  since 
convection  on  land,  during  the  summer  and  on  clear  days,  is  far  greater 
than  under  the  opposite-  conditions. 

Figure  70  shows  graphically  the  change  in  wind  velocity  during  the 
day  at  New  York,  St.  Louis,  and  San  Francisco  for  both  January  and 
July  and  illustrates  well  the  truth  of  the  statements  which  have  just 
been  made. 

There  is  also  a  slight  diurnal  variation  in  wind  direction.  This,  how- 
ever, is  so  masked  and  changed  by  local  conditions,  particularly  if  the 

station  is  located  near  the  seashore,  where  the  land  and  sea 
variatio/in  breezes  blow,  or  near  mountains,  where  the  mountain  and 
wind  direc-  valley  breeze  is  felt,  that  the  typical  diurnal  variation  is 
cause"*  hardly  noticeable.  In  order  to  observe  this  diurnal  variation 

in  all  its  simplicity,  it  would  be  necessary  to  have  a  station 
surrounded  on  every  side  by  practically  identical  conditions.  If  this  were 
the  case  it  would  be  found  that  the  wind  shifts  slightly  in  a  clockwise 
direction  during  the  daytime  and  shifts  back  again  in  a  counterclockwise 
direction  during  the  night.  The  reason  for  this  is  again  to  be  found  in 
convection.  The  upper  air  currents  always  blow  in  a  direction  turned 
somewhat  clockwise  as  contrasted  with  the  surface  winds.  Thus,  during 
the  day,  as  these  upper  air  masses  come  down,  they  will  bring  with 
them  a  wind  direction  turned  slightly  in  a  clockwise  manner. 

The  diurnal  variation  in  wind  velocity  and  in  wind  direction 

as  described  above  occurs  only  at  low  altitudes.  At  high 
on  the  altitudes,  at  the  level  of  the  clouds  for  example,  exactly  the 

variation        reverse  is  true;   that  is,  the  higher  values  of  wind  velocity 

occur  during  the  night  and  the  wind  direction  shifts  in  the 
opposite  direction. 

If  the  prevailing  wind  direction  for  the  various  hours  of  the  day 
and  the  normal  hourly  wind  velocities  for  any  station  are  known,  they 

may  be  expressed  graphically,  as  in  Fig.  71,  for  the  top  of 

the  Eiffel  Tower  at  Paris.  The  wind  direction  during  any 
tion  of  the  hour  of  the  day  may  be  found  by  connecting  the  corre- 
ft!anhiVari '  spending  hour  with  the  point  0,  and  the  wind  velocity  by 
wind  direc-  noting  the  length  of  the  line  connecting  the  hour  in  ques- 

tion  with  the  P°int  0-      li  wil1  be  seen  tnat  at  this  sliSnt 

elevation,  990  feet,  a  modification  of  the  surface  conditions 
is  already  apparent. 


PRESSURE  AND   CIRCULATION   OF  THE  ATMOSPHERE     151 


134.   If  the  normal  daily  or  the  normal  monthly  velocities  are  plotted 
to  scale,  the  graph  will  indicate  the  annual  variation  in  wind  velocity. 
In  general,  the  wind  blows  harder  in  winter  than  in  summer. 
The  maximum  usually  comes  in  the  very  late  winter,  Feb-  variation  in 


ruary,  March,  or  April,  and  the  minimum  during  the  summer,  ™*  y^'g 

July  or  August.     The  reason  for  this  is  twofold.     In  the  first  character- 

place,  when  the  trees  are  covered  with  leaves  and  vegetation  c^use*1*4 

is  most  luxuriant,  wind  velocities  are  lessened  much  more  by 

friction    than  during    the  winter,  when  the  trees  are  bare  and   the 

ground  is  snow-covered  and  frozen.     In  the  second  place,  the  poleward 

temperature  gradient  is  much  8  p  M 

greater    in    winter    than    in 

summer  (see  section  88),  and 

it  will  be  seen  later  that  it  is 

this  difference  in  temperature 

between    equator    and    pole 

which  drives  the  convectional 

circulation  which    is    at   the 

foundation     of    the    general 

wind    system    of    the    globe. 

The  greater  the  difference  in 

temperature      between      the 


FIG.  71. —  Diurnal  Variation  in  Wind  Direction 
and  Velocity  at  the  Top  of  the  Eiffel  Tower 
during  June,  July,  and  August. 
(After  -ANGOT.) 


equator  and  pole,  the  more 
energetic  will  be  this  circula- 
tion, and  thus  the  higher  will 
be  the  wind  velocities.  The  following  table  gives  the  normal  monthly 
wind  velocity  for  several  stations  in  the  United  States.  In  Fig.  72  the 
results  are  shown  graphically  for  four  stations,  Philadelphia,  Chicago, 
Phoenix,  and  San  Francisco.  The  graphs  for  Philadelphia  and  Chicago 
are  typical  for  the  northeastern  and  central  portion  of  the  country  and 
illustrate  the  description  of  the  annual  variation  which  has  just  been 
given.  In  the  Southern  States  and  on  the  Pacific  coast  the  character- 
istics are  very  different  because  the  whole  type  of  weather  and  weather 
control  are  different. 


152 


METEOROLOGY 


OAJ 

i 
oc 
£10 


\ 


\ 


Chicago 


Philadelphia 


San  Francisco 


Phoenix 


M.ONTK8 
FIG.  72.  —  The  Annual  Variation  in  Wind  Velocity. 


PRESSURE  AND  CIRCULATION   OF  THE   ATMOSPHERE      153 


THE  NORMAL  WIND  VELOCITY  IN  MILES  PER  HOUR  FOR  THE  VARIOUS 
MONTHS  AND  FOR  THE  YEAR 


«  3 

1 

°  i  ° 

A 

I 

iio 

3gg 

B§- 

Is 

»-s 

3 

- 

-' 
S 

a 

S 

1 

5 

o 

M 
OQ 

I 

o 
fc 

6 

p 

i 

B3J 

3 

Bismarck, 

N.  Dak. 

35 

34 

8 

9 

10 

12 

11 

10 

9 

9 

10 

10 

9 

8 

10 

Charleston, 

S.C. 

92 

38 

9 

12 

11 

12 

11 

10 

10 

9 

9 

9 

9 

9 

10 

Chicago,  111. 

274 

15 

18 

18 

19 

19 

17 

14 

14 

14 

16 

17 

18 

19 

17 

Columbus, 

Ohio 

222 

9 

10 

10 

9 

8 

7 

7 

6 

7 

8 

9 

9 

8 

El  Paso,  Tex. 

133 

10 

12 

14 

13 

12 

10 

7 

9 

9 

9 

9 

10 

10 

Indianapolis, 

Ind. 

164 

12 

11.7 

11.5 

12.1 

11.3 

9.9 

8.9 

8.2 

7.4 

8.3 

9.4 

10.4 

11.5 

10.0 

Key  West,  Fla. 

53 

38 

11 

11 

11 

11 

9 

8 

8 

7 

8 

11 

11 

11 

10 

New  Orleans, 

La. 

121 

39 

8.8 

9.3 

9.4 

8.9 

7.6 

6.1 

6.1 

6.1 

7.4 

7.8 

8.3 

8.8 

7.9 

Omaha,  Neb. 

121 

37 

9 

9 

10 

11 

9 

8 

7 

7 

8 

8 

9 

9 

9 

Philadelphia, 

Pa. 

184 

11.1 

12.0 

11.6 

11.3 

10.2 

9.4 

9.0 

8.4 

8.9 

10.3 

10.4 

10.9 

10.3 

Phoenix,  Ariz. 

56 

15 

3.4 

4.0 

4.5 

4.6 

4.6 

4.4 

4.5 

4.0 

3.8 

3.5 

3.3 

3.2 

4.0 

Portland,  Ore. 

106 

37 

6 

7 

6 

6 

6 

6 

6 

5 

6 

5 

6 

6 

6 

St.  Louis,  Mo. 

317 

10 

12.0 

12.1 

12.4 

12.0 

10.9 

9.0 

8.8 

8.0 

9.2 

10.4 

11.8 

11.8 

10.7 

St.  Paul,  Minn. 

124 

36 

7.8 

8.3 

8.8 

9.3 

8.7 

7.7 

7.1 

7.1 

8.0 

8.5 

8.1 

7.8 

8.1 

San  Francisco, 

Cal. 

204 

21 

7.3 

7.5 

9.1 

10.4 

11.1 

13.0 

13.5 

12.6 

10.4 

8.1 

6.9 

6.9 

9.7 

Seattle,  Wash. 

151 

7  1 

73 

74 

68 

64 

6? 

55 

/t  8 

54 

56 

70 

70 

64 

The  year  1908  is  the  last  year  included  in  these  normals. 

There  is  also  an  annual  variation  in  wind  direction.     This,  however, 
depends  so  much  upon  the  location  of  the  place  on  the  earth's  surface 
that  no  general  rules  can  be  laid  down.     For  New  England  the 
prevailing  winds  of  winter  are  more  northwest  and  north  than 
during  the  summer,  when  they  shift  more  to  the  south  and  wind  direc- 
southwest.      The  reason  in  every  case  is  the  building  up  of  ^  cause. 
areas  of  high  pressure  over  the  continents  during  the  winter 
and  of  low  pressure  over  the  continents  during  the  summer.     It  is  to 
these  changing  areas  of  high  and  low  pressure  that  the  annual  change  in 
wind  direction  is  due. 

135.   In  addition  to  these  regular  diurnal  and  annual  changes  of  wind 
direction  and  velocity,  there  are  many  irregular  variations. 
In  the  first  place,  both  wind  direction  and  wind  velocity 
are  constantly  changed  slightly  from  moment  to  moment,  in  wind  di- 
-  We  characterize  this  by  saying  that  the  wind  is  usually  gusty, 
This  is  due  entirely  to  the  irregularities  in  the  surface  of  the 
earth  over  which  the  air  is  moving.     In  addition,  wind  Direction  and 


*™ 


154 


METEOROLOGY 


wind  velocity  are  very  different  on  different  days.  The  reason  for  this 
change  in  direction  and  velocity  is  to  be  found  in  the  various  storms,  which 
are  to  be  discussed  later  (Chapter  VI). 

136.  Wind  data.  —  In  addition  to  the  prevailing  wind  direction  and 
the  various  normals  in  connection  with  the  velocity  which  have  been 
Extremes  described  above,  very  few  results  are  computed  from  the  ob- 
of  velocity,  servations  of  wind.  Practically  the  only  one  of  any  great 
interest  is  the  extremes  of  velocity  which  have  occurred  at  various  places. 
At  nearly  all  stations  in  the  United  States  wind  velocities  have  gone  above 
fifty  miles  an  hour  for  a  period  of  five  minutes  and  at  nearly  every  sea- 
coast  station  velocities  above  seventy  miles  for  the  same  period  have  been 
recorded.  The  accompanying  table  gives  the  maximum  wind  velocity 
ever  recorded  at  a  number  of  stations  in  the  United  States.  At  the 
regular  stations  of  the  U.  S.  Weather  Bureau  there  are  five  tables  of  wind 
data  which  are  kept  constantly  up  to  date.  These  contain 

(1)  Total  movement  in  miles.     (The  data  are  given  for  the  various 
months  and  for  the  year  as  a  whole.) 

(2)  Prevailing  direction   and   average  hourly  velocity  (velocity  to 
tenths). 

(3)  Maximum  velocity,  direction,  and  date. 

(4)  Mean  hourly  wind  velocity  (miles  and  tenths  per  hour). 

(5)  Prevailing  wind  direction  (hourly). 


STATION 

MAXIMUM 
VELOCITY, 
MILES 

DIREC- 
TION 

DATE  OF 
OCCURRENCE 

LENGTH  OF 
RECORD 
IN  YEARS 

LAST  YEAR 
INCLUDED 
IN  RECORD 

Abilene,  Texas  .    .     . 

66 

sw 
sw 

May  8,  1892 
June  8,  1892 

22 
22 

1908 
1908 

Albany,  N.Y.    .    .    . 

70 

w 

E 

Feb.  2,  1876 
Oct.  23,  1878 

34 
34 

1907 
190$ 

Atlanta,  Ga.      .     .     . 

60 

JNW 
1NW 

Feb.  16,  1903 
April  8,  1907 

29 
29 

1908 
1908 

Atlantic  City,  N.J. 

72 

NE 

Sept.  10,  1889 

34 

1908 

Augusta,  Ga.     .     . 

52 

NE 

July  28,  1893 

37 

1908 

Baltimore,  Md. 

70 

W 

June  20,  1902 

37 

1908 

Binghamton,  N.Y. 

44 

W 

Jan.  20,  1907 

17 

1907 

Bismarck,  N.D. 

74 

NW 

Mar.  10,  1878 

33 

1908 

Block  Island,  R.I. 

90 

NE 

Oct.  27,  1898 

28 

1908 

Boise,  Idaho      .    . 

55 

SW 

May  13,  1900 

9 

1908 

fNE 

Mar.  3,  1891 

36 

1908 

Boston,  Mass.  .    .    . 

60 

N 
|NE 

Oct.  27,  1898 
Oct.  24,  1901 

37 
37 

1908 
1908 

I  E 

Nov.  5,  1900 

37 

1908 

Buffalo,  N.Y.    .    .    . 

90 

SW 

Jan.  13,  1890 

37 

1907 

Burlington,  Vt.      .     . 

60 

SE 

Jan.  20,  1907 

22 

1907 

PRESSURE   AND  CIRCULATION  OF  THE  ATMOSPHERE     155 


STATION 

MAXIMUM 
VELOCITY, 
MILES 

DIREC- 
TION 

DATE  OF 

OCCURRENCE 

LENGTH  OF 
RECORD 
IN  YEARS 

LAST  YEAR 
INCLUDED 
IN  RECORD 

Cairo,  111  

.  84 

w 

June  21,  1891 

36 

1908 

Charleston,  S.C.    .     . 

96 

E 

Aug.  28,  1893 

37 

1908 

Chattanooga,  Tenn. 

60 

W 

May  12,  1895 

29 

1908 

Chicago,  111.      ... 

84 

NE 

Feb.  12,  1894 

37 

1908 

Cincinnati,  O.   .     .    . 

52 

NW 

May  23,  1901 

37 

1908 

Cleveland,  O.    .     .     . 

73 

s 

Nov.  26,  1895 

37 

1908 

Columbus,    O.  .     .     . 

66 

NW 

Jan.  20,  1907 

29 

1907 

Davenport,  Iowa  .     . 

72 

SW 

Aug.  7,  1872 

37 

1908 

Denver,  Col.     .     .     . 

68 

NW 

May  1,  1902 

36 

1908 

Des  Moines,  Iowa.     . 

64 

SW 

April  1,  1892 

29 

1908 

Detroit,  Mich.  .     .     . 

76 

SW 

Oct.  26,  1895 

'     37 

1908 

Duluth,  Minn.  .     .     . 

78 

NE 

Aug.  16,  1881 

37 

1908 

El  Paso,  Texas  .     .     . 

78 

W 

Mar.  5,  1895 

29 

1908 

Galveston,  Texas  *    . 

84 

NE 

Sept.  8,  1900 

37 

1908 

Hatteras,  N.C.      .     . 

105 

N 

July  17,  1899 

33 

1908 

Havre,  Mont.   .     .     . 

76 

NW 

June  9,  1890 

26 

1908 

Helena,  Mont.  .    .    . 

60 

f   W 

1  w 

Feb.  6,  1890 
Dec.  25,  1890 

27 
27 

1908 
1908 

Honolulu 

55 

SE 

Dec.  31,  1906 

33 

1908 

Huron,  S.D.       .     .     . 

72 

NE 

Jan.  6,  1903 

26 

1907 

Indianapolis,  Ind. 

60 

W 

June  25,  1882 

37 

1908 

Jacksonville,  Fla.  .     . 

75 

SW 

Feb.  16,  1903 

36 

1908 

Kansas  City,  Mo. 

55 

NW 

July  10,  1902 

20 

1908 

Key  West,  Fla.      .     . 

88 

SW 

Oct.  19,  1876 

37 

1908 

Lexington,  Ky.      .     . 

68 

w 

Sept.  8,  1899 

23 

1908 

Los  Angeles,  Cal.  .     . 

48 

NE 

Jan.  28,  1882 

30 

1907 

Memphis,  Tenn.    .     . 

75 

SW 

Mar.  9,  1901 

37 

1908 

Milwaukee,  Wis.   .     . 

60 

f  SW 

1  sw 

July  24,  1874 
Oct.  16,  1880 

37 
37 

1908 
1908 

Minneapolis,  Minn.  . 

84 

NW 

July  20,  1904 

17 

1908 

New  Haven,  Conn.    . 

62 

SE 

Oct.  21,  1904 

35 

1908 

New  Orleans,  La.  .     . 

60 

E 

Aug.  19,  1888 

37 

1908 

New  York,  N.Y.    .     . 

80 

N 

Mar.  20,  1899 

37 

1908 

Omaha,  Neb.     .     .     . 

64 

NE 

July  13,  1905 

37 

1908 

Philadelphia,  Pa.  .     . 

75 

SE 

Oct.  23,  1878 

37 

1908 

Phosnix,  Ariz.    .     .     . 

48 

SE 

July  25,  1903 

12 

1908 

Portland,  Me.   .     .    . 

60 

f  SE 

1  sw 

Mar.  21,  1876 
Dec.  2,  10,  1878 

36 
36 

1908 
1908 

Portland,  Oregon  .     . 

55 

'    S 

Mar.  25,  1897 

37 

1908 

St.  Louis,  Missouri    . 

80 

NW 

May  27,  1896 

37 

1908 

St.  Paul,  Minn.      .     . 

102 

NW 

Aug.  20,  1904 

37 

1908 

Salt  Lake  City,  Utah 

66 

NW 

Nov.  15,  1906 

34 

1908 

San  Francisco,  Cal.   . 

64 

NE 

Nov.  30,  1906 

37 

1908 

Savannah,  Ga.  .     .     . 

76 

NW 

Aug.  21,  1898 

37 

1908 

Springfield,  Mo.     .     . 

64 

W 

May  29,  1905 

20 

1908 

Syracuse,  N.Y.      .     . 

66 

S 

Mar.  24,  1907 

5 

1908 

Washington,  D.C.     . 

66 

SE 

Sept.  29,  1896 

37 

1908 

Yuma,  Arizona      .     . 

54 

NW 

Mar.  17,  1894 

30 

1908 

*  Anemometer  blew  away  — 120  miles  by  estimation. 

137.    Prevailing  winds  of  the  world.  — Charts  XI  and  XII,  in  addition 
to  the  pressure,  indicate  the  prevailing  wind  direction  for  the  world  for 


156  METEOROLOGY 

* 
January  and  July.      In  preparing  these  charts,   that  wind  direction 

which  predominated  during  the  month  in  question  was  the 
acteristics  of  one  cnarted-  If  these  charts  are  carefully  compared,  it 
the  prevail-  will  be  found  that  the  following  generalizations  can  be 
the  world  °f  made.  i  I*1  the  northern  hemisphere  the  wind  blows  spirally 

inward  toward  areas  or  belts  of  low  pressure,  turning  in  a 
counterclockwise  direction.  From  areas  or  belts  of  high  pressure  the 
wind  blows  spirally  outward,  turning  in  a  clockwise  direction.  The 
results  are  the  same  for  the  southern  hemisphere  with  the  exception 
that  the  direction  of  rotation  is  the  converse.  In  the  centers  of  the 
areas  or  belts  of  high  and  low  pressure  calms  occur. 

138.  Other  wind  charts.  —  In  addition  to  the  two  charts  described  above 
many  other  wind  charts  might  be  prepared.    The  prevailing  wind  direc- 
other  wind     tion  for  all  the  months  in  the  year  for  the  world  as  a  whole 
charts.  or  £or  anv  separate  country  might  be  charted.      Also,  the 
normal  wind  velocity  or  the  extremes  of  velocity  for  all  the  months  in 
the  year  or  for  the  year  as  a  whole  might  be  charted  for  the  whole 
world  or  for  any  given  country. 

C.    THE  CONVECTIONAL  THEORY    AND    ITS    COMPARISON  WITH  OBSERVED 

FACTS 

THE  CONVECTIONAL  THEOKY 

139.  General  convectional  motion.  —  The  general  convectional  mo- 
tion of  the  atmosphere  can  best  be  illustrated  by  means  of  a  fluid  analogy. 

In  Fig.   73  let    NES  represent  a  long 
tank    filled    with    fluid    and 


x 


conve 

tionai  circu-    heated  at  the  bottom  and  in 

ff  s     fl*£nina      the    center.      The    layer    of 

fluid  next  the  bottom  will  be 
FIG.  73.  —  Diagram  illustrating  Con-     heated  by  conduction  and  will  expand, 

vection  in  a  Long  Tank  of  Fluid.  .  : 

thus  raising  the  layers  of  fluid  above  it 

and  causing  a  slight  bulging  of  the  upper  surface.  Gravity  acting 
upon  this  will  cause  the  fluid  to  flow  towards  the  ends  of  the  tank, 
thus  decreasing  the  pressure  in  the  center  and  increasing  it  at  the  ends. 
This  increased  pressure  at  the  ends  will  drive  in  the  colder  fluid  toward 
the  center,  forcing  the  lighter  expanded  fluid  to  rise  and  thus  starting 
the  convectional  circulation. 

The  atmosphere,  as  was  fully  discussed  in  Chapter  II,  is  heated  pri- 
marily at  the  bottom,  and  the  amount  of  heat  supplied  to  the  equator  is 


PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE     157 

nearly  three  times  the  amount  applied  at  the  poles,  the  exact  ratio 
being  347  to  143.  One  would  thus  expect  a  convectional  circulation 
to  take  place  between  the  equator  and  poles,  the  warm  air 
rising  at  the  equator,  flowing  poleward  on  the  outside  of 
the  atmosphere,  descending  in  the  polar  regions,  and  re-  lationbe- 
turning  toward  the  equator  along  the  earth's  surface.  The 
continents  are  warmer  than  the  adjoining  oceans  in  summer  * 
and  colder  in  winter ;  one  would  thus  expect  a  convectional  circulation 
between  continents  and  adjoining  oceans.  Along^  the  seashore  the 
land  is  warmer  than  the  water  during  the  day  and  colder  other  con- 
than  the  water  at  night ;  one  would  thus  expect  a  convec-  vectionai 
tional  circulation  between  the  land  and  the  near-by  water.  circulatlons- 
For  one  reason  or  another  a  certain  locality  might  be  warmer  than  near- 
by places ;  small  local  convectional  circulations  would  thus  be  expected. 
As  a  matter  of  fact,  as  will  be  seen  in  later  sections,  all  of  these  convec- 
tional circulations  actually  exist.  The  first  gives  rise  to  the  most  im- 
portant of  the  general  winds  of  the  world ;  the  second  is  the  cause  of  the 
monsoons ;  the  third  is  the  reason  for  land  and  sea  breezes ;  while  certain 
cloud  formations,  thunder  showers,  and  dust  whirlwinds  are  evidences 
of  the  last. 

140.  Arrangement  of  isobaric  surfaces  in  a  general  convectional 
circulation.  —  If  there  were  no  temperature  differences  and  no  winds, 
the  isobaric  surfaces  would 

be    parallel    to  ^ •-'"""  — ^^ 

the    hydro-  ^nli^ec1-4"          ^" ^^> is 


sphere    and    at  ti°n  before 

•i  •       convection 

the    same    dis-  starte(j. 
tance  above  it. 
A    vertical    section    along 
the  meridian  of  these  iso- 
baric surfaces  would  show       ^_ ^ 

a  series  of  lines  parallel  to 
the  earth's  surface.  These 
are  represented  in  Fig.  74 

,       ,  ,.  ,.  FIG.  74.  —  The  Meridional  Section  before  Convection 

by  heavy  continuous  lines.  started. 

Suppose  now  that  the  at- 
mosphere is  suddenly  heated  at  the  bottom  and  at  the  equator  more 
than  elsewhere.     The  air  will  expand  and  the  isobaric  surfaces  will  be 
warped  upward,  the  upper  surfaces  showing  an  increasing  amount  of 
bulge.     The  vertical  section  along  the  meridian  of  these  warped  sur- 


158 


METEOROLOGY 


faces  is  represented  by  the  dashed  lines  in  Fig.  74.     The  excess  of  air 
at  the  equator  will  now,  under  the  influence  of  gravity,  flow  off  toward 

the  poles,  decreasing  the 

sr       . v  equatorial    pressure    and 

increasing  the  pressure  at 
the  poles. 
This  increased 
pressure  at  the 


\ 


1  r\  > 

/ 

^  i 

i  {  '/ 

i  x 

•29  in. 


0  In, 


FIG.  75.  —  The  Meridional  Section  after  Convection  was 
permanently  established. 


The  meridi- 
onal section 
after  con- 
vection was 

permanently    poles  will  force 
established.     •        ,  i  ,  , 

in  the  colder 
air  along  the  earth's  sur- 
face and  cause  the  warm 
equatorial  air  to  rise,  thus 
starting  the  convectional 
circulation.  Figure  75 
shows  a  vertical  section 
along  a  meridian  of  the 
isobaric  surfaces,  and  the 

air  circulation  after  the  convectional  circulation  has  become  perma- 
nently established.  A  meridional  view  of  the  air  circulation  is  shown 
more  in  detail  in  Fig.  76.  One  half  of  the  atmosphere  is  below  three 
miles  of  elevation,  and  30°  N.  latitude 
marks  the  dividing  line  on  the  earth's 
surface,  one  half  of  the  at- 

A  meridi- 
onal view  of  mosphere  being  between  it 

and  the    equator    and    one 

half  between  it  and  the 
pole.  Thus,  between  30°  N.  latitude 
and  the  equator  the  air  would  be  rising, 
flowing  poleward  outside  of  the  three- 
mile  limit,  dropping  down  to  lower 
levels  between  30°  N.  latitude  and  the 
pole,  and  returning  equatorward  below 
the  three-mile  limit.  These  last  two 
diagrams  illustrate  the  convectional  circulation  between  equator  and 
pole  which  would  be  expected  on  a  non-rotating  earth  heated  at  the 
equator  and  colder  at  the  poles.  In  all  three  of  the  above  diagrams 
the  vertical  scale  is  much  exaggerated  as  compared  with  the  horizontal. 
141.  Condition  of  steady  motion.  —  The  condition  of  steady  motion 
can  be  stated  in  a  single  sentence.  As  long  as  in  the  atmosphere  differ- 


FIG.  -76. 


A  Meridional  View  of  the 
Air  Circulation. 


PRESSURE  AND   CIRCULATION  OF  THE  ATMOSPHERE     159 


ences  of  temperature  are  maintained,  a  steady  convectional  circulation 
will  continue.      In    the    case  of   the  earth,  the  equatorial 
portions  are  constantly  maintained  at  a  higher  temperature  tion  of 
than  other  regions,  and  the  atmosphere  is  heated  at  the  stea.dy 
bottom.     Thus  a  permanent  convectional  circulation  be- 
tween equator  and  pole  is  to  be  expected. 

142.  Barometric    gradients.  —  The   barometric    gradient   is    denned 
in  exactly  the  same  way  as  the  two  temperature  gradients  which  have 
already  been  described  (sections  53  and  88).     The  vertical  _ 

Definition 

temperature  gradient  was  denned  as  the  change  in  tempera-  Of  the  baro- 
ture  with  elevation  above  the  earth's  surface;  the  poleward  metr.ic 
temperature  gradient  was  defined  as  the  change  in  tempera- 
ture with  distance  in  going  from  equator  to  pole.     Similarly,  the  baro- 
metric or  pressure  gradient  may  be  defined  as  the  change  in  barometric 
pressure  with  distance.     This  is  ordinarily  expressed  as  a  change  of  so 
many  hundredths  of  an  inch  of  pressure  in  500  miles,  or  so  many  milli- 
meters per  latitude  degree  (69.5  statute  miles).     It  can  be  HOW 
found  by  noting  the  difference  in  pressure  between  two  places  exPressed. 
and  the  distance  between  them,  and  then  reducing  this  to  the  difference 
per  500  miles  or  per  latitude  degree. 

143.  Relation  of  wind  direction  to  pressure  gradient.  —  The  relation 
of  wind  direction  to  the  barometric  or  pressure  gradient  can  be  expressed 
best  by  means  of  a  diagram.     In  Fig. 

77,  let  A,  B,  and  C  represent  respec- 
tively three    isobaric    lines, 


with    a    pressure    of    30.3, 
30.0,  and  29.7  inches  respec- 


Relation  of 
wind  direc- 
tion to  pres- 
sure gra- 

tively.    Suppose  that  a  mass  dients  and 

of  air  M  is  located  on  the  jj^"16 

30-inch  line.     The  pressure 

on  the    K  face  of  this  mass  of  air  is 

slightly  greater  than  30  inches,  because 

it   lies    between  the   30-inch  line  and 

the  30.3-inch  line.     In  the  same  way, 

the  pressure  of  the  L  face  of  this  mass 

of  air  is  less  than  30  inches  because  it 

lies  between  the  30-inch  line  and  the 

29.7-inch  line.     There  is  thus  an  unbalanced  pressure  on  this  mass  of 

air  tending  to  push  it  from  the  30-inch  line  to  the  29.7-inch  line. 

This  may  be  generalized  by  saying  that  air  tends   to   move  along 


30.0 


29.7 


FIG.  77.  —  The  Relation  of  Wind  Direc- 
tion to  Pressure  Gradients. 


160 


METEOROLOGY 


pressure  gradients  and  at  right  angles  to  isobaric  lines.     If  this  princi- 
ple is  applied  to  the  air  in  an  area  of  high  and  low  pressure, 
about  as  illustrated  in  Fig.  78,  it  will  be  seen  that  the  air  should 

"highs"    ^  move  directly  outward   from  areas  of    high    pressure   and 
directly  inward  toward  areas  of  low  pressure. 


The  first 
recognition 
of  the  effect 
of  the 
earth's  ro- 
tation. 


FIG.  78.  —  Air  Motion  about  "  Highs"  and  "  Lows  "  on  a  Non-rotating  Earth. 

144.    Effects   of  the    earth's  rotation    on    wind    direction    and    pres- 
sure. —  The  introduction  into  meteorology  of  the  idea  that  the  rotation 
of  the  earth  might  influence  the  direction  of  moving  air 
masses  was  slow.     The  first  use  of  this  principle  to  explain 
observed  wind  directions  was  made  by  Hadley  in   1735. 
It  had  been  known,  for  a  considerable  time  previous  to  this, 
that  the  trade  winds  did  not  blow  directly  towards  the 
equator,   but   had  an  oblique  movement.     This  had  been  observed 
especially  in  the  case  of  the  trade  winds  from  30°  N.  latitude  to  the 
equator   which    blow   from   the   northeast.     His    explanation    of    the 
oblique  movement  was  essentially  as  follows  :    A  mass  of 
a*r  Carting  from  30°  N.  latitude  directly  toward  the  equator 
would  be  passing  over  regions  which  would  nave  a  greater 
tne°winas    easterly  velocity  of  motion  due  to  the  earth's  rotation,  than 
the  places  from  which  it  had  come.     As  a  result  it  would  lag 
behind  the  rotating  earth  and  thus,  instead  of  moving  due  south,  it 
would  deviate  to  the  right  and  become  a  northeast  wind. 
takes  in         Hadley's  explanation  contains  the  germ  of  the  truth,  but  is 
Hadiey's  ex-   erroneous  in  at  least  two  directions.     In  the  first  place,  it 
tacitly  assumes  that  the  effect  of  the  earth's  rotation  would 
be  felt  only  by  air  masses  moving  north  or  south,  and  not  by  masses 


the  direc- 
°f 


PRESSURE  AND  CIRCULATION  OF  THE   ATMOSPHERE     161 


moving  in  an  easterly  or  westerly  direction ;  and  in  the  second  place, 
it  is  a  natural  corollary  of  Hadley's  explanation  that  the  wind  velocity 
must  increase  as  the  equator  is  approached,  for  with  the  motion  from  the 
north  must  be  compounded  the  motion  from  the  east  due  to  lag.  Both 
of  these  are,  however,  mistakes,  since  the  earth's  rotation  influences  air 
moving  in  any  direction,  and  the  velocity  does  not  increase  as  the  equator 
is  approached. 

The  deflective  effect  of  the  earth's  rotation  on  moving  bodies  on  its 
surface  was  worked  out  later  by  various  mathematicians,  but  its  com- 
plete application  to  the  motion  of  air  masses,  and  the  first  Fen-el's 
rational  explanation  of  the  general  wind  system  of  the  globe,  work- 
was  begun  by  Ferrel  in  1856      He  was  a  school  teacher  at  Nashville, 
Tenn.,  and  was  a  self-taught  mathematician  of  remarkable  originality 
and  ability.      It  is  probably  not  too  much  to. say  that  Ferrel's  work 
caused  a  revolution  in  the  science  of  meteorology. 

145.   The  law  which  expresses  the  effect  of  the  earth's  rotation  on 
moving  air  can  be  briefly  stated  as  follows :   If  a  mass  of  air  starts  to 
move  on  the  earth's  surface,  it  deviates  to  the  right  in  the  The  law 
northern  hemisphere  (to  the  left  in  the  southern  hemisphere),  which  ex- 
and  tends  to  move  in  a  circle  the  radius  of  which  depends  effTcTof^e 
upon  its  velocity  and  the  latitude  of  the  place.     The  accom-  earth's  ro- 
panying  table  indicates,  for  several  velocities  and  latitudes,  * 
the  radius  of  this  circle.     It  will  be  seen  that  the  earth  exercises  a 
deviating  influence  on  air  moving  east  or  west  of  exactly  the  same  kind 
as  if  the  air  moved  north  or  south. 

RADIUS  OF  CURVATURE  (IN  MILES)  FOR  FRICTIONLESS  MOTION  ON  THE 
EARTH'S  SURFACE 


Latitude  

0° 

5° 

10° 

20° 

30° 

40° 

50° 

60° 

70° 

80° 

90° 

77 

20  miles  an  hour  . 

00 

880 

442 

224 

153 

119 

100 

88 

82 

78 

10  miles  an  hour  .  . 

oo 

440 

221 
110 

112 

76 

59 

50 

44 

41 

39 

38 

5  miles  an  hour  .  . 

oo 

220 

56 

38 

30 

25 

22 

20 

19 

19 

146.  The  effect*  of  this  deviation  to  the  right  on  the  air  masses  which, 
due  to  convection,  are  moving  from  the  equator  toward  the  poles  on  the 
outsid^  of  the  atmosphere  must  now  be  considered.  Instead  of  mov- 
ing directly  from  the  equator  poleward,  the  air  masses  will  be  deviated 
to  the  right  in  the  northern  hemisphere  and  become  more  and  more  a 


162 


METEOROLOGY 


west  air  current  encircling  the  pole  in  a  great  whirl.  It  is  a  principle 
of  mechanics  that  whenever  a  rotating  body  is  not  acted  upon  by  out- 
side forces,  the  moment  of  momentum  must  remain  a  con- 
stant. The  formula  for  the  moment  of  momentum  is  SM  VR, 
where  M  represents  the  mass  of  each  particle  of  the  rotating 
body,  V  the  velocity  of  the  particle,  and  R  the  radius,  that 
is,  the  distance  of  the  particle  from  the  center  of  rotation. 
This  product  M  VR  must  be  summed  up  for  all  the  particle 
of  the  rotating  body,  and  thus  %MVR  represents  the  moment 
of  momentum  of  the  body.  As  this  ring  of  whirling  air  about  the  pole 
approaches  it,  the  mass  remains  constant,  the  radius  is  decreasing, 
anc^  thus  ^ne  velocity  must  steadily  increase,  and  it  has  been 
computed  that,  if  the  velocities  were  not  held  down  by  fric- 
tion, they  would  amount  to  hundreds  of  thousands  of  miles 
per  hour.  A  whirl  of  air  with  these  high  velocities  must  cause  cen- 
trifugal force,  and  this  centrifugal  force  will  hold  air  away  from  the  pole, 
thus  causing  a  diminution  in  the  barometric  pressure.  The  amount  of 
land  at  the  north  pole  is  much  greater  than  at  the  south  pole.  One 
would  thus  expect  wind  velocities  and  the  diminution  in  pressure  to  be 
larger  at  the  south  pole  than  at  the  north  pole.  An  exact  analogy  to 
this  process  can  be  seen  in  the  escape  of  water  from  a  washbowl  through 
a  central  vent.  Due  to  some  cause,  the  escaping  water 
usually  takes  up  a  motion  of  rotation.  As  the  water  ap- 
proaches the  vent  to  escape,  the  velocity  of  rotation  becomes  greater 
and  greater,  and  the  centrifugal  force  developed  is  often  sufficient  to  hold 
the  water  away  from  the  center  and  cause  an  empty  core  above  the  vent. 


The  effect 
of  the 
earth's  ro- 
tation on 
the  air 
masses  ap- 
proaching 
the  poles. 


Low  polar 
pressures 
explained. 


An  analogy. 


30  in. 

FIG.  79.  —  The  Meridional  Section  of  the  Isobaric  Surfaces  in  a  Convectional  Circulation 

on  a  Rotating  Earth. 


The  vertical  section  along  a  meridian  of  the  isobaric  surfaces  on  a 
rotating  earth  should  thus  have  the  appearance  of  Fig.  79. 
On  a  non-rotating  earth,  the  pressure  at  the  equator  would 
be  low  and  at  the  poles  high.  This  was  illustrated  in  Fig.  75. 
It  is  the  centrifugal  force  due  to  the  circumpola*  whirls  whi'-h 
has  held  the  air  away  from  the  poles  and  turned  the  high 
pressure  into  the  low  pressure.  The  larger  velociti  :it 
the  south  pole  are  responsible  for  the  lower  barometric 
pressure  there. 


The  verti- 
cal section 
along  a 
meridian  of 
the  isobaric 
surfaces  on 
a  rotating 
earth. 


PRESSURE   AND   CIRCULATION  OF  THE  ATMOSPHERE 


163 


147.   The  effect  of  the  earth's  rotation  on  the  motion  of  air  in  connec 
tion   with   areas   of   high    and  low  pressure   must  also  be 
considered.     In  Fig.  80  the  dashed  lines  represent  the  air  Ofthe  * 
motion  on  a  non-rotating   earth    (see  Fig.   78)  ;    the    full  e&T^h's  r°- 
lines  represent  the  air  motion  on  a  rotating  earth.      The  air  motion 


deviation  to  the  right  in  each  case  is  evident.  These 
diagrams  apply  to  the  northern  hemisphere.  In  the 
southern  hemisphere  the  direction  of  deviation  is  the  opposite. 


FiG.,80.  —  Air  Motion  about  "  Highs  "  and  "  Lows  "  on  a  Rotating  Earth. 


148.  .Buys  Ballot's  law.  —  About  1850  Buys  Ballot,  after  a  careful 
study  of  the  air  circulation  about  storms,  generalized  a  law  which  was  of 
great  practical  value.     The  usual  statement  of  the  law  is  Buys  Bal- 
that  if  one  stands  with  his  back  to  the  wind,  the  pressure  on  lot>s  law- 
his  left  hand  is  lower  than  on  his  right.    This  law  derived  from  observa- 
tions found  its  way  into  text  books  on  meteorology,  was  used  by  the 
captains  of  vessels,  and  had  a  very  wide  practical  application.     This  is 
its  chief  importance,  for  considered  in  connection  with  the  preceding 
sections,  it  is  simply  the  inevitable  result  of  the  rotation  of  The  reason 
the  earth  on  air  moving  in  towards  areas  of  low  pressure.     In  for  lts  truth* 
Fig.  80,  if  one  applies  Buys  Ballot's  law,  its  truth  becomes  at  once 
evident. 


164  METEOROLOGY 

COMPARISON  OF  THE  CONSEQUENCES  OF  THE  CONVECTIONAL  THEORY 
WITH  THE  OBSERVATIONS  OF  PRESSURE  AND  WIND 

149.  In  the  first  two  subdivisions  of  the  present  chapter,  the  material 
was  presented  from  the  inductive  standpoint  (see  section  30).     The 

instruments  for  making  various  observations  were  described, 
The  mduc-     ^e  various  observations  taken  were  then  stated,  and  from 

live  method 

of  gaining  these  observations  several  generalizations  were  made.  It 
a^ouiTthe011  was  ^oun(^'  ^or  instance,  that  there  was  a  belt  of  low  pressure 
pressure  at  the  equator,  belts  of  high  pressure  at  35°  N.  and  30°  S. 
amfair^011  ^itude,  and  low  pressure  at  the  poles,  the  pressure  at  the 
motion.  south  pole  being  much  lower,  than  at  the  north  pole.  It  was 
furthermore  seen  that  air  tended  to  move  spirally  outward 
from  areas  of  high  pressure  and  spirally  inward  toward  areas  of  low 
pressure,  turning  clockwise  about  areas  of  high  pressure,  counterclock- 
wise about  areas  of  low  pressure  in  the  northern  hemisphere.  These 
conclusions  were  simple  generalizations  from  observations  and  serve  as 
a  good  example  of  the  inductive  method  of  gaining  information. 

In  the  third  subdivision  of  this  chapter,  the  deductive  method  of 
reasoning  was  followed.  Two  general  principles  were  used;  namely, 
The  deduc-  that  the  atmosphere  was  heated  at  the  bottom  and  more  at 
tive  method,  ^e  equator  than  elsewhere,  and  secondly,  that  the  earth 
turned  eastward  on  its  axis.  From  these  two  general  principles  it  was 
determined  what  the  pressure  distribution  over  the  earth  ought  to  be 
and  what  the  air  circulation  about  areas  of  high  and  low  pressure  ought 
to  be.  If  these  deductions  from  the  two  principles  are  tested,  by  com- 
paring them  with  the  generalizations  derived  from  the  observations, 
The  com-  exact  agreement  will  be  found.  This  stamps  the  whole 
parison.  process  of  reasoning,  as  well  as  the  conclusions,  as  correct. 
The  present  illustration  of  the  inductive  and  deductive  method  of 
reasoning  is  the  best  one  to  be  found  in  the  realm  of  meteorology. 

D.    A  GENERAL  CLASSIFICATION  OF  THE  WINDS          ^ 

THE  CLASSIFICATION  OF  THE  WINDS 

150.  The  chief  characteristics  of  a  general  convectional  circulation 
between  equator  and  pole  on  a  rotating  earth  have  just  been  stated ;  it 
The  general    now  remams  to  treat  in  detail  the  various  components  of 
winds  of        this  circulation.     These,  together  with  certain  other  winds, 

due  in  most  cases  to  convection  on  a  smaller  scale,  are  usually 
spoken  of  as  the  general  winds  of  the  globe.     These  were  first  classified 


PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE     165 

by  Dove,    a    German  meteorologist,  nearly  one  hundred   years   ago. 
He  divided  them  into  permanent,  periodic,   and  irregular;  the  per- 
manent being  those  whose  direction  remained   the    same  The  Dove- 
throughout  the  year,  the  periodic  being  those  whose  direc-  classifica- 
tion changed  during  a  certain  interval  of  time,  the  irregular 
being  those  which  showed  marked  irregular  changes  in  either  direction 
or  velocity.     In  the  American  Meteorological  Journal  for  March,  1888, 
Professor  W.  M.  Davis  published  a  classification  based  upon  another 
principle.     This  classification  is  also  treated  in  the  seventh  The  Davis 
chapter   of   his    book,    Elementary   Meteorology,    Ginn    and  ciassifica- 
Company,  1894.     In  this  classification,  the  winds  are  divided  taon' 
into  sun-caused,  moon-caused,  and  earth-caused  winds.     In  neither  of 
.these  classifications  is  any  account  taken  of  the  importance,  or  intensity, 
or  the  amount  of  the  earth's  surface  covered  by  these  winds.     In  the 
following  sections  these  general  winds  of  the  globe  will  be  treated  roughly 
in  the  order  of  their  importance,  although  the  treatment  is  based  largely 
upon  the  Davis  classification.     In  each  case  the  position  of  the  wind  in 
both  the  Dove  and  Davis  classification  will  be  given. 

PLANETABY  WINDS 

151.   Typical  system.  —  By  planetary  winds  are  meant  those  winds 
which  would  occur  on  any  planet  heated  at  the  equator  and  turning 
eastward  on  its  axis.     Any  migration  of  the  point  of  appli-  Definition 
cation  of  the  greatest  heat  and  all  irregularities  of  surface  of  planetary 
are  here  left  out  of  consideration.     The  typical  system  of  wmds* 
planetary  winds  can  be  best  illustrated  by  means  of  a  series  of  diagrams. 
In  section  146  it  was  shown  that  a  planet  heated  at  the  Thepres- 
equator  and  turning  eastward  on  its  axis  would  develop  a  sure  belts, 
belt  of  low  pressure  at  the  equator,  belts  of  highjpressure  in  the  tropical 
regions,  and  -  cups  of  low  pressure  at  the  poles^  These  deductions  are 
also  in  exact  agreement  with  the  results  of  the-  pressure  observations 
made  ih/different  parts  of  the  earth.     One  -would  thus  expect  calms  at 
the  equator  and  in  the  tropical  regions,  and  winds  blowing  outward  from 
the  belts  of  high  pressure,  turning  to  the  right  in  the  northern  The  air  cir- 
hemisphere   and  ta  the  left  in  the  southern.     These  are  cuiation. 
illustrated  in  Fig.  81.     The  equatorial  belt  of  calms  is  called  the  dol- 
drums;   the  tropical  belts  of  calms  are  called  the  horse  The  names 
latitudes;   the  northeast  winds  blowing  from  the  tropical  of  the 
belt  of  high  pressure  in  the  northern  hemisphere  and,Jbhe 
southeast  winds  blowing  from  the  tropical  belt  of  high  pressure  in  the 


166 


METEOROLOGY 


HORSE  LATITUDES 


HORSE  LATITUDES 


PREVAILING  WESTERLIES 


PREVAILING  WESTERLIES 


The  extent 
of  this  sur- 
face distri- 
bution. 


FIG.  81.  —  Surface   Distribution   of   the   Planetary 
Winds;  0  to  3000  ft. 


southern  hemisphere  toward  the  equator  in  each  case  are  called  the 
trade  winds ;  the  southwest  wind,  blowing  in  the  northern  hemisphere 
from '  the  belt  of  high  pressure  poleward,  and  the  northwest  wind 

blowing  from  the  high  pres- 
sure  belt   in    the    southern 
hemisphere     poleward     are 
called  the  prevailing  wester- 
lies.    This  surface  distribu- 
tion of  the  winds  extends  to 
an     altitude     of 
about  3000  feet. 
At    this    height, 
as   was    seen    in 
connection  with  the  merid- 
ional section  of  the  isobaric 

surfaces  (Fig.  60)  the  equatorial  belt  of  low  pressure  disappears  and 
the  isobaric  lines  become  practically  straight  with  a  slight  droop  at 
the  poles. 

152.  On  the  outside  of  the  atmosphere,  that  is,  from  a  height  of 
about  13,000  feet  on,  the  air  currents  move  from  the  equator  toward 

the  pole  in  both  hemispheres,  turning  to  the  right  in  the 
northern  hemisphere  so  as  to  become  a  southwest  air  current, 
and  to  the  left  in  the  southern  hemisphere  so  as  to  become 
a  northwest  air  current.  These  air  currents  approaching  the 
poles  in  spiral  paths  give  the  impression  of  circumpolar  whirls 

in  each  hemisphere.     These  are  indicated  in 

Fig.  82.     Right  at  the  equator  the  air  moves 

from  east  to  west,  and  the  reason  for  this  can 
be  readily  stated.  The  air,  rising 
at  the  equator,  in  order  to  start  its 

east  air  cur-  circumpolar  journey,  goes  into  re- 
gions which  have  a  larger  eastward 
velocity  of  motion,  due  to  the  earth's 

rotation,  than  the  earth's  surface  itself.     As  a 

result,  these  rising  air  masses  lag  behind  and 

give  the  impression  of  an  east  wind. 

153.  From  the  tropical  belt  of  high  pressure 
to  the  pole  in  the  northern  hemisphere,  both 

the  upper  air  currents  and  the  surface  winds  blow  from  the  south- 
west. If  the  air  is  not  to  beroma  massed  at  the  pole,  a  return  current 


The  air  cur- 
rents in  the 
outer  layer 
of  the  atmos- 
phere. 


rent  at  the 
equator 


FIG.  82.  —  The  Air  Currents 
in  the  Outer  Layer  of  the 
Atmosphere;  13,000  ft. 
on. 


PRESSURE   AND   CIRCULATION   OF   THE   ATMOSPHERE      167 


in  an  intermediate  layer  is  necessary.     The  direction  of  the  air  motion 
in  this  intermediate  layer  from,  say,  3000  feet  to  13,000  is  illustrated 
in  Fig.  83.     The  air  would  start  from  the  poles 
with  a  slight  velocity,  and  would  tend  to  be 
deviated   toward   the    right    in   the  „ 

The  air  mo- 

northern  hemisphere    and    thus  be-  tioninthe 
come  a  northeast  wind,  but  it  is  im-  intermediate 

layer. 

prisoned  between  two  layers  of  air, 
both  of  which  have  a  high  velocity  from  some 
westerly  point.     As  a  result,  it  is  carried  east- 
ward and  becomes  a  northwest  re-  The  expia_ 
turn  current  in  spite  of  the  deviation  nation  of  its 
to  the  right  caused  by  the  earth's  d 
rotation.     After   passing   the    tropical  belt   of 
high  pressure  the  surface  wind  becomes  a  north- 
east wind  and  the  intermediate  layer  then  also  obeys  the  deviating 
force  of  the  earth's  rotation  and  becomes  a  northeast  wind. 

154.   The  north-south  component  of  the  circulation  of  the  atmosphere 
can  be  illustrated  by  projecting  it  upon  the  plane  of  the  meridian.     This 

is  shown  in  Fig.  84.     On  - 


FIG.  83.  — The  Air  Circu- 
lation in  the  Interme- 
diate Layer;  3000  to 
13,000  ft. 


the  outside  of  the  atmos- 


\ 


The  atmos- 
pheric cir- 

phere,  that  is,  from  13,000  cuiationin 
feet     on,    the     motion     is  section, 
northward,  while  in  the  in- 
termediate layer,  from  3000  to  13,000 
feet,  the  motion  is  southward.     The 
surface  winds  move  southward  from 
the  tropical  belt  of  high  pressure  to- 
ward the  equator,  and  northward  from 
this  belt   toward   the   pole.     At   the 
equator  there  are  rising  air  currents 
and  in  the  tropical  belt  of  high  pres- 
sure  descending    air    currents.      The 
circulation  in  the  southern  hemisphere  is  exactly  the  same. 

155.   This  general  circulation  of  the  atmosphere  must  be  modified  in 
two  ways,  as  will  be  seen  later,  to  adapt  it  to  the  actual  earth,  and  it  is 
also  invaded  by  storms,  principally  in  the  region  of  the  The  typical 
prevailing  westerlies  which  cause  much  confusion  and  re-  system  must 
duce  wind  velocities  to  a  marked  extent. 

If  the  earth  did  not  rotate  on  its  axis,  the  air  motion  would  be  a 


EQUATOR 

FIG.  84. —  The  North-south  Component 
of  the  Circulation  of  the  Atmosphere. 


168  METEOROLOGY 

simpler  direct  north  and  south  motion,  as    illustrated    in    Fig.    78, 
and  not  an  oblique  circulation  as  at  present.     The  effect  of  the  deflec- 
tive force  due  to  the  earth's  rotation  is  to  increase  some- 
onthewinds  w^at  tne  win(i  velocities;  but  by  causing  the  circulation  to 
if  the  earth     be  oblique,  it  also  retards  the  exchange    of    air   between 
rotating         equator  and  pole  and  thus  causes  greater  temperature  dif- 
ferences to  exist  between  equator  and  pole  than  otherwise 
would.     In  the  Dove  system  all  these  winds  would  be  classified  as  per- 
manent, in  the  Davis  classification  they  would  be  considered  sun-caused. 

156.  Trade  winds.  —  The  trade  winds  blow  from  the  tropical  belts 
of  high  pressure  toward  the  equator ;  as  a  northeast  wind  in  the  northern 
Description     hemisphere  and  a  southeast  wind  in  the  southern  hemisphere, 
of  the  trade    They  cover  nearly  half  of  the  earth's  surface  and  their  name 

is  derived,  not  from  their  importance  to  commerce,  but  from 
the  steadiness  with  which  they  blow.  They  blow  with  moderate  to 
brisk  velocity  and  particularly  over  the  ocean  have  an  unchanged  direc- 
tion for  perhaps  weeks  at  a  time.  They  carry  but  few  clouds  by  day, 
and  by  night  the  sky  is  usually  cloudless.  As  they  approach  the  equator 
the  velocity  steadily  increases,  and  the  amount  of  moisture  contained 
in  them  also  becomes  greater.  The  trade  winds  were  first  described  by 
Halley  in  1686,  and  since  that  time  many  observations  have  been  made 
of  them.  Their  thickness  is  about  13,000  feet.  This  information  is 

gained  by  noting  the  motion  of  the  cirrus  and  cirro-cumulus 
of  the  trades  clouds  which  occur  at  this  level  or  higher,  from  kite  and 
and  how  balloon  observations,  and  also  from  the  observations  made 

found. 

on  mountain  tops  which  exceed  this  elevation.  In  some 
cases  the  smoke  coming  from  volcanoes  has  been  observed  to  drift  from 
the  southwest  in  the  air  currents  of  the  outer  layer  of  the  atmosphere, 
while  the  lower  clouds  drifting  in  the  trade  winds  moved  from  the 
northeast. 

157.  Doldrums.  —  The  doldrums  are  the  equatorial  belt  of  calms. 
The  air,  laden  with  moisture  and  at  a  high  temperature,  brought  in  by 
Description     ^ne  trade  winds,  here  loiters  and  finally  rises  to  commence 
of  the  its  poleward  journey  on  the  outside  of    the   atmosphere. 

ms'  This  rising  air  cools,  reaches  the  dew  point,  and  then  yields 
cloud  and  precipitation.  The  doldrums  are  thus  characterized  by 
light,  baffling  breezes,  frequent  calms',  overcast  sky  and  heavy  rains, 
often  in  the  form  of  thunder  storms  and  squalls. 

158.  Horse  latitudes'.  —  In  the  tropical  belts  of   high  pressure,  at 
35°  N.  and  30°  S.  latitude,  are  located  the  horse  latitudes.     These  stand 


PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE     169 

in  marked  contrast  to  the  doldrums,  although  the  wind  is  light  and 
variable   and   tne   velocity  small.     Calms    are   frequent.     The   sky, 
however,  is  nearly  always  clear  and  the  amount  of  moisture  The  horse 
in  the  atmosphere  is  small^  This  is  due  chiefly  to  the  fact  latitudes- 
that  here  we  have  to  do  with  descending  instead  of  ascending  air  currents. 

159.  Prevailing  westerly  winds.  —  In  the  northern  hemisphere,  from 
the  tropical  belt  of  high  pressure  poleward,  the  wind  blows  from  the  south- 
west or  some  westerly  quarter.  In  the  southern  hemisphere  The  pre_ 
the  wind  direction  is  from  the  northwest.  Both  these  vailing  west- 
winds  are  spoken  of  as  the  prevailing  westerly  winds.  In  the  erly  wmds* 
southern  hemisphere  the  velocity  is  larger,  sometimes  amounting  to  a 
gale,  and  for  this  reason,  particularly  among  sailors,  thesQ  winds  are 
often  spoken  of  as  the  "  brave  west  winds,"  and  the  region  of  their  occur- 
rence in  the  southern  hemisphere  called  the  "  roaring  forties."  Sailing 
vessels,  in  going  from  Europe  to  Australia  often  make  the  outward  jour- 
ney around  the  Cape  of  Good  Hope  and  the  return  journey  around  Cape 
Horn,  thus  using  the  prevailbp  westerlies  which  encircle  the  south  pole. 
The  prevailing  westerlies  are  more  invaded  by  storms  than  any  other 
of  the  permanent  winds,  and  in  many  parts  of  the  United  States  the 
succession  of  storms  is  so  rapid  that  the  prevailing  westerly  wind  only 
makes  itself  manifest  in  the  general  averages. 

1  60.    Upper  currents.  —  The  upper  currents  move  from  the  equator 
as  a  southwest   air    current    in   the    northern   hemisphere 
and  a  northwest  air  current  in  the  southern  hemisphere,  tionofmo- 
spirally  poleward^  and  are  often  spoken  of  as  the  anti-trades.  tion  and 
The  velocities  are  high  and  much  greater  in  winter  than  in  istics  of  the 
summer.     The  motion  of  these  air  currents  has  been  fre- 


quently  determined  by  the  drifting  of  smoke  from  lofty  vol- 

canoes, by  the  cirrus  clouds  which  occur  at  these  levels,  by  observa- 

tions on  high  mountains,  and  by  kite  and  balloon  observations. 

/v 
TERRESTRIAL  WINDS  \^ 

161.   Definition.  —  The  typical  system  of  planetary  winds  must  be 

modified  in  two  ways.     In  the  first  place,  the  axis  of  the  The  planet- 

earth  is  inclined  23i°  to  the  plane  of  the  ecliptic,  which  ary  system 

causes  a  change  in  the  presentation  of  the  northern  hemi-  mu^njes 

sphere  to  the  ra\  "  of  the  sun.     On  account  of  this  the  sun  modified  in 
migrates  47°  in  the  course  of  a  year,  being  farthest  north  on 

the  21st  of  June  and  farthest  south  on  the  21st  of  December.  As  will  be 


170  METEOROLOGY 

seen  later,  this  migration  modifies  the  wind  system.  In  the  second 
place  the  earth's  surface  is  not  uniform,  but  is  very  cHversified,  being 
composed  of  both  land  and  water  and  with  numerous  mountain  ranges. 
Definition  ^his  diversity  of  surface  also  affects  the  typical  system  of 
of  terrestrial  planetary  winds.  By  terrestrial  winds  are  meant  the  typical 
planetary  system  modified  by  taking  account  of  the  first 
condition ;  namely,  that  the  sun  migrates  47°  in  the  course  of  a  year. 

162.  Annual  migration  of  the  winds. — This  migration  of  the  sun 
through  47°  causes  a  migration  of  the  heat  belt.     The  heat  belt-migrates 

less  than  the  sun  and  lags  behind  it  from  four  to  six  weeks. 
The  migra-  The  migration  of  the  equatorial  belt  of  high  temperature 
sun,  heat  causes  the  equatorial  belt  of  low  pressure,  of  which  it  is  the 
belt,  equa-  cause,  to  migrate.  This  migration  in  turn  lags  behind  the 
pressure  heat  belt  and  migrates  over  a  smaller  distance.  The  mi- 
belt,  and  gration  of  the  equatorial  belt  of  low  pressure  causes  the 
system.  permanent  wind  system  of  the  globe  to  migrate.  The 

migration  of  the  wind  system  lags  some  two  months  behind 
the  migration  of  the  sun,  and  on  the  average  covers  a  distance  of 
The  amount  5  or  6°.  The  following  table,  which  gives  for  the  Atlantic 
of  the  mi-  and  Pacific  oceans  the  boundaries  of  the  trade  winds  and 

doldrums  for  March  and  September,  makes  clear  this  migra- 
tion of  the  wind  system. 

ATLANTIC  OCEAN  PACIFIC  OCEAN 

March  September  March  September 

N.E.  Trades    26°  N.  -   3°  N.  35°  N.  -11°N.  25°  N.  -   5°  N.  30°  N.  -  10°  N. 

Doldrums           3°  N.  -   0°  11°  N.  -    3°  N.  5°  N.  -    3°  N.  10°  N.  -    7°  N. 

S.E.  Trades       0°N.-25°$.  3°N.-25°S.  3°N.-28°S.  7°N.-20°S. 
t 

163.  Subequatorial  and  subtropical  wind  belts.  —  This  migration  of 
the  wind  system  over  5  or  6°  in  the  course  of  a  year  gives  rise  to 

three  belts  on  the  earth's  surface  which  require  particular 
equatorial  consideration.  These  are  called  the  subequatorial  and  the 
and  sub-  subtropical  belts,  and  they  are  illustrated  in  Fig.  85.  In 

^e  case  °^  *ke  northern  subtropical  belt,  when  ^the  horse 

latitudes  are  farthest  north,  most  of  the  belt  is  covered  J 
the  northeast  trade  winds.      When  the  horse  latitudes  are  fartlu^t 

south,  most  of  the  belt  is  covered  by  the  prevailing  we 
winds  found  lies-     Thus  places  in  this  belt  in  the  course  of  a  year  will 
in  each          experience  calms,  southwest  winds,   and   northeast  win 

The  corresponding  subtropical  belt  in  the  southern  hemi- 
sphere will  experience  calms,  southeast  trade  winds,  and  northwest 


PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE 

winds.  ^Ln  the  case  of  the 
subequatorial  belt  a  further 
complication  arises.  When 


the  doldrums  are  farthest 
north,  the  southeast  trade 
winds  from  the  southern 
hemisphere  cross  the  equa- 
tor. After  crossing  the 
equator,  they  come  under 
the  deflective  influence  of 
the  earth's  rotation  and 
become  a  southwest  wind. 
Thus,  places  in  the  subequa- 
torial belt  north  of  the  equator 
experience  calms,  northeast 
winds,  and  southwest  winds 
in  the  course  of  a  year.  A 
place  south  of  the  equator 
in  this  belt  will  experience 
calms,  southeast  trade  winds, 
and  northwest  winds. 


171 


PREVAILING  I   I 

WESTERLIES          "*T*1 


CALMS 

s.w.  WINDS 

N.E.  WINDS' 


CALMS 
.E.  WINDS 
WINDS 
CALMS 
N.W.  WINDS 
S.E.  WINDS 


TRADE 
WINDS 


PREVAILING 
WESTERLIES 


FIG.  85.  —  The    Subequatorial    and    Subtropical 
Wind  Belts. 


CONTINENTAL  WINDS 

164.    Definition.  —  The   typical   system   of   planetary   winds   which 
must,  exist  on  every  planet  heated  at  the  equator  and  turning  eastward 
on  its  axis  must  be  modified  in  two  directions  in  order  to  fit  Tne  two 
it  more  exactly  to  our  earth  as  it  actually  exists.    'The  first  modifica- 
one  of  these  two  modifications  has  already  been  discussed.  f^caiV^- 
The'  sun  migrates  47°  in  the  course  of  a  year  and,  as  a  tem  of  plan- 
result  of  this  migration,  the   equatorial  jylt  ofjrigh  tem-  etary  winds* 
perature,  the  equatorial  belt  of  Iowj3£e.ssure,  and  the  wind  system  of 
the  world  migrate  north  and  south,  lagging  behind  the  sun  and  each 
migrating  over  a  less  distance.     This  migration  of  the  wind  system 
gives  rise  to  three  belts  known  as  the  subequatorial  and 
the  two  subtropical  belts,  where  calms  and  winds  blowing  of  terrestrial 
from  two  opposite  directions  in  the  course  of  a  year  exist,  and 
The  planetary  winds  thus  modified  are  called  the  terrestrial 
winds.     The  second  modification  is  due  to  tne  fact  that  the 
earth's  surface  is  not  uniform.     It  is  not  an  all  land  surface,  or  an 


172  METEOROLOGY 

all  water  surface,  but  is  highly  diversified,  consisting  of  par^mnd  and 
part  water.  In  addition  the  surface  of  the  ocean  has  not  the  same 
temperature  along  a  parallel  of  latitude  due  to  the  presence  of  ocean 
currents.  The  terrestrial  winds  modified  by  taking  account  of  this 
diversity  of  surface  are  called  continental  winds. 

In  the  chapter  on  temperature,  the  fact  was  emphasized  that  the 
continents  during  the  winter  are  colder  than  the  adjoining  oceans  under 
the  same  latitude,  while  in  summer  they  are  warmer  than  the 
of  diversi-  adj  acent  oceans.  In  view  of  the  convectional  theory,  one 
ties  in  the  would  expect  the  continents  to  become  areas  of  low  pressure 
during  the  summer  and  of  high  pressure  during  the  winter. 
This  tendency  must  strongly  modify  and  distort  the  belts  of  pressure 
running  round  the  world.  The  oceans,  due  to  the  presence  of  hot  or 
cold  ocean  currents,  have  not  the  same  temperature  along  a  parallel  of 
latitude.  As  a  result  one  would  expect  highs  to  build  over  the  colder 
parts  of  an  ocean  while  lows  would  form  over  the  warmer  parts.  As  a 
result  of  this  the  belts  of  pressure  would  again  be  modified  or  broken  up. 
The  general  tendency,  then,  of  diversities  in  the  surface  would  be  to 
form  peaks  and  depressions  and  to  break  up  continuous  belts. 

In  section  119,  when  the  isobars  for  January  and  July  were  being  con- 
sidered, it  was  found  that  there  were  eight  areas  of  high  pressure  and  six 
areas  of  low  pressure  to  be  considered.  Six  of  these  highs  and  three  of 
these  lows  either  persist  throughout  the  whole  year  or  are  so  prominent 
during  a  large  portion  of  it  that  they  make  themselves  felt  in  the  annual 
averages  and  appear  on  the  chart  of  isobars  for  the  year.  These  peaks 
and  depressions  are  often  spoken  of  as  the  "  permanent  highs  and  lows  " 
or  as  "  the  centers  of  action  "  because  they  exert  such  a  marked  influence, 
as  will  be  seen  later,  on  the  wandering  areas  of  high  and  low  pressure 
which  constitute  our  storms  and  determine  the  weather  to  such  a  marked 
extent.  This  tendency  of  diversities  in  the  surface  to  break  up  the  belts 
of  pressure  modifies  to  such  an  extent  the  typical  planetary  wind  system 
that  it  is  sometimes  claimed  that  it  does  not  really  exist  on  the  poleward 
side  of  the  tropics,  but  is  replaced  by  an  air  circulation  about  these  highs 
and  lows. 

The  eight  highs  may  be  divided  into  two  groups.  There  are  five  which 
A  stud  of  are  l°cated  over  the  ocean  and  in  the  region  of  the  high 
the  eight  pressure  belts  at  35°  N.  and  30°  S.  latitude.  They  are  also 
nearly  permanent  in  position  and  persist  throughout  the 
year.  They  are  also  located  on  the  eastern  side  of  the  ocean. 
They  are  located  in  the  North  Pacific  opposite  California,  in  the  North 


Atlanti^^ 


PRESSURE  AND  CIRCULATION  OF  THE   ATMOSPHERE     173 


of  Spain,  in  the  South  Pacific,  in  the  South  Atlantic,  and 
in  the  Indian  Ocean.  In  each  case,  they  are  located  near  the  place  where 
a  cold  ocean  current  flowing  towards  the  equator  crosses  the  belts  of 
high  pressure  and  to  this  is  without  doubt  due  the  breaking  up  of  the 
belt  and  the  building  of  the  peak  of  pressure.  These  facts  have  just 
(1911)  been  brought  out  by  Humphreys  in  a  valuable  article  in  the  Mount 
Weather  Bulletin  and  for  further  details  the  reader  must  be  referred 
to  this  article.  The  other  three  highs  are  located  over  land  surfaces, 
namely,  over  Siberia,  over  North  America,  and  over  South  Africa.  In 
each  case  they  appear  during  the  winter  and  disappear  during  the 
summer.  The  Siberian  high  builds  to  such  an  immense  height 
during  the  winter  that  its  influence  is  felt  in  the  yearly  averages  and  it 
also  appears  on  the  chart  showing  the  isobars  for  the  year.  These  are 
without  doubt  due  to  the  low  temperatures  to  which  the  continents  fall 
during  the  winter.  During  the  summer,  when  the  land  surface  heats 
up,  they  disappear. 

The  six  lows  may  also  be  divided  into  two  classes.  Four  of  them 
are  located  in  the  equatorial  belt  of  low  pressure.  They  are  in  South 
Africa,  South  America,  Australia,  and  India.  In  each  case  they 
appear  during  the  summer  and  disappear  during  the  winter. 
They  are  due  without  doubt  to  the  excessive  heating  of  the 
land  surface  during  the  summer.  The  other  two  lows  are  located  near 
Iceland  and  just  south  of  Alaska,  in  each  case  over  the  ocean.  The 
Iceland  low  persists  throughout  the  year,  while  the  Aleutian  Islands 
low  disappears  during  the  summer.  These  are  the  two  depressions 
into  which  the  polar  cap  of  low  pressure  breaks  up.  These  are  without 
doubt  due,  as  Humphreys  has  also  pointed  out,  to  the  Gulf  Stream  and 
the  Japan  current.  The  warm  water  brought  to  the  far  north  by  the 
Gulf  Stream  stands  in  sharp  temperature  contrast  to  the  perpetual  ice 
and  snow  of  Greenland  and  Iceland.  As  a  result  a  low  persists  over  the 
ocean  throughout  the  year.  In  the  case  of  the  ocean  near  the  Aleutian 
Islands  the  same  thing  is  true  during  the  winter.  During  the  summer, 
both  Alaska  and  the  coast  of  Asia  heat  markedly.  As  a  result  the 
temperature  contrast  and  also  the  low  disappear. 

Charts  XIII  and  XIV  represent  the  air  circulation  of  the  Atlantic 
Ocean  for  January-February,  and  July- August.     The  vari-  Thg  ^  dr_ 
ous  components  of  the  planetary  wind  system  are  clearly  dilation  of 
evident.      The  trade  winds,  the  prevailing  westerlies,  the 
doldrums,  and  the  horse  latitudes  are  all  easily  recognized. 
The;  ted,  however,  by  lines  that  run  parallel  to  latitude 


174  METEOROLOGY 

lines ;  but  a  distinct  tendency  is  shown  to  blow  in  a  spirafflirectio 
around  these  areas  of  high  and  low  pressure. 

165.  Monsoons.  —  This  tendency  of  the  continents  to  become  the 
centers  of  high  pressure  during  the  winter  with  spirally  outflowing  wincls, 
Definition  of  and  the  centers  of  areas  of  low  pressure  during  the  summer 
a  monsoon.  w^h  spirally  inflowing  winds,  is  not  usually  sufficient  to  cause 
a  complete  reversal  of  the  wind  direction  between  winter  and  summer. 
The  temperature  differences  and  the  resulting  pressure  differences  are 
too  slight  to  bring  about  this  result.  The  usual  result  is  an  increase  or 
decrease  in  wind  velocity,  and  a  slight  change  in  wind  direction  between 
summer  and  winter.  There  are  several  instances,  however,  when  a 
complete  reversal  is  brought  about,  and  these  (winds  which  change 
direction  with  the  season^  are  spoken  of  as  monsoons.  The  word  is  of 
Arabic  origin  and  means  "  season."  Monsoons  are  particularly  notice- 
Where  mon-  able  in  connection  with  those  places  which  are  located  in  the 
soons  occur,  subtropical  or  subequatorial  wind  belts.  The  monsoonlike 
changes  in  the  wind  direction  brought  about  by  the  migration  of  the 
wind  system  when  aided  by  continental  influences  readily  develop  into 
pronounced  monsoons. 

The  most  typical  monsoon  in  the  world  occurs  in  India.     During  the 

winter  an  immense  peak  of  high  pressure  forms  over  southern  Siberia, 

with  a  central  pressure  of  30.50  inches.     The  wind  blowing 

and  de^86      spirally  outward  from  this  area  of  high  pressure  causes  north- 

scription  of     east  winds  over  India.     The  air  coming  over  southern  Siberia 

monsoon and  across  the  Himalaya  Mountains  is  dry,  and  this  is  spoken 

p  of  as  the  dry  or  winter  monsoon.  During  the  summer  the 
equatorial  belt  of  low  pressure  migrates  to  the  north  of  India,  and  the 
wind  blowing  spirally  inward  towards  this  belt  of  low  pressure  causes 
southwest  winds  to  blow  over  India.  The  air  coming  from  the  ocean  is 
moisture-laden,  and  when  forced  to  rise  in  this  area  of  low  pressure  and 
also  by  the  Himalaya  Mountains,  it  causes  drenching  rains  over  India. 
This  is  called  the  southwest  or  wet  monsoon.  The  coming  of  the  south- 
west monsoon  has  been  vividly  described  by  many  authors.  For  several 
weeks  previous,  the  air  has  been  nearly  calm  and  the  breezes  have  been 
light.  Then  the  wind  begins  to  blow  from  the  southwest,  at  first  fit- 
fully, but  gradually  attaining  greater  velocity  and  steadiness.  The 
dark  clouds  on  the  horizon  and  the  breaking  of  the  waves  on  the  shore 
announce  the  coming  of  the  steady  southwest  wind.  Soon,  with  light- 
ning flashes  and  thunder,  the  rain,  which  descends  in  torrents,  perhaps 
lasts  for  two  or  three  weeks  and  marks  the  beginning  of  the  monsoon.  The 


PRESSURE   AND  CIRCULATION   OF  THE  ATMOSPHERE     175 


FIG.  86.  —  Pressure  and  Wind  Distribution  over  India  during  January. 
(From  SALISBUKY'S  Physiography.) 

clouds  then  break  away,  and  wind  blows  steadily  and  freshly  from  the 
southwest,  and  rains  are  frequent  during  the  whole  of  the  monsoon 
period.  Figures  86  and  87  indicate  the  pressure  and  wind  distribution 
over  India  during  January  and  July. 

Monsoon  winds  are  also  noticeable  in  the  peninsula  of  Spain  and 
Portugal.     Figures  88  and  89  illustrate  the  pressure  and  wind  direction 
during  January  and  July.     It  will  be  seen  that  the  penin-  The  mon_ 
sula  is  the  seat  of  low  pressure  with  inflowing  winds  during  soons  of 
the  summer  and  the  seat  of  high  pressure  with  outflowing 
winds  during  the  winter.     Australia  alsov  exhibits  a  well-  and  of 
marked  monsoon,  as  illustrated  by  Fig.  90.     Other  portions 
of    the    world    also    exhibit    monsoon    tendencies,   but    not    to  such 
marked  extent  as  those  countries  just  mentioned. 


29.80 


FIG.  87.  —  Pressure  and  Wind  Distribution  over  India  during  July. 

(Fiom  SALISBURY'S  Physiography.) 


176 


METEOROLOGY 


1 66.  Other  land  effects.  —  The  various  results  due  to  the  fact  that 
the  earth  is  not  a  water  surface  throughout,  but  has  a  diversified  surface 
The  seven  made  up  of  part  land  and  water,  may  be  briefly  brought 
land  effects,  together  and  summarized  here :  (1)  The  belts  of  pressure 
are  broken  up  into  peaks  and  depressions  which  are  called  the  permanent 
highs  and  lows.  (2)  The  continents  are  the  cause  of  the  monsoons  which 


FIG.  88.  —  Pressure  and  Wind  Direction 
in  Spain  and  Portugal  during  January. 


FIG.  89.  —  Pressure  and  Wind  Direction 
in  Spain  and  Portugal  during  July. 


have  just  been  described.    (3)  The  diurnal  change  in  wind  velocity  and  wind 
direction  is  brought  about  by  the  presence  of  land.     During  the  day  the 

* 


SUMMER  WINTER 

FIG.  90.  —  Wind  Direction  in  Australia  during  January  and  July. 

wind  velocities  are  greater  than  during  the  night.  The  wind  direction 
also  changes  slightly  during  the  day  and  shifts  back  again  during  the 
night.  The  cause  of  this  is  convection,  which  causes  the  upper  air  during 
the  day  to  come  down  to  the  earth's  surface,  bringing  with  it  its  higher 
velocity  and  changed  direction.  Over  the  ocean  the  diurnal  change  in 
either  wind  velocity  or  direction  is  barely  noticeable.  (4)  The  conti- 

N 


PRESSURE  AND   CIRCULATION  OF  THE   ATMOSPHERE     177 

nents  also  cause  an  annual  change  in  wind  direction.  This  is  brought 
about  by  the  fact  that  they  become  centers  of  high  pressure  during  the 
winter  and  low  pressure  during  the  summer.  (5)  Mountain  chains 
cause  changes  in  wind  direction  and  velocity.  This  is  not  only  true  on  a 
small  scale  in  connection  with  local  barriers,  but  the  longer  -a'hd  higher 
mountain  chains  of  the  world  influence  the  general  winds  of  the  world 
both  as  regards  direction  and  velocity.  (6)  Wind  velocity  is  much 
reduced  by  friction  when  the  air  is  passing  over  land.  This  is  proven  by 
the  fact  that  the  normal  wind  velocity  over  the  ocean  is  nearly  twice 
what  it  is  over  the  land.  (7)  The  Arctic  winds,  if  they  exist,  are  without 
doubt  the  result  of  a  land  surface.  Only  few  observations  have  been  made 
in  the  polar  regions,  but  they  would  seem  to  show  that  the  barometric 
pressure  does  not  decrease  steadily  as  the  pole  is  approached,  but  that 
it  reaches  a  minimum  in  about  70°  or  80°  north  latitude,  and  then  in- 
creases slightly  toward  the  pole.  In  the  polar  regions,  winds  blowing 
from  the  north  instead  of  the  southwest  have  also  been  observed.  These 
would  be  the  inevitable  result  of  an  increase  of  pressure  toward  the  pole. 
If  these  observations  are  trustworthy,  the  explanation  is  without  doubt 
to  be  found  in  the  increasing  percentage  of  land  near  the  north  pole. 
This  would  decrease  the  wind  velocity  so  that  the  highest  values  of 
wind  velocity  and  thus  the  least  pressure  would  be  found,  not  at  the 
north  pole  itself,  but  perhaps  20°  or  30°  from  it.  The  observations, 
however,  are  not  sufficiently  numerous  or  trustworthy  to  make  an 
elaborate  explanation  or  treatment  of  the  subject  necessary. 

/ 
LAND  AND  SEA  BREEZES 

167.   At  the  seashore  the  land  is  heated  more  than  the  adjacent  water 
during  the  day  and  at  night  by  rapid  cooling  becomes  colder  than  the 
adjacent  water.     This  gives  rise  to   a  small  convectional 
circulation  known  as  land  and  sea  breezes.     The  sea  breeze  tion  of  land 
ordinarily  begins  about  ten  or  eleven  o'clock  in  the  morning  JJ1*  "* 
and   blows   gently   but  with  increasing  velocity  from  the 
ocean  toward  the  land.     It  does  not  usually  penetrate  more  than  ten 
or  fifteen  miles  inland,  and  it  reaches  its  maximum  velocity  ordinarily 
at  two  or  three  in  the  afternoon.     At  about  sunset  the  sea  breeze  dies 
out,  and  it  is  replaced  during  the  night  by  the  land  breeze,  which  blows 
from  the  land  toward  the  ocean.     The  velocity  of  the  sea  breeze  and  its 
regularity  are  usually  more  marked  than  in  the  case  of  the  land  breeze. 
The  lars^1  — J  sea  breezes,  as  has  been  fully  shown  hv  nhsprvations  made 


178 


METEOROLOGY 


by  kites  and  captive  balloons,  do  not  ordinarily  extend  to  a  greater 
height  than,  say,  a  thousand  feet.  The  phenomenon,  therefore,  of 
land  and  sea  breezes  is  confined  to  a  space  of  fifteen  miles  either  side  of 
the  coast  line,  and  to  the  lower  thousand  feet  of  atmosphere. 

The  explanation  of  the  land  and  see  breezes  may  be  thus  stated : 

During  the  early  hours  of  the  morning  the  land  heats  rapidly  under 

sunshine   and   becomes   warmer  than  the   adjacent  water. 

nation  of        The  air  expands,  flows  off  aloft  over  the  ocean,  thus  increas- 

land  and  sea  mg  fae  pressure  slightly  over  the  ocean,  and  decreasing  it 

over  the  land.     This  increased  pressure  over  the  ocean  drives 

in  the  air  in  the  form  of  the  sea  breeze.     At  night  the  land  and  the  layer 

of  air  next  to  it  cool  rapidly,  and  the  converse  process  takes  place,  the 

air  being  forced  to  move  by  the  increased  pressure  from  the  land  toward 

the  ocean. 

It  has  been  often  observed  that,  when  the  sea  breeze  first  makes  its 
appearance,  it  starts  some  distance  from  the  land,  as  is  shown  by  the 
rippled  surface  of  the  water,  and  then  slowly  beats  its  way 
in  toward  the  shore.  This  can  be  readily  explained.  As 
the  air  over  the  land  becomes  heated  during  the  early  morn- 
ing hours,  it  tends  to  expand  laterally  as  well  as  to  flow  off 
above  over  the  ocean.  This  lateral  expansion  would  tend 
to  hinder  the  coming  of  the  sea  breeze  near  the  shore.  Furthermore  it 
is  the  increased  pressure  over  the  ocean  which  drives  in  the  sea  breeze, 
and,  as  this  is  first  felt  at  a  considerable  distance  from  the  shore,  it  is 
there  that  one  ought  to  expect  the  first  beginnings  of  the  sea  breeze. 

In  many  places  the  sea  breeze  is  not  felt  as  a  breeze  blowing  directly 
from  the  ocean  during  the  day,  and  the  land  breeze  at  night  does  not 

blow  di- 
rectly from 
the     land, 
but     they 
are     com- 
pounded with  the  pre- 
vailing wind  and  thus 
merely  cause  changes 
in    the    direction    or 
velocity  of   the   pre- 
vailing wind.     On  the  shores  of  Long  Island  the  prevailing  wind  direc- 
tion is  west,  and  as  is  shown  in  Fig.  91,  the  sea  breeze  changes  this  to 
a  southwest  wind  by  day  and  the  land  breeze  is  combined  with  it  and 


Why  the  sea 
breeze  starts 
some  dis- 
tance from 
the  land. 


Land  and 
sea  breezes 
combine 
with  the  pre- 
vailing wind. 


PREVAILING  WEST  WIND 


AT  NIGHT 


FIG.  91.  —  The  Effect  of  Land  and  Sea  Breezes  on  the  Pre- 
vailing West  Wind  on  the  Shores  of  Long  Island. 


PRESSURE  AND   CIRCULATION  OF  THE  ATMOSPHERE      179 

becomes  a  northwest  wind  during  the  night.  On  the  coast  of  Cali- 
fornia and  Chile,  where  the  prevailing  wind  direction  is  west,  that  is, 
from  the  ocean  toward  the  land,  the  sea  breeze  during  the  day  causes 
a  marked  increase  in  the  wind  velocity,  while  the  land  breeze  during 
the  night  reduces  it  to  practically  a  calm.  This  is  said  to  be  partic- 
ularly pronounced  in  the  portions  of  Chile  near  Valparaiso.  Here  the 
wind  velocity  during  the  day  may  amount  almost  to  a  decided  gale, 
hindering  walking,  making  the  transaction  of  business  unpleasant,  and 
almost  cutting  off  intercourse  between  vessels  in  the  harbor  and  the 
shore.  At  night  an  almost  complete  calm  reigns. 

MOUNTAIN  AND  VALLEY  BREEZES 

1 68.  The  so-called  mountain  and  valley  breezes  are  well  known  in 
all  mountainous  countries,  but  they  are  particularly  noticeable  in  long, 
narrow  valleys  which  emerge  on  a  large  open  plain  below*  .  . . 

They  are  periodic,  sun-caused  winds  which  are  particularly  ofthemoun- 

well  developed  on  still  clear  days.     During;  the  night,  from  tain  and  val~ 

1  .  .ley  breeze, 

a  few  hours  after  sunset  until  early  morning,  the  mountain 

breeze  flows  down  from  the  mountains  through  the  valley  to  the  plain 
below.  It  is  not  only  easily  detected,  but  at  times  it  attains  a  moderate 
velocity;  and  those  who  camp  at  night  in  the  open  in  mountainous 
countries  soon  learn  to  build  camp  fires  on  the  downhill  side,  so  that  the 
mountain  breeze  may  blow  the  smoke  away  from  their  tents.  During 
the  day  the  valley  breeze  is  felt  as  a  gentle  breeze  blowing  up  through 
the  valley  and  up  the  mountain  slopes.  It  is  usually  hardly  noticeable 
and  is  never  as  well  developed  as  the  mountain  breeze.  • 

During  the  night,  the  layer  of  air  next  the  ground  becomes  cooled  by 
radiating  its  heat  to  the  colder  ground  and  to  the  sky,  and  also  by  con- 
duction to  the  cold  ground.  This  layer  of  cold,  and  thus  The  cause 
dense,  air  drains,  just  as  water  would,  into /the  valleys  and  ofthemoun- 
places  of  less  elevation  than  surrounding  regions.  As  a  * 
result,  depressions  are  filled  at  night  with  pockets  of  colder  air  which 
have  drained  into  them  from  the  surrounding  slopes.  In  a  long,  narrow 
valley  this  drainage  of  colder  air  makes  itself  felt  as  a  mountain  breeze: 
If  a  glacier  is  located  in  the  valley,  the  layer  of  air  next  it  may  be  cooled 
sufficiently  to  cause  a  mountain  breeze  even  during  the  daytime.  .  As 
the  mountain  breeze  moves  down  the  valley  the  air  is  compressed  and 
thus  is  heated  to  the  extent  of  1.6°  for  every  300  feet  of  descent.  If  the 
motion  down  the  valley  is  slow  so  that  there  is  sufficient  time  for  the  air 


180  METEOROLOGY 

to  radiate  its  heat  to  the  ground  and  sky,  or  to  lose  it  by  conduction,  it 
may  reach  the  open  plain  below  as  cold  or  even  colder  than  the  air  over 
the  plain.    If,  however,  the  descent  has  been  rapid,  the  air  will  probably 
reach  the  plain  with  a  higher  temperature  than  the  air  over  the  plain, 
and  to  this  fact  may  be  due  the  often  observed  result  that  frosts  are  less 
severe  on  a  plain  opposite  the  point  of  entering  of  a  long  valley. 

During  the  day  the  air  at  the  bottom  of  a  valley  is  heated  more  than 
the  air  along  the  mountain  slopes,  particularly  as  one  side  of  a  valley 

The  cause      is     nearly     always     in 
x^  —    «—  -  ____________  —  >      _-^*       of  the  vai-      shade.     As  a  result  of 

\-~-"""'  _        ^  --.  ../  eeze'     this    heating,   the    iso- 

\*     /        baric  surfaces  are  warped  upward.  . 


~*  In  Fig.   92,  which  shows  a  cross 


section    of   a   valley,  the  normal 

^_  _^  isobaric  surfaces  are   represented 

by  fujj  lines,  while  the  upward 
bulging  of  these  surfaces  is  indi- 
cated by  dotted  lines.  As  a  re  — 

FIG.  92.  —  Cross  Section  of  a  Valley,  showing  IA     .*  .J.  'TIT-  r^.u 

the  Isobaric  Surfaces.  suit  of  this  upward  bulging  of  the 

isobaric  surfaces,  the  air  flows  off 

toward  the  side;  and,  when  it  meets  the  sloping  sides  of  the  valley,  it 
is  deflected  upward,  thus  causing  valley  breezes  which  flow  up  the 
mountain  slopes  during  the  day. 

ECLIPSE,  LANDSLIDE,  TIDAL,  AND  VOLCANIC  WINDS 

169.   Eclipse  winds.  —  When,  during  an  eclipse  of  the  sun,  the  shadow 
of  the  moon  falls  upon  the  earth,  the  sun's  radiant  energy  is  withheld 

from  a  belt  which  represents  the  path  of  the  eclipse.  As  a 
acteristks  result  of  this,  the  air  ought  to  be  colder  here  than  in  the 
and  cause  of  surrounding  regions,  and  thus  a  belt  of  high  pressure  should 
wfnds?  be  developed  along  the  path  of  an  eclipse  of  the  sun..  The 

air  would  gently  descend  in  this  belt  of  high  pressure  and 
flow  outward  in  both  directions  from  it  along  the  earth's  surface.  The 
few  observations  which  have  been  made  in  connection  with  eclipses 
would  seem  to  show  that  these  conditions  are  actually  realized.  But' 
few  observations  have,  however,  been  made;  and  the  winds  due  to  an 
eclipse  are  of  such  seldom  occurrence,  of  such  little  importance,  and  of 
such  slight  intensity  that  they  are  hardly  worth  considering.  They 
would  be  classified  as  irregular  sun-caused  winds. 


PRESSURE  AND   CIRCULATION  OF  THE   ATMOSPHERE     181 

170.  Landslide    and   avalanche   winds.  —  Masses   of    air   are   often 
pushed  ahead  of  landslides  and  avalanches  in  sufficient   quantity  to 
cause  destruction  even  at  considerable  distances.     The  rush 

of  air  ahead  of  an  avalanche  is  sometimes  felt  at  a  distance  and  ava- 
of  a  mile,  and  destruction  has  been  caused  at  a  distance  of  a  la*che 
quarter  of  a  mile.     These  winds  are  ordinarily  classified  as 
irregular  earth-caused  winds.     In  the  case  of  an  avalanche  wind,  how- 
ever, it  is  indirectly  of  solar  origin,  for  it  is  the  energy  of  the  sun  which 
has  evaporated  the  water  and  caused  the  circulation  of  the  air  which  has 
deposited  the  snow  on  the  mountain  side  to  build  the  avalanche. 

171.  Tidal  winds. — In  some  gulfs  and  bays,  where  the  rise  and  fall 
of  the  tide  is  considerable,  a  slight  motion  of  the  air  away  from  the  gulf 
or  bay  when  the  tide  is  coming  in  and  toward  the  gulf  or 

bay  when  the  tide  is  going  out  has  been  observed.      This  teristics  and 
would  seem  to  be  caused  by  the  actual  raising  of  the  atmos-  c*^e  °.f 
phere  by  the  incoming  water  as  the  tide  comes  in  and  by  the 
falling  of  the  atmosphere  to  take  the  place  of  the  out-going  water  as 
it  recedes.     These  tidal  winds,  if  they  exist,  would  be  compounded  with 
the  land  and  sea  breeze  in  such  a  way  that  it  would  require  careful 
observations  at  many  stations  surrounding  the  gulf  or  bay  to  be  sure 
of  their  presence.      Such  breezes,  if  they  exist,  would  be  classified  as 
periodic  moon-caused  winds. 

172.  Volcanic  winds.  —  It  has  been  observed  in  connection  with  cer- 
tain volcanoes  while  in  eruption  that  there  seems  to  be  a  rising  air  column 
above  the  volcano  accompanied  by  inflowing  breezes  from  Volcanic 

all  directions.  There  are  two  causes  for  the  rising  of  air  over  winds- 
the  volcano.  In  the  first  place  it  may  be  caused  by  the  explosive  action 
of  the  eruption;  and  secondly,  the  heating  of  the  air  by  the  hot  lava  and 
other  material  ejected  from  the  volcano  may  cause  convectional  motion. 
A  volcano  in  violent  eruption  is  sometimes  capped  by  a  thundershower 
with  violent  thunder  and  lightning.  These  volcanic  winds  are  of 
telluric  origin  and  are  irregular  as  to  time  of  occurrence. 

CYCLONIC  STORMS 

173.  The  general  winds  of  the  earth  have  now  been  considered  in 
detail,  and,  as  a  result  of  what  has  been  said,  it  ought  to  be  possible  to 
state  for  any  portion  of  the  earth  exactly  what  changes  in  wind  direc- 
tion and  wind  velocity  ought  to  take  place  in  the  course  of  a  day  or 
a  year.     In  the  northeastern  portion  of  the  United  Stafes,  for  example, 


182  METEOROLOGY 

the  prevailing  wind  direction  should  be  from  some  westerly  quarter, 
since  this  portion  of  the  country  is  located  in  the  region  of  the  prevail- 
ing westerlies.  The  wind  ought  to  blow  more  from  the 
southwest  in  summer  and  from  the  northwest  in  winter, 
regards  di-  when  the  extensive  area  of  high  pressure  builds  over  North 
veioc°i°y^fd  America  due  to  the  low  temperatures.  The  wind  velocity 
the  winds  in  ought  to  be  higher  in  winter  than  in  summer,  reaching  its 
easterruSart  maximum  m  February  or  March  and  its  minimum  in  August 
of  the  or  September.  The  velocity  ought  to  be  somewhat  greater 

States1  during  the  daytime  than  at  night,  and  there  should  also 

be  a  slightly  diurnal  change  in  wind  direction.  If  a  place  is 
located  near  the  seashore  or  in  a  mountainous  region,  the  land  and  sea 
breezes  and  the  mountain  and  valley  breezes  ought  to  make  them- 
selves felt.  If  careful  observations  of  the  changes  in  wind  direction 
or  in  wind  velocity  are,  however,  made  for  any  period  of  time,  it  will 
be  at  once  noticed  that  the  irregularities  both  as  regards 

There  are          _.  .  _  .        .      . 

numerous      directiojAnd  velocity  are  numerous.      For  example,  during 
irregu-  t/ne  wi^H^Bheii  the  wind  ought  to  be  blowing  from  the 

northw^^^^h  moderate  velocity,  the  wind  will  perhaps  be 
blowing  from  ^he^HBFwith  high  velocity  accompanied  by  driving  snow. 
This  fcay  continue  during  a  whole  night  and,  instead  of  increasing,  die 
day  comes.  All  of  these  irregular  and  unlooked-for  wind 
directions  and  wind  velocities  are  due  to  the  presence  of 
ies  are  so-called  storms,  of  which  there  are  four  different  kinds : 
due  to  extratropical  cyclones  or  the  lows  of  our  weather  map ; 

storms,  of  .  . 

which  there  tropical  cyclones,  which  are  sometimes  called  hurricanes  in 
arejour  tne  West  inciies  or  typhoons  in  the  China  Sea;  thunder 
showers;  tornadoes.  These  four  storms  have  been  ar- 
ranged in  order  of  size,  for  the  extratropical  cyclones  sometimes 
cover  an  area  a  thousand  or  more  miles  in  diameter,  while  the 
tornadoes,  although  more  violent  than  any  of  the  others,  cover  but  an 
extremely  small  area.  These  are  usually  spoken  of  as  cyclonic  storms, 
because  the  air  in  each  case  is  usually  in  motion  in  a  spiral  direction. 
These  will  be  fully  considered  in  one  of  the  following  chapters. 

WINDS  OF  OTHER  PLANETS 

174.  A  careful  consideration  of  those  changes  in  the  general  winds  of 
our  own  earth  which  would  result  from  a  change  in  the  conditions  upon 
which  they  depend  will  serve  as  an  introduction  to  the  consideration 
of  the  wind  system  which  ought  to  exist  on  the  various  planets. 


PRESSURE  AND  CIRCULATION  OF  THE  ATMOSPHERE     183 

If  the  earth  turned  more  rapidly  on  its  axis,  the  deflective  force  would 
be  greater,  and  the  circulation  between  equator  and  pole  would  be 
more  oblique  than  at  present.     As  an  example,  the  trade  The  effect 
winds,  in  the  northern  hemisphere,  instead  of  blowing  from  on  the  wind 
the  northeast  would  blow  from  a  still  more  easterly  point.  systemo?a 

*j    *  more  rcipid 

The  effect  of  having  a  more  oblique  circulation  would  be  to  rotation  of 
cause  higher  wind  velocity,  to  retard  the  exchange  of  air  t 
between  equator  and  pole,  and  thus  to  accentuate  the  differences  in 
temperature  which  already  exist. 

If  the  earth's  axis  were  inclined  to  the  plane  of  its  orbit  at  a  greater 
angle  than  23.5°,  the  migration  of  the  wind  belts  would  be 
more  pronounced,  the  subtropical  and  subequatorial  belts 
would  be  wider,  and  the  tendency  to  monsoons  would  be  system  of  a 
felt  over  a  larger  portion  of  the  earth's  surface.  cUnation" 

If  the  earth's  surface  were  entirely  a  water  surface  instead 
of  being  a  diversified  land  and  water  surface,  land  and  sea  breezes,  and 
mountain  and  valley  breezes  would  be  absent^tJ^belts 
of  pressure  would  be  uniform  around  the  world^B^Hpons  on  ®nee  wjnd 
would  be  unknown ;    the  daily  change  in  wincHj^fcction  system  of  a 
and  wind  velocity  would  no  longer  exist,  and  theyearly  surface! 
change  in  wind  velocity  would  be  very  much  less. 

The  factors,  then,  upon  which  the  characteristics  of  the  wirfft  s 
depend  are  the  rotation  of  the  earth  on  its  axis,  the  location 
belt  of  highest  temperature,  the  migration  of  this  belt  of 
highest  temperature   during  the  year,  and  the  character- 
istics and  diversity  of  the  surface.    If  these  facts  were  known  termine  the 
in  detail  for  any  planet,  it  ought  to  be  possible  to  determine  ^tem> 
by  deduction  the  characteristics  of  the  wind  system  which 
should  be  present. 

175.  Mercury  and  probably  Venus  turn  the  same  face  toward  the 
sun  always,  and  thus-  rotate  on  their  axes  very  slowly,  Mercury 
making  a  complete  rotation  in  eighty-eight  days  and  Venus 
in  two  hundred  and  twenty-five  days.  As  a  result  the 
center  of  the  illuminated  hemisphere  ought  to  become  an  systems  of 
area  of  low  pressure  with  air  blowing  spirally  inward  along 
the  surface  of  the  planet,  rising  in  this  area  of  low  pressure, 
flowing  outward  on  the  outside  of  the  atmosphere,  and  descending  again 
in  an  extensive  area  of  high  pressure  which  would  form  on  the  dark 
hemisphere  of  the  planet.  In  the  case  of  Mars,  since  the  surface  features 
are  but  seldom  obscured  by  haze  or  cloud,  no  observations  are  available 


184  METEOROLOGY 

• 

for  determining  the  characteristics  of  the  air  circulation  on  that  planet. 
Since  it  turns  on  its  axis  in  about  the  same  time  as  the  earth,  and  has  an 
axis  inclined  at  about  the  same  angle  to  the  plane  of  its  orbit,  it  ought 
to  have  a  system  very  similar  to  ours.  In  the  case  of  Jupiter,  Saturn, 
Uranus,  and  Neptune,  the  planets  are  probably  continually  cloud-covered, 
and,  particularly  in  the  case  of  Jupiter  and  Saturn,  there  are  belts 
parallel  to  the  equator  which  are  easily  observable.  These  are  probably 
due  to  the  atmospheric  circulation,  but  since  no  facts  are  known 
concerning  the  surface  of  these  planets,  it  is  impossible  to  form  any 
definite  deductions  as  to  what  wind  system  ought  to  exist. 

QUESTIONS 

(1)  Why  is  pressure  an  important  meteorological  element?  (2)  Why  does 
the  atmosphere  exert  a  pressure?  (3)  Is  the  atmospheric  pressure  constant? 
(4)  How  is  the  pressure  of  the  atmosphere  measured  and  expressed?  (5)  Give 
a  detailed  history  of  the  invention  of  the  mercurial  barometer.  (6)  Describe  a 
mercurial  barometer  as  used  at  present.  (7)  Describe  in  detail  the  cistern  of  a 
mercurial  barometer.  (8)  What  are  the  two  steps  in  reading  a  mercurial 
barometer.  (9)  Name  and  treat  fully  the  three  corrections  to  be  applied  to  the 
reading  of  a  mercurial  barometer.  (10)  Describe  an  aneroid  barometer.  (11) 
What  corrections  must  be  made  to  the  reading  of  an  aneroid  barometer?  (12) 
Compare  the  accuracy  of  an  aneroid  and  mercurial  barometer.  (13)  What  is 
a  barograph?  (14)  Describe  the  Richard  Freres  barograph.  (15)  Describe 
the  construction  and  action  of  the  so-called  "  mouth  barometer."  (16)  Describe 
the  construction  and  action  of  the  "  chemical  weather  glass."  (17)  What 
causes  the  changes  in  its  appearance?  (18)  What  observations  of  pressure  are 
taken  at  the  various  Weather  Bureau  stations?  (19)  What  instruments  are  used 
and  where  are  they  located  ?  (20)  How  are  the  pressure  normals  computed  ? 
(21)  Describe  the  diurnal  variation  in  pressure.  (22)  With  what  does  the 
diurnal  variation  in  pressure  changer  (23)  Treat  fully  the  various  explanations 
of  this  diurnal  variation.  (24)  Describe  the  annual  variation  in  pressure  and 
state  its  cause.  (25)  What  other  barometric  data  may  be  computed  from  the 
observations  of  pressure?  (26)  Describe  the  old  method  of  reducing  pressure 
observations  in  order  to  compare  them.  (27)  Describe  the  modern  way  of 
reducing  pressure  observations  to  sea  level.  (28)  What  observations  are  neces- 
sary to  determine  altitude  by  means  of  the  barometer  ?  (29)  How  are  these 
observations  secured  in  practice  ?  (30)  Describe  in  detail  the  four  methods  of 
computing  the  difference  in  elevation  from  the  observations  made^  (31)  How 
are  isobaric  charts  constructed  ?  (32)  Describe  the  chief  characteristics  of  the 
isobars  for  the  year.  (33)  How  is  the  vertical  section  along  a  meridian  con- 
structed? (34)  Describe  the  course  of  the  thirty-inch  line.  (35)  Why  do 
differences  in  pressure  exist  over  the  earth's  surface?  (36)  What  is  meant  by 
an  isobaric  surface?  (37)  What  is  the  normal  form  of  an  isobaric  surface? 

(38)  What  is  the  form  of  isobaric  surfaces  over  areas  of  high  and  low  pressure? 

(39)  Describe  the  chief  characteristics  of  the  isobars  for  January  and  July. 

(40)  What   other   pressure   charts   might   be   constructed?     (41)  Define   wind 
and  state  what  three  things  can  be  measured.     (42)  Define  windward,  leeward, 
veering,  and  backing.     (43)  To  how  many  points  of  the  compass  is    the  wind 


PRESSURE   AND  CIRCULATION  OF  THE  ATMOSPHERE     185 

direction  determined?  (44)  What  is  the  relation  between  wind  velocity  and 
the  pressure  of  the  wind  ?  (45)  Describe  the  wind  vane  of  the  Weather  Bureau 
form.  (46)  What  are  the  advantages  of  this  form  of  wind  vane?  (47)  What 
is  the  anemoscope?  (48)  Describe  the  rotating  cylinder  form  of  anemoscope. 
(49)  Describe  the  electrical  contact  form  of  anemoscope.  (50)  What  is  meant 
by  a  wind  scale?  (51)  Describe  the  ten-point  and  the  Beaufort  wind  scale. 
(52)  State  the  three  groups  of  anemometers.  (53)  Describe  a  simple  deflection 
anemometer.  (54)  Describe  two  pressure  anemometers.  (55)  Describe  the 
Robinson  cup  anemometer.  (56)  How  is  this  instrument  tested  and  its  constant 
determined?  (57)  Describe  other  forms  of  anemometers.  (58)  How  can  small 
wind  velocities  be  determined?  (59)  What  are  the  chief  effects  of  surroundings 
on  wind  direction  and  velocity  ?  ^  (60)  Describe  the  influence  of  a  valley.  (61) 
Describe  the  effect  of  buildings,  (62)  Describe  the  effect  of  the  nature  of  the 
surface.  (63)  What  is  the  effect  of  altitude?  (64)  What  are  the  advantages 
of  mountain  observatories?  (65)  What  observations  of  wind  are  taken 
at  regular  Weather  Bureau  stations?  (66)  State  the  instruments  used 
and  their  location.  (67)  Define  prevailing  wind  direction.  (68)  How  may 
prevailing  wind  direction  be  expressed?  (69)  Describe  the  construction  of 
a  wind  rose.  (70)  How  are  normal  wind  velocities  computed?  (71)  De- 
scribe the  daily  variation  in  wind  velocity  and  state  its  cause.  (72)  Describe 
the  daily  variation  in  wind  direction  and  state  its  cause.  (73)  How  may  the 
diurnal  variations  in  wind  direction  and  velocity  be  expressed  graphically? 
(74)  Describe  the  annual  variation  in  wind  velocity  and  state  its  cause^  (75) 
Describe  the  annual  variation  in  wind  direction  and  state  its  cause.  (76)  What 
is  the  cause  of  irregular  variations  in  wind  direction  and  velocity?  (77)  What 
other  wind  data  may  be  determined  from  the  observations?  (78)  State  the 
different  characteristics  of  the  prevailing  winds  of  the  world.  (79)  What  other 
wind  charts  may  be  prepared?  (80)  Describe  the  general  convectional  motion 
in  a  long  tank  heated  at  the  bottom.  (81)  Why  may  a  convectional  circulation 
between  equator  and  pole  be  expected?  (82)  What  other  convectional  circu- 
lations would  be  expected?  (83)  Illustrate  by  means  of  two  diagrams  the  ar- 
rangement of  the  isobaric  surfaces  in  the  general  convectional  circulation  just 
before  its  beginning  and  after  it  has  become  permanently  established.  (84) 
What  is  the  meridional  view  of  the  air  circulation  ?  (85)  State  the  condition  of 
steady  motion.  (86)  What  is  meant  by  a  barometric  gradient?  (87)  How  is 
it  expressed?  (88)  What  is  the  relation  of  wind  direction  to  pressure  gradient 
and  isobaric  lines?  (89)  What  is  the  reason  for  the  relation?  (90)  Describe 
the  air  motion  about  highs  and  lows.  (91)  State  the  historical  rise  of  the  recog- 
nition of  the  effect  of  the  earth's  rotation  on  air  motion.  (92)  What  was  Hal- 
ley's  explanation  of  the  direction  of  the  trade  winds?  (93)  Describe  Ferrel's 
work.  (94)  State  the  effect  of  the  earth's  rotation  on  the.  air  motion.  (95) 
State  the  effect  of  the  earth's  rotation  on  the  air  masses  moving  poleward  on  the 
outside  of  the  atmosphere.  (96)  Explain  in  full  the  cause  of  the  polar  caps  of 
low  pressure.  (97)  Illustrate  by  means  of  two  diagrams  the  meridional  section 
of  pressure  on  a  rotating  and  on  a  non-rotating  earth.  (98)  Illustrate  by  means 
of  two  diagrams  the  air  motion  about  highs  and  lows  on  a  rotating  and  non-rotat- 
ing earth.  (99)  What  is  Buys  Ballot's  law?  (100)  What  is  the  reason  for 
its  truth?  (101)  Contrast  the  inductive  and  the  deductive  method  of  gaining 
information  as  illustrated  in  this  chapter.  (102)  What  is  meant  by  the  general 
winds  of  the  globe ?  (103)  What  is  the  basis  of  the  Dove  classification?  (104) 
What  is  the  basis  of  the  Davis  classification?  (105)  Define  planetary  winds. 
(106)  Illustrate  by  means  of  a  diagram  the  air  circulation  near  the  earth's  sur- 
face. (107)  Illustrate  by  means  of  a  diagram  the  air  circulation  in  the  upper 


186  METEOROLOGY 

layer  of  the  atmosphere.  (108)  Illustrate  by  means  of  a  diagram  the  air  motion 
in  the  intermediate  layer.  (109)  Illustrate  by  means  of  a  diagram  the  atmos- 
pheric circulation  in  cross  section.  (110)  What  are  the  effects  of  the  earth's 
rotation?  (Ill)  Describe  the  trade  winds.  (112)  Describe  the  doldrums. 
(113)  Describe  the  horse  latitudes.  (114)  Describe  the  prevailing  westerlies. 
(115)  Describe  the  upper  currents.  (116)  In  what  two  ways  must  the  planet- 
ary system  of  winds  be  modified?  (117)  Define  terrestial  winds.  (118)  What 
causes  the  yearly  migration  of  the  wind  system?  (119)  What  are  the  charac- 
teristics and  amount  of  this  migration?  (120)  Describe  the  subequatorial  and 
subtropical  wind  belts.  (121)  Illustrate  by  means  of  a  diagram  the  kind  of 
winds  found  in  each  belt.  (122)  Define  continental  winds.  (123)  What  is  the 
effect  of  a  diversified  surface  on  the  belts  of  pressure?  (124)  What  effect  does 
the  building  of  peaks  of  pressure  and  depressions  have  on  the  air  circulation? 
(125)  What  is  a  monsoon?  (126)  Describe  the  monsoons  of  India.  (127) 
State  in  full  the  cause.  (128)  At  what  other  place  on  the  earth's  surface  are 
monsoons  felt?  (129)  State  the  various  effects  of  land  on  wind  direction  and 
wind  velocity.  (130)  Treat  fully  the  so-called  Arctic  winds.  (131)  Describe 
the  land  and  sea  breezes.  (132)  What  is  the  explanation  of  land  and  sea  breeze? 
(133)  Why  does  the  sea  breeze  start  over  the  ocean.  (134)  In  what  way  is 
land  and  sea  breeze  combined  with  other  winds?  (135)  Describe  the  mountain 
and  valley  breezes.  (136)  Explain  in  detail  the  valley  breeze  during  the  day. 
(137)  Explain  in  detail  the  mountain  breeze  during  the  night.  (138)  Describe 
in  detail  the  air  motion  caused  by  eclipses,  landslides,  tides,  and  volcanoes. 

(139)  How  are  these  various  winds  classified  in  each  system  of  classification? 

(140)  What  is  the  cause  of  irregularities  in  the  general  winds  of  the  world. 

(141)  Name  the  four  storms.     (142)  What  would  be  the  effect  on  the  wind 
system  if  the  earth  turned  more  rapidly  on  its  axis?     (143)  What  would  be  the 
effect  on  the  wind  system  if  the  earth's  axis  were  inclined  at  a  greater  angle? 
(144)  Name  the  factors  upon  which  the  characteristics  of  the  wind  system  of  a 
planet   depend.     (145)  Describe   the   probable  wind   systems   of   the  various 
planets. 

TOPICS   FOR   INVESTIGATION 

(1)  The  history  of  the  mercurial  barometer. 

(2)  Modified  forms  of  mercurial  barometers. 

(3)  The  various  forms  of  barographs. 

(4)  The  construction  and  action  of  chemical  weather  glasses. 

(5)  The  diurnal  variation  in  barometric  pressure. 

(6)  The  barometric  determination  of  altitude. 

(7)  Wind  scales. 

(8)  Anemometers. 

(9)  Mountain  observatories  and  their  work. 

(10)  Ferrel' s  contributions  to  meteorology. 

(11)  The  monsoons  of  India. 


PRACTICAL   EXERCISES 

(1)  Test  carefully  an  aneroid  barometer.     This  might  include  its  accuracy  for 
a  given  range  of  pressures,  the  effects  of  temperature  changes,  and  the  effects  of 
jars  due  to  transportation. 

(2)  Study  a  chemical  weather  glass. 


PRESSURE  AND   CIRCULATION  OF  THE  ATMOSPHERE     187 

(3)  Learn  to  read  a  mercury  barometer  and  to  reduce  the  observations  by 
applying  all  the  corrections. 

(4)  Plot  the  graphs  showing  the  annual  variation  in  pressure  at  several  sta- 
tions and  explain  the  peculiarities  of  each  graph. 

(5)  Determine  a  difference  in  elevation  by  means  of  a  barometer. 

(6)  Work  up  some  or  all  of  the  barometric  or  pressure  data  mentioned  in 
section  110  for  several  stations.     Those  stations  may  be  chosen  in  which  the 
student  has  a  particular  interest.     A  barograph  record  of  considerable  length  is 
almost  essential.     Extremes  of  pressure  and  the  frequency  and  magnitude  of  the 
irregular  variations  would  be  the  most  interesting.     The  attempt  should  be  made 
to  draw  general  conclusions  by  contrasting  different  places,  summer  and  winter, 
etc. 

(7)  Determine  the  wind  velocity  on  the  stillest  nights  of  winter. 

(8)  Construct  several  wind  roses  for  different  places  for  the  same  month  or 
year  and  for  the  same  place  for  different  months. 

(9)  Work  up  some  or  all  of  the  wind  data  mentioned  in  section  136. 

(10)  Perform  the  experiment  illustrated  in  Fig.  70. 

(11)  Investigate  some  valley  for  mountain  and  valley  breezes. 


REFERENCES 

For  the  description,  illustration,  construction,  and  use  of  apparatus  to  measure 

pressure  and  wind,  see  : 
ABBE,  Meteorological  Apparatus  and  Methods   (Washington,  1888) ;    pp.  Ill  to 

179  and  183  to  336  treat  of  pressure  measuring  and  wind  measuring  instru- 
ments respectively. 

MOORE,  JOHN  W.,  Meteorology,  2d  ed.,  pp.  120  to  138  and  265  to  292. 
PLYMPTON,  The  Aneroid  Barometer  (D.  Van  Nostrand  Company,  New  York). 
WALDO,  Modern  Meteorology,  pp.  59  to  135. 
WILD,  H.,  Ueber  die  Bestimmung  des  Luftdruckes,  Rep.  fur  Met.  Ill,  N.  1,  pp. 

1-145. 
Report  on  the  Barometry  of  the  United  States,  Canada,  and  the  West  Indies, 

Report  of  the  Chief  of  the  Weather  Bureau,  1900-1901,  Vol.  II. 
MARVIN,  C.  F.,  Circulars  A,  D,  F,  Instrument  Division  of  the  U.  S.  Weather 

Bureau. 

Instructions  for  Cooperative  Observers  (U.  S.  Weather  Bureau,    Washington). 
Apparatus  catalogues  of  various  firms.      See  p.  110. 

(See  also  the  various  guides  to  observers  mentioned  in  Appendix  IX,  in 

group  2(B).) 
For  observations  of  pressure  and  wind  consult  the  publications  mentioned  on 

p.  no. 

For  normal  values  of  pressure  and  wind,  see : 
BUCHAN,  ALEXANDER,  Report  on  Atmospheric  Circulation. 
HANN,   Klimatologie. 
Report  of  the  Chief  of  the  Weather  Bureau  particularly  for  1891-1892,  1896- 

1897. 
Climatology  of  the  United  States   (Bulletin  Q  of  U.  S.  Weather  Bureau  by 

A.  J.  HENRY,  1906). 
Summary  of  the  Climatological  Data  for  the  United  States,  by  Sections  (106 

are  to  be  published). 
For  charts  of  pressure  and  wind,  see : 
BARTHOLOMEW,  Physical  Atlas,  Vol.  III. 


188  METEOROLOGY 

BUCHAN,  ALEXANDER,  Report  on  Atmospheric  Circulation  (Report  on  the  Scien- 
tific Results  of  the  Voyage  of  H.M.S.  Challenger). 
HANN,  Atlas  der  Meteorologie,  1887. 
HILDEBRANDSSON,  H.   H.,  ET  TsissERENC  OB  BORT,   Les  bases  de  la    meteor- 

ologie  dynamique,  Paris,  1900-1907. 
Segelhandbuch  der   Deutschen   Seewarte ;    Hierzu  ein  Atlas  (for  the  Atlantic, 

Pacific,  and  Indian  Oceans). 
Summary  of  International  Meteorological  Observations  (Bulletin  A  of  the  U.  S. 

Weather  Bureau). 

ELIOT,  SIR  JOHN,  Climatological  Atlas  of  India,  Edinburgh,  1906. 
Russia,  Atlas  climatologique  de  V empire  de  Russie,  St.  Petersburg,  1900. 
BLODGET,  LORIN,  Climatology  of  the  United  States,  Philadelphia,  1857. 
GREELY,  American  Weather. 

Report  of  the  Chief  of  the  Weather  Bureau  for  1900-1901,  Vol.  II. 
Climatic  Charts  of  the  United  States  (U.  S.  Weather  Bureau). 
Climatology  of  the  United  States  (Bulletin  Q  of  the  U.  S.  Weather  Bureau). 
For  the  daily  change  in  barometric  pressure,  see  : 
ALT,  EUGEN,  Die  Doppeloszillation  des    Barometers   insbesondere  im  Arktischen 

Gebiete.     (Inaug.  Diss.),  22  pp.,  1909. 
ANGOT,  H.,  "Etude  sur  la  Marche  Diurne  du  Barometre,"  Ann.  Bu.  Cent.  Met., 

Paris,  1889. 
COLE,  FRANK  N.,  The  Daily  Variation  of  Barometric  Pressure,  W.  B.  Bulletin 

No.  6,  1892. 
FASSIG,  OLIVER  O.,  The  Westward  Movement  of  the  Daily  Barometric  Wave, 

Bulletin  No.  31  of  U.  S.  Weather  Bureau  or  Monthly  Weather  Review  for 

November,  1901. 
HANN,  JULIUS,  "  Theory  of  the  Daily  Barometric  Oscillation,"  Quart.  Journ.,  Vol. 

20,  p.  40. 
HANN,  JULIUS,    Untersuchungen   liber    die    tdgliche    Oscillation   des    Barometers^ 

Wien,  1889. 
WAGNER,  ARTHUR,  "Die  Temperatur    verhaltnisse  in  der  freien  Atmosphare," 

Beitrage  zur  Physik  der  freien  Atmosphare,  Band  III,  Heft  2/3. 
WILD,  HEINRICH,  Repetorium  fur  Meteorologie,  Vol.  VI,  No.  10  (La  marche 

diurne  du  barometre  en  Russie  par  M.  Rykatchew). 
For  the  barometric  determination  of  altitude,  see : 
Smithsonian  Meteorological  Tables.     See  section  111  of  this  book. 
WHYMPER,  How  to  Use  the  Aneroid  Barometer  (John  Murray),  London. 
WILSON,  Topographic  Surveying  (Wiley  and  Sons). 
For  a  classification,  description,  and  explanation  of  the  winds  and  the  general 

circulation  of  the  atmosphere,  see : 
ABBE,   CLEVELAND,    The   Mechanics   of    the    Earth's    Atmosphere,   a   Collection 

of  Translations.     Second  Collection  ;  Washington,  1891.     Third  Collection ; 

Washington,  1910. 
BIGELOW, FRANK  H.,  Report  on  the  International  Cloud  Observations  (Washington, 

1900). 
BRILLOUIN,  MARCEL,  Memoires  originaux  sur  la  circulation  ginerale  de  I'atmos- 

phere  (Paris,  1900). 

COFFIN,  J.  H.,  The  Winds  of  the  Globe. 
DAVIS,  American  Meteorological  Journal,  March,  1888. 
FERREL,  A  Popular  Treatise  on  the  Winds.     The  preface  of  this  book  contains 

a  list  of  his  previous  articles  on  the  same  and  kindred  subjects. 


CHAPTER  V 

THE  MOISTURE  IN  THE  ATMOSPHERE 

A.   THE  WATER  VAPOR  OF  THE  ATMOSPHERE 
EVAPORATION    . 

Water  vapor,  176. 

Latent  heat,  177. 

Amount  of  evaporation,  178. 

Distributlon-of  the  water  vapor,  ro,. 

THE  CONDITION  OF  .THE  ATMOSPHERE  AS  REGARDS  MOISTURE 

Capacity  of  air  for  water  vapor,  180. 

Saturation,  181. 

Humidity ;  absolute  and  relative  humidity,  182. 

Dew  point,  183. 

Problems,  184. 

THE  DETERMINATION  OF  THE  MOISTURE  OF  THE  ATMOSPHERE 

Hygrometers  for  determining  absolute  humidity,  185. 
Hygrometers  for  determining  relative  humidity,  186,  187. 
Dew  point  hygrometer,  188. 
Psychrometer,  189. 
Recording  hygrometers,  190. 

THE  RESULTS  OF  OBSERVATION 

The  observations,  191. 

Normal  hourly,  daily,  monthly,  and  yearly  values  of  absolute  and  relative  humidity, 

192. 

Daily  and  annual  variation  in  absolute  humidity,  193,  194. 
Geographical  variation  in  absolute  humidity,  195. 
Daily  and  annual  variation  in  relative  humidity,  196,  197. 
Geographical  variation  in  relative  humidity,  198. 
Other  moisture  data  and  charts,  199. 

THE  EFFECT  OF  WATER  VAPOR  ON  THE   GENERAL  CIRCULATION,  200 

B.    DEW,  FROST,  FOG 


DEW 


Condensation,  201. 
Dew,  202,  203. 

The  part  played  by  latent  heat,  204. 
Conditions  for  the  formation  of  dew,  205. 

189 


190  METEOROLOGY 

FROST 

Frost,  206,  207. 

Prediction  of  frost,  208,  209,  210. 

Protection  from  frost,  211. 

Frost  observations,  frost  data,  and  charts,  2x2. 

FOG 

The  nature  of  fog,  213. 

Fog  observations,  fog  data,  and  charts,  214. 

C.     CLOUDS 

THE  CLASSIFICATION  OF  CLOUDS 

Early  history,  215. 
The  international  system,  216. 
The  thirteen  cloud  forms,  217. 
The  sequence  of  cloud  forms,  218. 

THE  OBSERVATION  OF  CLOUDS  AND  CLOUDINESS  AND  THE  RESULTS 
OF  OBSERVATION 

Height  of  clouds,  219. 

Direction  and  velocity  of  motion,  220. 

Cloudiness,  221. 

Sunshine  records,  222,  223. 

Observations  of  clouds,  cloudiness,  and  sunshine,  224. 

Normal  values,  data,  and  charts,  225,  226. 

THE  NATURE  OF  CLOUDS 

Nuclei  of  condensation,  227. 

Size  and  constitution  of  cloud  particles,  228. 

Haze,  229. 

THE  FORMATION  OF  CLOUDS 

Introduction,  230. 

Condensation  in  warm  winds  blowing  over  cold  surfaces  (method  i),  231. 

Condensation  in  ascending  currents  due  to  convection  (method  2),  232. 

Condensation  in  forced  ascending  currents  (method  3),  233. 

Condensation  caused  by  diminishing  barometric  pressure  (method  4),  234. 

Condensation  in  atmospheric  waves  (method  5),  235. 

Condensation  caused  by  radiation  (method  6),  236. 

Condensation  due  to  conduction  (method  7),  237. 

Condensation  by  mixing  air  (method  8),  238. 

Condensation  by  diffusion  of  water  vapor  (method  9),  239. 

Conditions  that  favor  a  clear  sky,  240. 

D.     PRECIPITATION 

THE  KINDS  OF  PRECIPITATION 

Rain,  241. 

Snow,  242. 

Hail,  243. 

Ice  storms,  244. 

Rain-making,  245.  <_ 

Cooling  produced  by  precipitation,  246 


THE  MOISTURE   IN  THE   ATMOSPHERE  191 

THE    DETERMINATION    OF    PRECIPITATION     AND    RESULTS    OF    OB- 
SERVATION 

The  measurement  of  rainfall,  247. 

The  measurement  of  a  snowfall,  248. 

Observations  of  precipitation,  249. 

Normal  values  and  precipitation  data,  250-256. 

THE  DISTRIBUTION  AND  EFFECTS  OF  PRECIPITATION 

Geographical  distribution  of  precipitation,  257-260. 

Other  precipitation  charts,  261. 

Variation  in  the  amount  of  precipitation  with  altitude,  262. 

Relation  of  rainfall  to  agriculture,  263. 

Relation  of  rainfall  and  forests,  264. 

Effects  of  snowfall,  265. 

A.   THE  WATER   VAPOR    OF  THE   ATMOSPHERE 

EVAPORATION 

176.  Water  vapor.  —  The  two  terms,  the  moisture  of  the  atmosphere 
and  the  water  vapor  in  the  atmosphere,  are  synonymous,  and  Definition  of 
both  refer  to  the  water  in  invisible  gaseous  form  which  is  water  Vflp°r- 
always  present  in  the  atmosphere,  but  in  amounts  varying  at  the  earth's 
surface  from  almost  nothing  to  4  per  cent  as  a  maximum.  Evaporation 
The  change  from  the  solid  or  liquid  state  to  this  invisible  gase-  and  conden- 
ous  state  is  called  evaporation ;  its  opposite  is  condensation. 

Water  vapor  is  supplied  to  the  atmosphere  from  a  variety  of  sources. 
Nearly  three  fourths  of  the  earth's  surface  is  a  water  surface, 
and  the  oceans,  lakes,  and  other  bodies  of  water  supply  by  of  t^esou 
evaporation  the  larger  portion  of  the  water  vapor  to  the  water  vapor 
atmosphere.     Other  sources  of  water  vapor  are  the  surface  of  mospehere. 
the  ground  which  is  nearly  always  moist,  the  leaves  of  plants 
and  vegetation  in  general,  and  the  air  exhaled  from  the  lungs  of  animals. 
Water  vapor  is  lighter  than  air,  the  ratio  being  100 :  62.     That  is  a 
cubic  foot  of  water  vapor  at  a  certain  temperature  and  _ 

p  Relative 

under  a  certain  pressure  has  .62  of  the  mass  of  a  cubic  foot  lightness 
of  air  at  the  same  temperature  and  under  the  same  pres- 
sure.      Moist  air  is  thus  lighter  than  dry  air  because  a  por- 
tion of  the  dry  air  has  been  replaced  by  water  vapor,  which  is  lighter. 

177.  Latent  heat.  —  When  evaporation  takes  place  from   a  water 
surface,  the  molecules  near  the  surface  break  away  from  the  Definition 
attractions  of  their  neighbors  and  make  their  way  into  the  air  of  latent 
above.     This  breaking  of  the  bond  of  adhesion  between  the 
molecules  requires  an  expenditure  of  energy;  and  the  energy  used  up  in 


192  METEOROLOGY 

evaporation,  that  is,  in  separating  the  molecules,  is  called  latent  '. 
This  energy  may  be  supplied  from  two  sources.  The  molecular  energy 
The  two  °f  the  body  itself  may  be  used  up  and  in  this  case  the  body 
sources  of  becomes  cold ;  or  radiant  energy  in  the  form  of  ether  waves 
latent  heat.  may  ^  use(j  up  directly  to  supply  the  latent  heat.  The 
amount  of  latent  heat  involved  in  the  change  from  the  solid  to  the  liquid, 
or  from  the  liquid  to  the  gaseous  state,  is  large.  If  the  Centigrade 
Values  of  system  of  measuring  temperatures  is  used  and  the  unit  of 
latent  heat.  heat  js  defined  as  the  amount  of  heat  required  to  raise  the 
temperature  of  a  gram  of  water  1°  C.,  then  the  amount  of  latent  heat 
required  to  change  a  gram  of  ice  to  a  gram  of  water  at  the  same  tempera- 
ture is  79,  and  the  number  of  heat  units  required  to  change  the  gram  of 
water  to  a  gram  of  water  vapor  at  the  temperature  of  boiling  water  is 
536.  If  the  Fahrenheit  system  of  measuring  temperature  is  employed 
and  the  heat  unit  is  defined  as  the  amount  of  heat  required  to  raise  one 
gram  of  water  1°  F.,  then  the  latent  heat  in  changing  from  the  solid  to 
the  liquid  state  is  143,  and  from  the  liquid  to  the  gaseous  state  966. 
If  evaporation  takes  place  at  a  lower  temperature  than  the  boiling  point 
of  water,  a  larger  amount  of  latent  heat  is  required.  For  example, 
in  the  Fahrenheit  system  of  thermometry  it  requires  1092  heat  units 
to  change  a  gram  of  water  at  32°  to  water  vapor  at  the  same  temperature. 
This  latent  heat  makes  itself  felt  in  many  different  ways.  One  reason 
why  the  ocean  rises  so  little  in  temperature  under  the  direct  rays  of  the 
sun  is  because  so  large  an  amount  of  the  radiant  energy  of 

Illustrations  .  .  . 

of  the  part      the  sun  is  used  up  in  causing  evaporation  instead  of  in.  heat- 
played  by       jng  the  water.     One  reason  why  the  air  temperature  at  the 

latent  heat.  .  J. 

north  pole  is  so  low  in  summer  in  spite  01  the  large  amount 
of  insolation  received  is  because  the  surface  is  a  snow  and  ice  surface  and 
the  insolation  is  used  up  as  latent  heat  in  melting  the  snow  and  ice 
instead  of  in  raising  the  temperature  of  the  air.  After  it  has  rained  in 
summer,  the  air  usually  remains  cooler  for  a  considerable  time.  The 
reason  is  that  after  a  rainfall  the  ground  and  all  surfaces  are  wet,  and 
the  drying  of  these  surfaces  uses  up  the  radiant  energy  of  the  sun,  and 
this  prevents  a  rapid  rise  in  the  air  temperature. 

178.  Amount  of  evaporation.  —  The  amount  of  evapora- 
depends  upon  a  great  variety  of  things :  such  as  the 
tion  depends  nature  of  the  surface  from  which  the  evaporation  is  taking 
place,  the  amount  of  water  vapor  already  in  the  atmosphere, 
the  temperature,  the  velocity  of  the  wind,  and  the  baro- 
metric pressure.  Each  surface  has  its  own  rate  of  evaporation.  Areas 


THE  MOISTURE   IN  THE  ATMOSPHERE 


193 


covered  with  vegetation  under  the  same  conditions  evaporate  about 
one  third  more  water  than  a  free  water  surface.  In  measuring  evapora- 
tion, a  free  water  surface  is  usually  taken  as  the  standard.  The  more 
water  vapor  there  is  already  in  the  atmosphere,  the  slower  the  evapora- 
tion, while  high  temperature  and  high  wind  velocity  favor  evaporation. 
Evaporation  is  least  when  the  barometric  pressure  is  largest.  If  the 
barometric  pressure  is  higher,  the  number  of  molecules  in  each  cubic 
foot  of  air  resting  upon  the  evaporating  surface  is  larger 
and  the  molecules  of  water  find  more  hindrances  and 
greater  difficulty  in  making  their  way  into  the  air 
above.  Thus  the  higher  the  pressure,  the  slower  the 
evaporation. 

The  amount  of  evaporation  is  usually  determined  by 
exposing  large  pans  of  water  in  the  open  and  measuring 
the    amount   evaporated    in    a   given   time. 
These  evaporating  pans  should  have  a  large  of 


area  and  considerable  depth,  so  that  the  ing  the 
temperature  will  be  approximately  the  same  evaporated 
as  the  temperature  of  the  surrounding  areas. 
Values  of  the  amount  evaporated  have  also  been  deter- 
mined by  measuring  the  evaporation  from  inclosed 
tanks,  reservoirs,  etc.  Relative  values  of  the  amount 
evaporated  may  be  determined  by  means  of  an  inge- 
nious little  instrument  pictured  in  Fig.  93,  and  The  piche 
called  a  Piche  evaporimeter.  It  consists  evaporim- 
essentially  of  a  long  glass  tube  graduated 
to  cubic  centimeters  or  cubic  inches.  A  piece  of  rough 
paper  is  held  against  the  open  end  of  this  glass  tube  by 
means  of  a  brass  spring  and  plate.  The  rough  paper 
is  kept  wet,  and  as  the  water  evaporates  from  it,  it  is 
replaced  by  water  from  the  glass  tube.  The  amount 
evaporated  from  the  rough  paper  in  a  given  time 
can  thus  be  determined  by  simply  reading  the  amount  of  water 
remaining  in  the  glass  tube.  By  means  of  this  piece  of  apparatus, 
which  is  easily  managed  and  quickly  read,  relative  values  of  the 
amount  of  evaporation  on  days  of  different  types  and  at  different 
times  of  the  year,  and  in  different  places,  may  be  readily  deter- 
mined. In  order  to  determine  by  means  of  this  instrument 
the  absolute  amount  evaporated,  it  must  be  standardized  in 
terms  of  a  free  water  surface  near  it,  under  the  same  conditions. 
o 


194  METEOROLOGY 

Automatically  recording  evaporimeters  have  also  been  recently 
devised.1 

The  amount  of  evaporation  varies  all  the  way  from  a  few  inches  to 
several  hundred  inches  per  year  in  dry,  hot  countries.  The  amount  of 
The  amount  evaporation  is  greater  during  the  day  than  at  night  and 
evaporated,  greater  during  the  summer  than  in  winter,  except  under  very 
special  conditions.  Unless  the  atmosphere  is  to  become  unduly  filled 
with  water  vapor,  the  amount  which  condenses  from  it  in  the  form  of 
precipitation  must  equal  the  amount  which  evaporates  in  it.  Thus  for 
the  earth  as  a  whole  the  amount  of  evaporation  must  equal  the  amount 
of  precipitation. 

179.  Distribution  of  the  water  vapor.  —  The  water  vapor  which  is 
supplied  to  the  atmosphere  by  evaporation  is  distributed  by  three  pro- 
Water  vapor  cesses>  —  diffusion,  convection,  and  wind.  Diffusion  is 
is  distri-  the  process  whereby  the  molecules  of  water  vapor,  due  to 
diffusion  their  own  motion,  make  their  way  slowly  from  point  to 
convection,  point  between  the  molecules  of  the  air.  It  is  at  best  a  slow 
process,  and  due  to  this  cause  alone,  water  vapor  would 
be  transported  but  a  few  feet  in  many  hours.  Air  rising  due  to  con- 
vection carries  the  water  vapor  along  with  it,  and  in  this  way  it  is  dis- 
tributed throughout  the  lower  three  to  five  miles  of  the  atmosphere. 
The  wind  and  air  currents  complete  the  distribution  by  transporting  the 
water  vapor  immense  distances  and  scattering  it  widely. 


THE  CONDITION  OF  THE  ATMOSPHERE  AS  REGARDS  MOISTURE 

1 80.  Capacity  of  air  for  water  vapor.  —  By  the  capacity  of  air  for 
Definition       water  vapor  is  meant  the  amount  of  water  vapor  which  a 
of  capacity,     given   quantity  of   air   can   hold.      The   capacity   depends, 
It  depends      upon  the  temperature  only.     With  increasing  temperature 
on  temper-      the   amount   of  water  vapor  which   the   air   can   hold   in- 

y'      creases  rapidly ;  in  fac/t,  it  increases  at  an  increasing  rate. 

181.  Saturation.  —  If  a  given  (Quantity  of  air  contains  all  the  water 
vapor  which  it  can  hol*d;   in  ojther  words,  if  its  capacity  is  entirely 
Definition  of  satisfied,  then  it  is  said  to  be  in  a  state  of  saturation.     The 
saturation,      amount  of  water  vapor  which  saturates  a  given  quantity  of 
air  may  be  expressed  in  grams  per  cubic  foot  or  grams  per  cubic  nfeter, 
or  it  may  be  expressed  in  terms  of  the  pressure  which  it  exerts  in  milli- 

1  For  recent  works  on  evaporation,  see  articles  by  BIGELOW  in  the  Monthly  Weather 
Review  since  1905. 


THE  MOISTURE   IN  THE  ATMOSPHERE 


195 


meters  or  inches.     The  following  table,  which  applies  to  saturated  air, 
gives  for  the  Fahrenheit  scale  the  amount  of  moisture  in  grains  per 
cubic  foot,  the  pressure  in  inches,  and  the  mass  of  the  satu-  Table  of 
rated  air  in  grains  per  cubic  foot.      It  also  gives  for  the  values- 
Centigrade  scale  the  amount  of  moisture  in  grams  per  cubic  meter,  the 
pressure  in  millimeters,  and  the  mass  of  the  saturated    air  in  kilo- 
grams per  cubic  meter. 


TEMPERATURE 

VAPOR  PRESSURE 

AMOUNT  OF  WATER 

MASS  OF  SATURATED 

VAPOR  IN  GRAINS 

AIR  IN  GRAINS 

DEGREES  F. 

INCHES 

PER  Cu.  FT. 

PER  Cu.  FT. 

-30° 

0.010 

0.12 

650 

-20 

0.017 

0.21 

634 

-10 

0.028 

0.35 

620 

0 

0.045 

0.54 

606 

+10 

0.071 

0.84 

593 

20 

0.110 

1.30 

580 

30 

0.166 

1.97 

568 

40 

0.246 

2.86 

556 

50 

0.360 

4.09  •• 

544 

60 

0.517 

5.76 

533 

70 

0.732 

7.99 

521 

80 

1.022 

10.95 

509 

90 

1.408 

14.81 

497 

+100 

1.916 

19.79 

487 

TEMPERATURE 

VAPOR  PRESSURE 

AMOUNT  OP  WATER 

MASS  OF  SATURATED 

VAPOR  IN  GRAMS 

AIR  IN  KILOGRAMS 

DEGREES  C. 

MM. 

PER  Cu.  METER 

PER  Cu.  METER 

-30° 

0.38 

0.44 

1.45 

-20 

0.94 

1.04 

1.40 

-10 

2.15 

2.28 

1.35 

0 

4.57 

4.87 

.30 

+10 

9.14 

9.36 

.25 

20 

17.36 

17.15 

.20 

30 

31.51 

30.08 

.15 

+40 

54.87 

50.67 

.11 

In  order  to  compute  the  amount  of  water  vapor  or  saturated  air 
which  may  be  present  in  any  given  space,  it  should  be  held  in  mind 
that  7008  grains  constitute  one  pound  avoirdupois.     Sup-  niustration 
pose  a  room  15  feet  square  and  10  feet  high  were  filled  with 
saturated  air  at  a  temperature  of  70°  F.     Its  volume  would  be  2250 
cubic  feet  and  it  would  thus  contain  2.6  pounds  of  water  vapor  and 
167  pounds  of  moisture-laden  air. 

182.    Humidity ;  absolute  and  relative  humidity.  —  Humidity  is  defined 
as  the  state  of  the  atmosphere  as  regards  moisture.     If  the  air  were 


196  METEOROLOGY 

absolutely  dry,  its  humidity  would  be  spoken  of  as  zero.     It  is  the 
humidity,  as  much  as  the  temperature,  which  adds  to  the  uncomfort- 
Definition  of  ableness  of  a  sultry  day  in  summer.     In  fact,  the  human 
Humidity.-     body  feels  three  things:   temperature,  wind,  and  moisture, 
and  not,  as  is  sometimes  popularly  supposed,  temperature  only.     The 
cold  on  a  windy  day  in  winter  is  more  penetrating  than  still  cold, 
because^ the  wind  drives  the  cold  air  through  the  clothing 
body  feels11    m*°  contact  witn  the  skin.     The  cold  is  also  more  penetrat- 
temperature,  ing  on  a  damp  day  than  on  a  dry  day.    The  reason  is  because 
amTwind.      ^ne  moisture  makes  the  clothing  a  better  conductor  and  thus 
lessens  the  heat  of  the  body.     A  moist,  hot  day  in  summer  is 
much  more  oppressive  than  a  dry,  hot  day,  because  the  moisture  in  the 
atmosphere  prevents  that  free  evaporation  of  the  perspiration  from  the 
human  body  which  cools  it.  *If  an  instrument  could  be  invented  which 
could  indicate  the  feelings  of  the  human  body,  it  would  be  necessary  to 
take  account  of  temperature,  wind,  and  moisture  in  its  construction. 
£  Absolute  humidity  is  defined  as  the  actual  quantity  of  moisture 
present  in  a  given  quantity  of  air.j  It  may  be  expressed  as  a  certain 
.          number  of  grains  per  cubic  foot  or  a  certain  number  of  grams 
absolute  and  per  cubic  meter.  \By  relative  humidity  is  meant  the  ratio 
relative          of  ^he  actual  amount  of  water  Vapor  present  in  the  atmos- 
phere to  the  quantity  which  could  be  there  if  it  were  satu- 
rated.) Relative  humidity  is  always  expressed  in  per  cent. 

183.  Dew  point.  —  If  the  temperature  of  a  quantity  of  air  contain- 
ing moisture  is  lowered,  a  temperature  will  be  finally  reached  when  the 
Definition  of  given  quantity  of  air  is  saturated  with  moisture  and  is  con- 
dew  point,      taining  all  the  moisture  that  it  can  hold.     This  temperature 
is  spoken  of  as  the  dew  point,  and  any  further  reduction  in  temperature 
must  result  in  the  condensation  of  some  of  the  moisture  in  the  form  of 
dew,  frost,  fog,  cloud,  or  precipitation. 

184.  Problems.  —  The  four  quantities,  absolute  humidity,  relative 
The  inter-      humidity,  dew  point,  and  temperature,  are  so  linked  together 
relation  of      by  the  table  given  in  section  181,  that  if  any  two  of  these 
dCe™P6ohitrC>  ^our  Quantities  have  been  observed  or  determined,  the  other 
absolute  hu-  two    may    be    obtained    by    computation.     The    following 
relative and     examples  will  make  clear  the  meaning  of  these  four  terms 
humidity.       and  their  interrelations. 

Example  I.    If  the  temperature  is  70°  F.  and  the  dew  point  is  50°  F.,  find 
the  relative  and  absolute  humidity. 

From  the  table  it  is  seen  that  air  at  50°  F.  can  hold  4.09  grains  per  cubic  foot. 


THE  MOISTURE   IN  THE  ATMOSPHERE  197 

This  same  amount  of  moisture  was  present  at  70°  F.  The  absolute  humidity 
is  thus  4.09  grains  per  cubic  foot.  This  simply  says  that  if  air  containing  4.09 
grains  at  a  temperature  of  70^F.  is  cooled  down  to  50°  F.,  it  is  containing  all 
the  moisture  it  can,  it  is  saturated,  and  has  reached  the  dew  point. 

Air  at  70°  F.  can  contain  7.99  grains  per  cubic  foot.    The  relative  humidity  is 


thus  or  51  %. 


Example  II.     If  the  temperature  is  40°  F.  and  the  relative  humidity  (R.  H.) 
is  70  %,  find  the  absolute  humidity  (A.  H.)  and  the  dew  point. 
Air  at  40°  F.  can  hold  2.86  grains  per  cubic  foot.    Thus 

.70  =   ^5\          A.  H.  =  2.00. 
2.86 

From  the  table  it  is  seen  that  2.00  grains  saturates  air  with  a  temperature 
a  little  above  30°  F.  The  dew  point  thus  lies  between  30°  and  31°  F. 

Example  III.  If  the  absolute  humidity  is  4.09  grains  and  the  relative 
humidity  is  80  %,  find  the  dew  point  and  temperature.  From  the  table  it  is 
seen  that  4.09  grains  saturates  air  at  50°  F.  50°  F.  is  thus  the  dew  point. 

.80  =  -  ^5  -  .         Possible  amount  =  5.11  grains. 
Possible  amount 

The  air  must  have  a  temperature  between  50°  F.  and  60°  F.  to  hold  this  amount. 
Interpolation  would  give  a  value  of  about  56°  F. 

Since  there  are  six  possible  combinations  of  four  things  taken  two  at  a  time, 
there  are  six  possible  examples.  Only  three  have  been  solved,  but  the  solution 
of  the  other  three  would  be  along  the  same  lines  as  indicated  above.  The 

formula  R.  H.  =  -         -  and  the  table  are  sufficient  to  solve  all  six. 
Possible  Amount 

THE  DETERMINATION  OF  THE  MOISTURE  OF  THE  ATMOSPHERE 

185.    Hygrometers  for  determining  absolute  humidity.  —  It  has  just 
been  shown  that  the  four  quantities,  temperature,  absolute  The  four 
humidity,  relative  humidity,  and  dew  point,   are  so  linked  jjJ^J^!8' 
together  by  means  of  a  table  that  if  any  two  of  the  four  are  ture,  reia- 
known,  the  other  two  may  be  determined  by  computation.  |^e^bus^!d" 
The  methods  for  determining  the  real  air  temperature  have  lute  humid- 
already  been  fully  discussed  in  Chapter   III.     The  neces-  *£.£££* 
sary  apparatus  is  either  a  thermometer  in  a  thermometer  ail  be  deter- 
shelter,  a  sling  thermometer,  or  a  ventilated  thermometer.  ™"ends^ 
The  experimental  methods  and  the  necessary  apparatus  for  suitable 
determining  the  other  three  quantities  must  now  be  care-  appa™*118- 
fully   considered.      These  instruments   for   determining  the   moisture 


198  METEOROLOGY 

of  the  atmosphere  are  called,  in  general,  hygrometers 1  or  moisture 
measurers. 

Absolute  humidity  is  determined  ordinarily  by  means  of  the  so-called 
chemical  hygrometer.     This  consists  usually  of  two  U-tubes  containing 

calcium  chlorid  (CaCl2)  and  sulfuric  acid  (H2S04).  Anhy- 
The  chemi-  (jrOus  phosphoric  acid  is  perhaps  better  than  sulfuric  acid, 
eter  for  A  known  quantity  of  the  moisture-laden  air  is  drawn  through 
determining  these  U-tubes  so  slowly  that  the  moisture  is  absorbed  and 
humidity.  thus  by  weighing  the  tubes  both  before  and  after  the  given 

quantity  of  air  was  drawn  through,  the  amount  of  moisture 
in  it,  and  thus  the  absolute  humidity,  may  be  determined.  In  perform- 
ing the  experiment  care  must  be  taken  to  have  the  air  pass  so  slowly 
through  the  U-tubes  that  all  of  the  moisture  will  be  absorbed,  and 
furthermore,  certain  precautions  are  necessary  in  order  to  prevent  any 
moisture  from  gaining  access  to  the  tubes  except  from  the  air  which  has 
passed  through  them. 

Another  method  of  determining  absolute  humidity  is  to  inclose  a 

given  quantity  of  the  moisture-laden  air  in  a  glass  vessel 
methods  of  and  then  to  extract  the  moisture  by  chemical  means.  By 
determining  measuring  the  diminution  in  pressure,  or  if  the  pressure 
humidity.  is  kept  the  same,  by  noting  the  diminution  in  volume,  the 

absolute  humidity  may  be  determined. 

1 86.    Hygrometers    for    determining    relative    humidity.  —  Relative 

humidity  is  determined  ordinarily  by  means  of  the  hair  hygrometer. 

.  This  consists  essentially  of  a  long  human  hair,  from  which 

struction  of     the  oil  has  been  extracted  by  soaking  it  in  alcohol  or  a  weak 

alkali  solution.     A  weak  solution  of  caustic  potash  (KOH) 

or  caustic  soda  (NaOH)  is  ordinarily  used.  The  hair  thus 
treated  changes  its  length  with  changes  in  moisture,  and  it  has  been 
found  by  experiment  that  these  changes  in  length  are  nearly  propor- 
tional to  changes  in  relative  humidity.  As  shown  in  Fig.  94,  one  end 
of  the  hair  is  fastened  rigidly  to  the  frame,  while  the  other  passes  over  a 
cylinder  and  is  held  taut  by  a  weight.  An  index  is  attached  to  the 

cylinder,  which  moves  over  a  dial  graduated  from  0  to  100 
rtrumtnt"1"  Per  cen^>  and  this  indicates  the  relative  humidity.  The 
may  be  instrument  may  be  standardized  by  comparing  it  with 
ardized.  some  other  accurate  hygrometer  or  by  determining  the  0  and 

100  per  cent  points.  The  0  point  is  verified  by  exposing 
the  hygrometer  to  air  which  has  been  entirely  desiccated  by  chemical 

=  moist;  ptrpov  =  measure. 


THE  MOISTURE  IN  THE  ATMOSPHERE 


199 


means.  The  instrument  must  be  left  a  sufficient  time  in  this  moisture- 
free  air  to  take  up  a  constant  reading.  The  100  per  cent  point  may  be 
verified  by  exposing  the  hygrometer  to  com- 
pletely saturated  air.  Air  may  be  completely 
saturated  by  blowing  live  steam  into  it  and 
then  cooling  it  down  to  an  ordinary  tempera- 
ture, or  by  spraying  moisture  into  it  by 
means  of  an  atomizer.  The  hair  hygrometer 
is  also  affected  somewhat  by  temperature 
changes,  but  as  these  are  slight  they  are 
ordinarily  neglected.  At  best,  the  its 
hair  hygrometer  is  an  inaccurate  accuracy- 
instrument.  If  recently  standardized,  its  in- 
dications may  be  trusted  perhaps  to  2  or  5 
per  cent.  If  this  is  not  the  case,  its  indications 
are  not  reliable  to  within  10  or  15  per  cent. 
Another  form  of  hygrometer  which  works 
on  the  same  principle  is  in  very  common  use. 
The  outer  case  is  usually  circular  . 

.  Another 

and  has  a  diameter  of  an  inch  or  form  with 
two.     The  working  part  consists  thf  saine 

-          ~  .  principle. 

of  a  fine  piece  of  spring  copper 


FIG.  94.— The  Hair 
Hygrometer. 


which  has  been  bent  into  the  form  of  a  spiral 
spring.  This  is  coated  with  some  hygrometric  material.  A  bamboo 
preparation  is  often  used  for  this  purpose  and  it  is  generally  colored 
red.  This  hygrometric  material  changes  length  with  moisture  changes, 
and  thus  causes  the  spiral  spring  to  wind  up  or  unroll.  These  motions 
are  communicated  to  a  pointer  which  moves  over  a  dial  graduated  to 
indicate  relative  humidity. 

187.   There    are    two    other    instruments    for    determining   relative 
humidity,  but  they  are  scientific  toys  rather  than  accurate  meteoro- 
logical instruments.     One  is  the  so-called  weather  house.  TWO  scig»- 
This  consists  of  a  little  box  usually  in  the  shape  of  a  house  tifictoy8^ 
and  ordinarily  provided  with  two  openings  in  which  are  figures  of  a  man 
and  of  a  woman.     It  is  so  arranged  that  if  the  air  is  dry,  the  woman 
appears ;   while  if  the  air  is  damp,  the  man  appears.     These  two  figures 
are  held  by  a  twisted  strand  of  hygrometric  material  which  winds  up 
or  unwinds  with  moisture  changes,  and  this  causes  the  motion  of  the 
figures. 

It  has  been  found  that  a  solution  of  cobalt  chlorid  of  the  proper 


200  METEOROLOGY 

strength  will  turn  pink  if  the  air  is  damp,  and  bluish  in  color  if  the  air  is 
dry.  A  piece  of  filter  paper  or  cheese  cloth  saturated  with  a  solution 
of  the  proper  strength  thus  becomes  a  rough  indicator  of  the  relative 
humidity.  An  attempt  has  been  made  by  standardizing,  the  strength 
of  the  solution  and  arranging  a  scale  of  color,  to  make  the  instrument 
give  quantitative  results. 

1 88.  Dew   point    hygrometer.  —  The   simplest    possible   instrument 
for  determining  the  dew  point  consists  of  a  bright  tin  cup,  provided  with 
The  con-        a  thermometer  as  a  stirring  rod  and  partly  filled  with  water, 
struction        If  ice  water  or  cold  water  is  added  to  the  water  in  the  cup, 
o?th*Cdew      ^s  temperature  will  be  steadily  reduced  and  there  will  come 
point  hy-        a  moment  when  the  outside  of  the  cup  will  become  coated 

with  moisture.  This  means  that  the  layer  of  air  in  con- 
tact with  the  cup  has  been  cooled  below  the  dew  point,  and  the 
moisture  has  been  deposited  in  the  form  of  dew.  The  reading  of  the 
thermometer  will  thus  indicate  the  dew  point.  A  more  accurate  deter- 
mination can  be  made  if  the  water  in  the  cup  is  now  allowed  to  rise  in 
temperature  until  the  moisture  on  the  outside  disappears.  The  average 
of  the  two  temperatures  will  give  a  very  close  approximation  to  the  dew 
point.  This  simple  method  of  determining  the  dew  point  has  been 
modified  in  various  ways.  The  cooling  may  be  produced  by  causing 
ether  in  the  cup  to  rapidly  evaporate,  and  black  glass  has  also  been  often 
used  instead  of  the  bright  metal.  In  all  the  instruments,  however,  the 
fundamental  principle  remains  the  same. 

189.  Psychrometer.  —  The  psychrometer  l  is  an  instrument  which 
indicates  directly  neither  absolute  humidity,  relative  humidity,  nor  dew 
Description     Pomt,  but  all  three    of  these  may  be  indirectly  determined 
of  a  psy-        by  means  of  its  indications.     It  consists  of  two  identical 

thermometers  attached  to  a  frame.  One  remains  in  its 
ordinary  condition,  while  the  bulb  of  the  other  is  covered  with  a  linen 
jacket  to  which  is  attached  a  wick  which  extends  down  to  a  vessel  of 
water.  The  instrument  is  pictured  in  Fig.  95.  The  evaporation  from 
the  wet  bulb  thermometer  cools^it  and  thus  causes  a  difference  in  the 
readings  of  the  two  thermometers.  The  underlying  principle  is  this : 
the  larger  the  amount  of  moisture  in  the  atmosphere  the  less  the  evap- 
The  under-  oration,  and  thus  the  smaller  the  difference  between  the 
lying  prin-  indications  of  the  two  thermometers.  This  difference,  how- 
ever, is  affected  by  pressure  and  by  wind,  as  well  as  by  the 
moisture  present  in  the  atmosphere.  The  higher  the  barometric  pres- 

1  i]/vxpfc  =  cold  ;  p-trpov  =  measure. 


THE  MOISTURE  IN  THE   ATMOSPHERE 


201 


sure,  the  larger  the  number  of  molecules  in  each  given  volume,  and,  as  a 
result,  the  smaller  the  evaporation.    The  effect,  however,  of  the  pressure 
on  the  rate  of  evaporation  is  so  slight  that  it  is  ordinarily      . 
neglected.   If  the  air  in  contact  with  the  wet  bulb  thermometer  affected  by 
stagnates,  it  will  soon  become  filled  with  moisture  and  further  Pressu5e 
evaporation  will  cease.     It  is  therefore  necessary  to  main- 
tain a  constant  supply  of  fresh  air  in  contact  with  the  wet  bulb  ther- 


FIG.  95.  —  The  Psychrometer. 


FIG.  96.— The  Whirled  Psychrometer. 


mometer.     This  may  be  done  by  rapidly  whirling  the  instrument,  or  by 
blowing  air  against  it  by  means  of  a  fan,  or  by  placing  it  in  HQW  ^ 
a  tube  and  drawing  air  rapidly  through  the  tube.     Ordinarily,   effect  of 
the  thermometer  is  whirled  rapidly,  and  such  an  instrument 
is  illustrated  in  Fig.  96.      The  instrument  is  practically  use- 
less below  32°  F.    When  the  water  in  the  wick^and  linen  jacket  freezes, 
evaporation  still  takes  place,  but  the  supply  can  no  longer  be  Difficulties 
kept  good.     Furthermore,  the  compression  caused  by  the  below  32° F' 
formation  of  the  ice  around  the  bulb  of  the  thermometer  often  causes  it 
to  indicate  a  temperature  from  one-half  to  a  degree  too  high.    Psychrom- 
eter readings  are  for  this  reason  often  discontinued  after  the  tempera- 


202 


METEOROLOGY 


ture  goes  below  32°  F.  The  accompanying  table  gives  for  various 
temperatures  and  various  differences  between  the  wet  and  dry  bulb 
thermometer,  the  relative  humidity  and  the  dew  point.  For  the  reduc- 
tion of  a  long  series  of  observations,  a  more  elaborate  table  will  be  found 
more  convenient. 


DIFFERENCE 
OF  READINGS 
OF  DRY  AND 
WET  BULBS 

TEMPERATURE  OF  AIR  —  FAHRENHEIT 

-10° 

0° 

10° 

20° 

30° 

40° 

50° 

60° 

70° 

80° 

90° 

100° 

, 

D.P.  .   . 

-22 

-7 

5 

16 

27 

38 

48 

58 

69 

79 

89 

99 

1 

R.H.  .   . 

55 

71 

.  80 

86 

90 

92 

93 

94 

95 

96 

96 

97 

oJ 

ID.  P.  .   . 

-76 

-18 

-1 

12 

24 

35 

46 

57 

67 

77 

87 

98 

2 

R.H.  .   . 

10 

42 

60 

72 

79 

84 

87 

89 

90 

92 

92 

93 

o 

D.P.   .   . 



-39 

-9 

7 

21 

33 

44 

55 

66 

76 

86 

96 

31 

R.H.  .   . 

- 

13 

41 

58 

68 

76 

80 

84 

86 

87 

88 

90 

4! 

D.P.   .   . 





-22 

1 

17 

30 

42 

53 

64 

74 

85 

95 

4 

LR.H.  .   . 

— 

— 

21 

44 

58 

68 

74 

78 

81 

83 

85 

86 

fi 

!D.P.  .   . 

_ 

_ 

_ 

-18 

7 

24 

37 

49 

61 

72 

82 

93 

6. 

R.H.  .   . 

— 

— 

— 

16 

38 

52 

61 

68 

72 

75 

78 

80 

c 

D.P.  .   . 

_ 

_ 

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_ 

-8 

16 

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45 

57 

68 

79 

90 

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R.H.  .   . 

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_ 

_ 

4 

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77 

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in 
10 

R.H.  .   . 

- 

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ID.  P.   .   . 

_ 

— 

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5 

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45 

58 

70 

82 

14 

R.H.  .   . 

- 

- 

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30 

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57 

D.P.   .   . 

_ 

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_ 

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_ 



-20 

20 

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16 

R.H.  .   . 

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1  o 

D.P.   .   . 



_ 

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_ 

8 

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63 

76 

io 

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60 

73 

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D.P.  .   . 

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_ 

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15 

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22 

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- 

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23 

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26 

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190.    Recording  hygrometers.  —  It  is  just   as   desirable  to  keep   a 

The  record-    con^muous  record  of  the  moisture  of  the  atmosphere  as  of 

ing  hygrom-    the  other  meteorological  elements.    It  is  the  relative  humidity 

of  which  the  continuous  record  is  ordinarily  kept,  and  it  is  ac- 

humidity.       complished  by  means  of  a  recording  Richard  Freres  hygrom- 


THE  MOISTURE   IN  THE  ATMOSPHERE 


203 


eter.     As  pictured  in  Fig.  97,  this  consists  of  a  strand  of  hair  fastened 
at  each  end  and  held  taut  in  the  middle  by  means  of  a  hook.     The 


FIG.  97.  —  The  Recording  Hygrometer.     (The  case  has  been  removed.) 

changes  in  length  of  this  strand  are  magnified  by  a  system  of  levers  and 
communicate  to  a  pen  which  rises  and  falls  over  the  revolving  drum. 

THE  RESULTS  OF  OBSERVATION 

191.  The  observations.  —  At  the  regular  stations  of  the  U.  S.  Weather 
Bureau,  psychrometer  observations  are  made  at  8  A.M.  and  8  P.M.,  the 
hours   at   which   observations  of  the  other  meteorological  Theobserva- 
elements  are  made.      In  addition,  a    continuous  record  of  tions  taken, 
relative  humidity,  by  means  of  the  Richard  Freres  recording  hygrom- 
eter, is  usually  kept.     At  the  cooperative  stations  no  observations  of 
moisture   are  required.     At   a   great  many  foreign  stations   the   hair 
hygrometer  is  used  instead  of  the  psychrometer  for  temperatures  below 
the  freezing  point. 

192.  Normal  hourly,  daily,  monthly,  and  yearly  values  of  absolute 
and  relative  humidity.  —  Since  absolute  humidity  and  relative  humidity 
are  expressed  by  mere  numbers,  the  various  average  and  The  aver_ 
normal  values  may  be  computed  in  exactly  the  same  way  as  ages  and 
the  corresponding  averages  and  normals  for  the  other  meteor-  ^computed 
ological  elements.     In  computing  the  normal  hourly  values  intheregu- 
it  is  customary  not  to  compute  the  value  for  each  hour  of 

every  day  in  the  year,  but  to  compute  the  value  for  each  hour  of  the 
dav  for  the  various  months  of  the  year. 


204  METEOROLOGY 

193.  Daily  and  annual  variation  in  absolute  humidity.  —  The  daily 
variation  in  absolute  humidity  may  be  shown  graphically  by  plotting 
The  char-       *o  SGS^e  the  normal  hourly  values.     If  these  are  not  known, 
acteristics       a  fairly  good  idea  of  the  characteristics  of  the  variation  may 
variation^     ^e  obtained  by  noting  the  actual  change   in  the  absolute 
absolute         humidity  on  some  day  when  the  other  meteorological  ele- 

]  ments  have  followed  as  closely  as  possible  a  normal  course. 

The  maximum  usually  occurs  in  the  late  afternoon  and  the  minimum 
at  the  time  of  sunrise.  During  the  day  evaporation  has  gone  on 
rapidly  from  all  \\ater  surfaces  and  from  the  damp  ground,  and  in- 
creased activity  on  the  part  of  the  plants  and  animals  during  the  day  has 
also  added  a  large  amount  of  moisture  to  the  atmosphere.  As  a  result 
the  absolute  humidity  is  greatest  in  the  late  afternoon  or  early  evening. 
During  the  night,  a  large  quantity  of  water  vapor  comes  out  of  the  atmos- 
phere in  the  form  of  dew,  and  evaporation  is  also  less.  As  a  result,  the 
minimum  is  reached  about  the  time  of  sunrise. 

During  the  summer  months  in  some  warm,  moist,  low-lying  countries, 
there  is  a  secondary  minimum  in  the  middle  of  the  afternoon  which 
causes  two  maxima,  one  during  the  morning  and  the  other  in  the  early 
evening.  The  reason  for  this  is  to  be  found  in  convection,  which  be- 
comes very  energetic  during  the  afternoon  and  carries  up  the  warm, 
moisture-laden  air,  replacing  it  with  dryer  air  from  above.  The  result 
is  a  decrease  in  the  absolute  humidity  which  gives  rise  to  the  secondary 
minimum  and  the  two  maxima. 

194.  The  annual  variation  in  absolute  humidity  is  found  by  plotting 
to  scale  the  normal  monthly  values.     The  maximum  usually  occurs  in 
The  charac-    ^ne  ^a^e  summer    and    the  minimum    during    the   winter, 
teristics  of      The  reason  for  this  is  the  increase  in  evaporation  during 
variation        ^ne  summer  due  to  the   higher  temperatures  and  the  fact 
in  absolute     that  the  ground  is  not   frozen.      Plant  life  is  also  much 

lty'       more  luxuriant.     In  the  accompanying  table  will  be  found 
a    series    of   values   of    absolute    humidity    for   the   various    months 

illustrations    °^   ^e    ^ear    anc^    ^or    ^*e    vear   as   a  whole   for  various 

stations  in  the  United  States.     In   Fig.  98  some  of  these 

results  are  shown  graphically  and  illustrate  the  points  just  mentioned. 

195.    Geographical  variation  in  absolute  humidity.  —  The 

of°thelaenn     geographical  variation  in  absolute  humidity  is  closely  corre- 

erai  wind        lated  with  the  general  wind  system.    The  largest  values  occur 

absolute**1'1     a^  ^e  eQuator>  because  temperatures  there  are  highest  and 

humidity.       the  air  is  relatively  calm.     The  amount  decreases  through 


_  MOISTURE   IN  THE  ATMOSPHERE 


205 


NORMAL  ABSOLUTE  HUMIDITY  IN  GRAINS  PER  CUBIC  FOOT  FOR  THE  VARIOUS 
MONTHS  AND  FOR  THE  YEAR 


STATION 

LENGTH  OF 
RECORD 

fc 

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H-S 

1 

1 

1 

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u 

& 

ANNUAL 

Albany,  N.Y.     }  f  ££ 

15 
15 

1.07 
1.23 

1.15 
1.27 

1.55 
1.71 

2.35 

2.47 

3.66 
3.86 

5.26 
5.51 

5.87 
6.26 

5.43 
5.70 

4.65 

4.86 

3.08 
3.28 

2.12 

2.22 

1.41 
1.55 

3.13 
3.33 

Bismarck,            j  8  A.M. 

15 

0.46 

0.46 

0.78 

1.82 

2.79 

4.20 

4.91 

3.84 

2.96 

1.92 

1.08 

0.70 

2.16 

N.  Dak.           I  8  P.M. 

15 

0.55 

0.59 

1.07 

2.28 

3.25 

4.86 

5.28 

4.02 

2.85 

2.39 

1.32 

0.79 

2.44 

•D     '          T^    "k 

8  A.M. 

5 

1.24 

1.59 

1.82 

2.21 

2.77 

3.09 

2.80 

2.91 

2.66 

2.08 

1.87 

1.59 

2.22 

jjOISC,  .LClcl.no 

8  P.M. 

5 

1.46 

1.61 

1.49 

2.07 

2.61 

3.09 

2.93 

3.21 

2.72 

2.58 

2.15 

1.79 

2.35 

"R  ncrf  art     TVT  f\  SQ 

8  A.M. 

15 

1.18 

1.18 

1.50 

2.21 

3.39 

4.88 

5.67 

5.50 

4.58 

3.14 

2.19 

1.50 

3.08 

JjUbLOU,    ivltibo. 

8  P.M. 

15 

1.30 

1.30 

1.70 

2.42 

3.53 

4.98 

5.93 

5.79 

4.82 

3.19 

2.24 

1.60 

3.23 

Buffalo,  N.Y. 

8  A.M. 

15 

1.17 

1.07 

1.41 

2.18 

3.26 

4.86 

5.79 

5.43 

4.31 

2.91 

1.98 

1.39 

2.98 

8  P.M. 

15 

1.25 

1.20 

1.54 

2.24 

3.34 

4.91 

5.96 

5.43 

4.44 

3.06 

2.08 

1.48 

3.08 

Charleston, 

8  A.M. 

2.80 

3.00 

3.62 

4.61 

6.18 

7.67 

8.38 

8.43 

6.98 

5.08 

3.75 

2.93 

5.29 

S.C. 

8  P.M. 

3.06 

3.28 

3.95 

4.70 

6.38 

8.27 

8.75 

8.86 

7.63 

5.33 

4.17 

3.32 

5.64 

Chicago  111. 

8  A.M. 

15 

1.14 

1.14 

1.55 

2.30 

3.51 

5.15 

5.82 

5.36 

4.25 

2.99 

1.89 

1.35 

3.04 

8  P.M. 

15 

1.31 

1.48 

1.80 

2.57 

3.53 

5.14 

6.07 

5.96 

4.85 

3.14 

2.11 

1.60 

3.30 

Columbus, 

8A.M. 

15 

1.43 

1.35 

1.82 

2.62 

3.89 

5.40 

5.98 

5.65 

4.54 

3.04 

2.17 

1.61 

3.29 

Ohio 

8  P.M. 

15 

1.56 

1.56 

2.04 

2.86 

4.07 

5.53 

5.96 

5.89 

4.56 

3.21 

2.29 

1.80 

3.44 

Denver,  Col. 

8A.M. 

15 

0.82 

0.91 

1.13 

1.64 

2.38 

3.16 

3.75 

3.38 

2.40 

1.65 

1.11 

0.95 

1.94 

8  P.M. 

15 

1.12 

1.16 

1.33 

1.75 

2.49 

2.99 

3.82 

3.29 

2.47 

2.04 

1.35 

1.18 

2.08 

El  Paso,  Tex. 

8  A.M. 

15 

1.41 

1.46 

1.42 

1.52 

2.01 

2.97 

4.94 

4.87 

3.87 

2.62 

1.71 

1.42 

2.52 

8  P.M. 

15 

1.44 

1.40 

1.18 

1.14 

1.51 

2.23 

4.08 

4.20 

3.61 

2.39 

1.72 

1.49 

2.20 

Galveston, 

8A.M. 

15 

3.76 

4.08 

4.78 

6.16 

7.20 

8.69 

9.92 

8.97 

8.17 

6.30 

4.87 

4.07 

6.42 

Tex. 

8  P.M. 

15 

3.98 

4.31 

5.10 

6.26 

7.20 

8.42 

8.87 

8.99 

7.98 

6.41 

5.12 

4.35 

6.42 

Havre,  Mont. 

8  A.M. 

15 

0.74 

0.68 

0.95 

1.67 

2.63 

3.37 

4.00 

3.30 

2.57 

1.77 

1.21 

0.94 

1.99 

8  P.M. 

15 

0.86 

0.87 

1.35 

1.86 

2.76 

3.54 

3.60 

3.50 

2.98 

2.20 

1.56 

1.11 

2.18 

Helena,  Mont. 

8  A.M. 

15 

0.77 

0.85 

1.09 

1.52 

2.17 

2.74 

3.06 

2.81 

2.31 

1.69 

1.22 

0.99 

1.77' 

8  P.M. 

15 

0.88 

0.96 

1.25 

1.82 

2.22 

2.92 

2.90 

2.45 

2.35 

1.81 

1.45 

1.14 

1.85 

Indianapolis, 

8  A.M. 

15 

1.39 

1.36 

1.75 

2.70 

4.00 

5.36 

5.69 

5.61 

4.52 

2.96 

2.01 

1.62 

3.25 

Ind. 

8  P.M. 

15 

1.62 

1.56 

2.00 

2.91 

4.28 

5.71 

6.23 

5.86 

4.71 

3.17 

2.27 

1.89 

3.51 

Key  West,  Fla. 

8  A.M. 

15 

5.94 

6.26 

6.34 

6.62 

7.50 

8.84 

8.87 

8.87 

8.95 

8.38 

7.03 

6.34 

7.49 

8  P.M. 

15 

6.18 

6.22 

6.38 

6.62 

7.60 

8.57 

8.72 

8.99 

8.79 

8.02 

7.16 

6.30 

7.46 

Los  Angeles, 

8A.M. 

15 

2.46 

2.77 

3.15 

3.46 

4.08 

4.52 

5.17 

5.34 

4.71 

3.91 

2.90 

2.49 

3.75 

Cal. 

8  P.M. 

15 

3.32 

3.38 

3.73 

3.87 

4.27 

4.71 

5.28 

5.53 

5.20 

4.77 

3.99 

3.56 

4.30 

New  Orleans, 

8  A.M. 

15 

3.42 

3.72 

4.36 

5.21 

6.59 

8.07 

8.69 

8.53 

7.43 

5.43 

4.21 

3.55 

5.77 

La. 

8  P.M. 

15 

3.59 

3.97 

4.59 

5.35 

6.47 

7.87 

8.60 

8.34 

7.40 

5.60 

4.54 

3.84 

5.85 

New  York, 

8  A.M. 

15 

1.42 

1.39 

1.68 

2.48 

3.67 

5.13 

6.14 

6.06 

5.19 

3.38 

2.27 

1.65 

3.37 

N.Y. 

8  P.M. 

15 

1.59 

1.56 

1.89 

2.66 

3.94 

5.45 

6.39 

6.38 

5.45 

3.57 

2.46 

1.75 

3.59 

OmaVin    Nph          ^  A.M. 

15 

0.88 

0.95 

1.46 

2.45 

3.77 

5.36 

6.07 

5.96 

4.12 

2.67 

1.60 

1.15 

3.04 

mana,  iNeb.        g  p  M 

15 

1.16 

1.21 

1.76 

2.71 

4.06 

5.52 

6.36 

6.17 

4.48 

2.83 

1.91 

1.52 

3.31 

Philadelphia,      (  8  A.M. 

15 

1.49 

1.40 

1.70 

2.58 

4.02 

5.56 

6.41 

6.18 

6.28 

3.32 

2.32 

1.70 

3.58 

Pa.                    |  8  P.M. 

15 

1.54 

1.63 

1.99 

2.72 

4.18 

5.38 

6.46 

6.36 

5.31 

3.52 

2.48 

1.78 

3.61 

Phrpni-jr    AnV       i  ^  A-M- 

8 

1.96 

2.12 

2.01 

2.11 

2.26 

2.55 

5.19 

5.38 

4.07 

2.83 

2.17 

1.68 

2.86 

.mx,  Ariz,     j  g  p  M 

8 

2.20 

1.90 

1.98 

1.91 

2.09 

2.18 

4.27 

4.80 

4.04 

2.48 

2.47 

1.90 

2.72 

-p       .        . 

8  A.M. 

1.01 

1.04 

l.?5 

2.04 

3.31 

4.61 

5.43 

5.36 

4.30 

2.78 

1.87 

1.21 

2.86 

Jrortiandj  iVle. 

8  P.M. 

1.01 

1.10 

1.54 

2.18 

3.44 

4.73 

5.65 

5.61 

4.44 

2.93 

1.89 

1.28 

2.98 

PortlnnrJ    Orp> 

8A.M. 

15 

2.18 

2.33 

2.54 

2.80 

3.39 

3.80 

4.21 

4.62 

4.27 

3.58 

2.86 

2.44 

3.25 

jruitidnci,  v^re. 

8  P.M. 

15 

2.31 

2.49 

2.62 

2.91 

3.43 

4.17 

4.21 

4.49 

4.19 

3.78 

3.15 

2.61 

3.36 

Qf    T             A/r 

8  A.M. 

16 

1.46 

1.45 

2.03 

2.95 

4.66 

6.24 

6.65 

6.33 

5.11 

3.34 

2.19 

1.70 

3.68 

OC.  AjOUlS,  IVlO. 

8  P.M. 

16 

1.62 

1.72 

2.36 

3.41 

4.79 

6.20 

6.76 

6.43 

5.50 

3.51 

2.39 

1.90 

3.88 

^t     "Ponl     IVTinn          "  A.M. 

15 

0.65 

0.66 

1.05 

2.06 

3.12 

4.63 

5.47 

5.10 

3.84 

2.45 

1.38 

0.95 

2.62 

ot.  i  aui,  iviinn.     o 

15 

0.75 

0.90 

1.32 

2.28 

3.24 

4.76 

5.55 

5.15 

3.81 

2.62 

1.51 

1.08 

2.75 

Salt  Lake  City,     8  A!M! 

15 

1.19 

1.37 

1.54 

1.75 

2.41 

2.67 

2.98 

2.98 

2.36 

1.98 

1.66 

1.35 

2.02 

Utah                   8  P.M. 

15 

1.48 

1.56 

1.81 

2.01 

2.51 

2.62 

2.96 

3.12 

2.55 

2.31 

1.94 

1.60 

2.47 

San  Francisco.    (  8  A.M. 

15 

3.04 

3.12 

3.35 

3.46 

3.80 

4.03 

4.26 

4.51 

4.46 

4.22 

3.85 

3.23 

3.79 

Cal.                   !  8  P.M. 

15 

3.28 

3.28 

3.44 

3.56 

3.87J4.00 

4.42 

4.69 

4.48 

4.14 

3.81 

3.39 

3.86 

Seattle,  Wash,      fA-JJ; 

13* 

13* 

2.42 
2.48 

2.42 
2.49 

2.51 

2.47 

2.77  3.2313.63 
2.72  3.3313.85 

4.07 
4.34 

4.36 

4.31 

3.98 
4.14 

3.42 
3.54 

2.80 
2.00 

2.57 
2.70 

3.18 
3.27 

Washington, 
B.C. 

8  A.M. 
8  P.M. 

15 
15 

1.49 
1.63 

1.52 
1.62\ 

1.98 
2.14 

2.764.315.87 

2.96i4.-".' 

B.76J6.81  5.31 
B.95l«.92|5.68 

3.50  2.33 
3.61  2.44 

1.66 
1.77 

3.69 

3.88 

The  last  year  included  in  these  normals  is  1903.     The  time  used  is  Eastern  Standard  Time. 


206 


METEOROLOGY 


the  trade  wind  belts  both  on  account  of  the  lower  temperatures 
and  on  account  of  the  mixing  caused  by  the  larger  wind  velocities. 
In  the  horse  latitudes  the  moisture  is  slightly  less,  both  because  of 


\ 


San  Francisco 
8P.M. 

Washington 
8P.M. 
8A.M. 


New  Orleans 
8P.M. 


St.  Louis 
8P.M. 


Bismarck 
8P.M. 


2      d 


MONTHS 
FIG.  98.  —  The  Annual  Variation  in  Absolute  Humidity. 

lower  temperatures  and  because  we  have  to  do  here  with  descending 
air  currents  which  are  always  relatively  dry.  In  the  region  of  the 
prevailing  westerlies  the  amount  of  moisture  decreases  steadily  both 


THE  MOISTURE   IN  THE  ATMOSPHERE 


207 


on  account  of  the  still  lower  temperatures  and  the  rapid  motion  of 

the  air.     It  must  not  be  supposed  that  the  absolute  humidity  is  the 

same  for  all  places  which  have  the  same  latitude  or  lie  in  the 

same  wind  belt.  (The  various  factors  which  determine  the 

amount  of  absjolujfcejnimidity  are  the  temperature,  distance  the  absolute 

from  the  ocean,  the  inclosure  by  mountains,  and  the  altitude?  * 

On  the  whole,  the  higher  the  temperature,  the  larger  the 

amount  of  moisture  in  the  atmosphere.     The  amount  decreases  with 

distance  from  the  ocean  and  is  markedly  less  if  the  place  is  surrounded 

by  mountains.     It  also  decreases  markedly  with  altitude  and  practically 

disappears  at  a  height  of  ten  miles. 

196.    Daily  and  annual  variation  in  relative  humidity.  —  The  daily 
variation  in  relative  humidity  is  found  by  plotting  to  scale  the  normal 
hourly  values.     The   minimum   usually   occurs    during  the  The  chajv 
early  hours  of  the  afternoon  and  the  maximum  just  before  actenstics 
sunrise.     During  the  morning  the  amount  of  moisture  in  the  vitiation  In 
atmosphere  rapidly  increases,  but  with  the  rising  tempera-  reiativ 
ture  the  capacity  of  the  air  for  moisture  increases  so  much 
more   rapidly  that    the    relative    humidity    decreases    and   reaches  a 


T7\\j    4     ,    S    10  rft  j    \          8     ofl  2    4 


l  2    4    /    g'   10  jt        j    <?.  8   l4Xl  2    ,j    /?    *J    *  M    j 


m 4  / 


FIG.  99.  —  Relative  Humidity  at  St.  Louis,  Mo.,  September  16-19,  190S. 
(U.  S.  Weather  Bureau.) 

minimum    in     the    early    afternoon.       Soon    after    sunset    the    air 
cools  rapidly  and  thus  the  capacity  of  the  air  for  moisture  is  rapidly 
lessening.      In    most  well-watered    countries    the    saturation    point 
reached  in  the  early  evening.     That  means  that  the  relative  humidity 
has  obtained  a  value  of  100  per  cent  and  it  usually  remains  at  this 


208 


METEOROLOGY 


all  during  the  night  until  the  rise  of  temperature  occurs  again  on  the 

following  morning.  In  Fig.  99  the  trace  of  a  recording  hygrometer 
for  September  16  to  19,  1908,  at  St.  Louis,  Mo.,  is  given. 
Here  the  characteristics  which  have  just  been  mentioned 

are  particularly  pronounced. 

197.  The  annual  variation  in  relative  humidity  may  be  found  by 
plotting  to  scale  the  normal  monthly  values.  The  maximum 
usually  occurs  in  the  autumn,  when  the  falling  tempera- 
tures are  causing  a  great  decrease  in  the  capacity  of  the  air 
for  water  vapor  or  during  the  winter  itself.  The  minimum 
usually  occurs  during  the  spring,  when  the  rapidly  rising 
temperatures  are  causing  a  great  increase  in  the  capacity 

of  the  air  for  water  vapor  or  during  the  summer  itself. 


Illustration. 


The  char- 
acteristics 
of  the 
annual 
variation  in 
relative 
humidity. 


Phoeriix 


T 

\8 


\ 


8P.M. 


.-Jflew  Orleans 

8P.M. 
Washington 
8P.M. 
Bismarck 
8P.M. 


FIG.  100.  —  The  Annual  Variation  in  Relative  Humidity. 


The  following  table,  which  gives  the  normal  values  of  relative  humidity 
for  the  various  months  and  for  the  year  for  several  stations 
in  the  United  States,  illustrates  these  characteristics.  In 
Fig.  100  some  of  these  results  are  shown  graphically. 

198.  Geographical  variation  in  relative  humidity.  —  The 
geographical  variation  in  relative  humidity  is  again  closely 
correlated  with  the  general  wind  system.  The  normal  yearly 
value  in  the  equatorial  regions  is  more  than  80  per  cent,  it 


The  charac- 
teristics of 
the  geo- 
graphical 
variation  in 
relative 
humidity. 


THE  MOISTURE   IN  THE  ATMOSPHERE 


209 


NORMAL  VALUES  OF  RELATIVE  HUMIDITY  IN  PERCENTAGES  FOR  THE 
VARIOUS  MONTHS  AND  FOR  THE  YEAR 


0  Q 

^ 

STATION 

j! 

4 

1 

« 
1 

1 

% 

w 
g 

1 

o 
p 

3 

CQ 

1 

o 

P 

<s 

*  Albany,  N.Y.  {  f  p'JJ' 

15 
15 

83 
79 

78 

80 
75 

74 

65 

73 
65 

75 

69 

76 
69 

75 
67 

81 
74 

84 

75 

83 
78 

83 
80 

79 
73 

Bismarck,     (  8  A.M. 

21 

77 

78 

78 

76 

76 

81 

80 

80 

79 

81 

80 

79 

79 

N.  Dak.     f  8  P.M. 

21 

66 

67 

65 

55 

54 

59 

52 

49 

52 

60 

63 

67 

59 

*  Boise,  Idaho  j  |  p'JJ' 

5 
5 

84 

72 

82 
63 

74 
46 

72 
36 

73 
36 

66 
32 

64 
21 

56 
26 

63 
31 

73 

45 

76 
63 

82 
73 

72 
45 

Boston,  Mass. 

14 

72 

71 

68 

66 

71 

72 

71 

75 

77 

5 

75 

71 

72 

*Buflfalo,  N.Y.  1  1  p'^- 

15 
15 

79 

77 

79 
77 

76 
73 

71 
68 

72 
69 

74 
70 

75 
70 

75 

68 

75 
70 

74 
70 

75 

73 

72 
70 

75 
71 

Charleston,    (  8  A.M. 

38 

81 

78 

79 

76 

77 

78 

79 

82 

83 

79 

80 

80 

79 

S.C.        i  8  P.M. 

38 

77 

76 

75 

76 

77 

79 

80 

81 

80 

76 

77 

79 

78 

Chicago,  111.   jf££ 

20 
20 

85 
80 

84 
80 

81 

77 

76 
70 

76 
69 

76 
71 

74 
67 

74 
69 

75 

68 

76 
68 

80 
73 

83 
79 

78 

72 

Columbus,     (  8  A.M. 

84 

82 

80 

74 

75 

77 

76 

79 

80 

81 

82 

83 

79 

Ohio       J  8  P.M. 

77 

74 

69 

61 

61 

63 

69 

61 

62 

64 

70 

76 

66 

18AM 

15 

58 

64 

61 

62 

65 

63 

63 

63 

59 

58 

66 

56 

61 

*  Denver,  Col.  ]  g  P'M' 

15 

49 

49 

42 

36 

38 

34 

36 

33 

30 

34 

41 

48 

39 

El  Paso,  Tex.  J  |  ^JJ' 

16 
16 

62 
34 

55 

27 

43 

18 

36 
13 

55 
13 

41 
16 

60 
29 

63 

32 

63 

33 

60 
30 

58 
31 

58 
34 

53 

26 

Galveston,    f  8  A.M. 

86 

87 

86 

85 

82 

82 

81 

82 

82 

79 

82 

84 

83 

Texas      (  8  P.M. 

82 

83 

83 

81 

77 

77 

74 

75 

73 

73 

78 

81 

78 

Havre,  Mont. 

22 

80 

81 

77 

62 

63 

63 

57 

66 

62 

68 

75 

79 

69 

Helena,  Mont.  {  |  p'JJ' 

72 
66 

73 

63 

71 
56 

65 
42 

66 
42 

67 
41 

60 
31 

59 
30 

64 
39 

66 

48 

68 
56 

71 
65 

67 
48 

Indianapolis,  Ind. 

21 

79 

77 

73 

66 

67 

68 

65 

67 

68 

68 

73 

77 

71 

Key  West,  Fla.  {  |  p'JJ' 

82 
80 

81 

78 

78 
76 

73 
U 

74 
75 

76 

77 

73 
75 

74 
75 

77 
78 

79 

78 

80 
79 

82 
80 

77 
77 

Los  Angeles,   f  8  A.M. 

22 

67 

75 

80 

83 

87 

88 

90 

88 

83 

79 

71 

60 

79 

Cal.        1  8  P.M. 

22 

64 

65 

65 

63 

67 

64 

62 

64 

64 

68 

71 

61 

64 

New  Orleans,  f  8  A.M. 

21 

84 

84 

85 

83 

81 

81 

82 

84 

83 

80 

84 

84 

83 

La.        1  8  P.M. 

21 

73 

'<  2 

71 

68 

68 

71 

74 

75 

73 

68 

73 

74 

72 

*  New  York,   J  8  A.M. 

15 

76 

75 

74 

70 

73 

76 

77 

78 

79 

77 

77 

75 

76 

N.Y.       1  8  P.M. 

15 

72 

71 

69 

65 

69 

71 

70 

73 

73 

71 

72 

71 

71 

Omaha,  Neb.  {§££ 

21 

21 

81 
71 

81 

70 

76 
64 

73 
53 

75 
55 

79 
57 

78 
57 

80 
59 

80 

58 

76 
55 

77 
63 

81 

70 

78 
61 

Philadelphia,  Pa. 

73 

72 

69 

64 

68 

68 

70 

72 

74 

71 

71 

72 

70 

Phoenix,  Ariz,  j  §  A'*J' 

(  O  P.M. 

8 
8 

64 
37 

62 

28 

53 

24 

45 

18 

38 
15 

32 
12 

49 
21 

54 
25 

51 
25 

51 

26 

57 
32 

57 
32 

51 
25 

Portland,  Me. 

14 

75 

74 

72 

69 

76 

76 

76 

80 

81 

.79 

77 

75 

75 

Portland,  Ore. 

21 

84 

79 

74 

70 

69 

69 

64 

66 

71 

79 

84 

85 

74 

St.  Louis,  Mo. 

21 

75 

72 

72 

66 

69 

69 

67 

69 

70 

67 

70 

76 

70 

St.  Paul,  Minn.  I  §  A>M: 

(  o  P.M. 

21 
21 

84 
76 

84 
75 

81 
68 

75 
55 

75 
64 

79 

58 

79 
55 

83 
56 

83 
60 

81 
63 

81 
69 

83 
76 

81 
64 

Salt  Lake  City,  Utah 

22 

74 

68 

58 

48 

48 

39 

34 

34 

40 

51 

52 

71 

52 

San  Francisco,  f  8  A.M. 
Cal.       1  8  P.M. 

19 
19 

87 
75 

86 

72 

85 
70 

85 
69 

87 
72 

89 

72 

92 

77 

93 
79 

88 
73 

86 
71 

85 
71 

H 

88 
73 

Seattle,  Wash,  {  f  p-JJ' 

88 
81 

86 

74 

85 
66 

85 
58 

86 
59 

85 
56 

85 
51 

87 
54 

89 
63 

90 

74 

88 
81 

88 
82 

87 
67 

*  Washington,  (  8  A.M. 
D.C.       1  8  P.M. 

15 
15 

77 
69 

75 

66 

75 
65 

70 
59 

75 

68 

76 
71 

77 
72 

80 
74 

81 
76 

80 
72 

79 
69 

76 

77 
69 

The  time  is  expressed  in  Eastern  Standard  Time.     Ordinarily  the  last  year  included  in 

Q  i-> .^f-m n  1  o  I'Q   1  QflQ         Tf  -fV>o  a+a+irvn  liaa  a   *    f.hp.  last.  Vfia.r  IS  1903. 


the  normals  is  1908.    If  the  station  has  a  *,  the  last  year  is  1903, 


210  METEOROLOGY 

is  less  in  the  trade  wind  belts,  and  reaches  a  minimum  of  about  70  per 
cent  in  the  horse  latitudes.  It  then  increases  again  in  value,  and  in 
the  polar  regions  is  between  80  and  90  per  cent,  thus  surpassing  the 
value  at  the  equator.  Locally,  it  is  influenced  by  the  same  four 
factors  which  determine  the  absolute  humidity. 

199.    Other  moisture  data  and  charts.  —  In  connection  with  mois- 
ture, the  average  and  normal  values  for  the  various  months  and  for  the 

year  are  the  only  results  which  are  computed  from  the  ob- 
made  of  servations  taken.  In  the  case  of  the  U.  S.  Weather  Bureau, 
the  obser-  the  relative  humidity  at  8  A.M.  and  8  P.M.  are  the  only  data 

which  are  kept  constantly  computed  to  date.  If  these  normal 
values  are  known  for  a  country  or  for  the  world,  corresponding  charts 
may  be  prepared.  Charts  XV  and  XVI  give  the  relative  humidity 
for  the  United  States  for  January  and  July. 


THE  EFFECT  OF  WATER  VAPOR  ON  THE  GENERAL  CIRCULATION 

200.  As  was  stated  in  section  176,  water  vapor  is  lighter  than  air 
under  the  same  conditions  of  pressure  and  temperature,  the  ratio 
The  mois-  of  mass  being  62  to  100.  Thus,  moisture-laden  air  is 
tureacts  lighter  than  dry  air.  Now  the  amount  of  moisture  in  each 
temperature  cubic  foot  of  air  at  the  equator  is  normally  six  times  the 
in  causing  amount  in  the  same  volume  of  air  at  the  pole.  This  excess 
ferencesand  of  moisture  at  the  equator  will  thus  cause  the  air  at  the 
wind-  equator  to  be  lighter  than  at  the  pole,  and  will  thus  operate 

in  the  same  way  as  the  higher  equatorial  temperatures  fo  accentuate 
the  pressure  differences  between  the  equator  and  pole,  and  to  cause 
a  general  circulation  of  the  atmosphere. 


B.     DEW,  FROST,  AND  FOG 

DEW 

201.    Condensation.  —  Condensation  is  the  opposite  of  evaporation; 
Definition.      ^  *s  ^ne  passage  of  the  moisture  of  the  atmosphere  from 
the  invisible  gaseous  form,  water  vapor,  into  some  visible, 
'forml^of11      solid    or    liquid   form.      When   condensation   takes    place, 
condensa-      the  water  vapor  takes  the  form  of  dew,  frost,  fog,  cloud, 
rain,  snow,  or  hail.     There  are  thus  seven  forms  of  con- 
densation, and  each  of  these  forms  must  receive  careful  consideration. 


THE  MOISTURE  IN  THE  ATMOSPHERE  211 

There  are  two  ways  in  which  the  condensation  of  some  of  the  water 
vapor  in  a  given  quantity  of  air  may  be  brought  about:  by  com- 
pression or  by  cooling.  Of  these,  the  first  can  be  performed 
in  the  laboratory,  and  never  occurs  in  nature.  Con- 
densation  by  cooling  is  the  only^  process  which  occurs  in  given  quan- 
nature.  Suppose  a  vessel  contains  two  cubic  feet  of  air  at 
a  temperature  of  70°  F.,  and  suppose,  furthermore,  that  it  densed  by 
contains  7  grains  of  moisture  per  cubic  foot.  Let  this  air 
be  compressed  until  its  volume  is  only  one  cubic  foot,  and 
suppose  that  the  temperature  has  been  held  at  70°  by  cooling  the  con- 
taining vessel.  This  cubic  foot  of  air  will  now  contain 
14  grains  of  moisture,  and  by  referring  to  the  table  in  sec- 
tion  181,  it  will  be  seen  that  a  -cubic  foot  of  air  at  70°  can  tion  b7  com- 
contain  only  7.99  grains.  Therefore  the  excess  of  moisture, 
namely,  6.01  grains,  must  come  out  of  the  air  in  some  form.  In  nature 
this  can  never  occur,  because  there  is  no  means  by  which  the  compressed 
air  may  be  kept  at  a  constant  temperature.  Descending  air  currents 
are  so  warmed  by  compression  that  the  capacity  for  water  vapor  in- 
creases much  more  rapidly  than  the  increase  in  the  amount  of  moisture 
in  each  cubic  foot.  Thus,  descending  air  currents  soon  become  dry. 

If  a  given  quantity  of  air  is  cooled,  it  will  soon  reach  a  temperature 
at  which  the  amount  of  moisture  in  it  saturates  it,  and  any  further  cooling 
will  cause  moisture  to  come  out  of  it  in  some  one  of  the  seven  The  three 
forms  of  condensation.     This  cooling  of  the  air  in  nature  ways  in 
may  be  brought  about  by  three  processes.     It  may  be  caused  maybe* 
by  ^xpansion,   and  in  this  case  the  air  grows  colder  with-  cooled  in 
out  the  addition  or  subtraction  of   heat  from  any  outside  nature* 
objects ;  secondly,  it  may  be  cooled  by  mixture  with  colder _airj  thirdly, 
it  may  be  cooled  by  conducting  its  heat  to  surrounding  objects  which 
are  colder  or  by  radiaWng  its  heat  to  space  or  surrounding  objects. 
^pfQ2.    DewT" —  In  the  early  morning,  more  particularly  in  summer, 
shortly  after  sunrise,  the  temperature  rises  rapidly  and  the  amount  of 
moisture   added  to   the   atmosphere   by   evaporation  from  Description 
bodies  of  water  and  the  moist  ground  also  increases  rapidly,  pf  how  dew 
Activity  on  the  part  of  plants  and  animals  also  adds  large 
quantities  of  water  vapor  to  the  atmosphere*     As  a  result,  all  during 
the  day,  the  amount  of  moisture  in  the  atmosphere,  that  is,  the  number 
of  grains  in  each  cubic  foot,  is  steadily  increasing.     In  the  early  evening, 
just  after  sunset,  the  leaves  of  trees  and  plants,  the  grassy  covering  of 
the  ground,  the  ground  itself,  and  in  short  all  material  objects  lose  their 


212  METEOROLOGY 

heat  rapidly  by  radiation  to  space  and  become  colder  than  the  overlying 
layer  of  air.  The  layer  of  air  loses  its  heat  by  conduction,  and  to  a 
certain  extent  by  radiation,  and  soon  reaches  the  saturation  point. 
Any  further  reduction  in  temperature  will  cause  the  excess  moisture  to 
come  out  on  any  solid  object  as  dew.  Dew  has  been  studied  for  a 
little  more  than  a  century,  but  the  first  correct  explanation  was 
given  by  Wells  in  1814.  It  was  essentially  the  explanation  just  given 
above. 

The  formation  of  water  drops  on  the  outside  of  an  ice  pitcher  in  a 
warm  room  on  a  summer's  day  is  also  an  illustration  of  this  process, 
ice  pitcher  The  ice  pitcher  corresponds  to  the  ground  which  has  become 
illustration.  co\^}  <}ue  to  radiation.  The  layer  of  air  next  it  corresponds 
to  the  layer  of  air  near  the  surface  of  the  ground,  and  the  water  which 
collects  on  the  outside  of  the  pitcher  corresponds  to  the  dew. 

203.  There  are  three  sources  of  the  moisture  which  go  to  make  up 
dew.     A  large  part  of  it  comes  from  the  lo_wer  layer  of  the  atmosphere 

th  itself.     Some  of  it  comes  from  the  leaves^of  trees  and  plants. 

sources  of  The  moisture  exudes  from  these,  and,  since  evaporation  is 
the  mois-  not  possible,  it  collects  on  them  and  adds  to  the  supply  of 

11UTC* 

dew.  Furthermore,  a  certain  amount  of  jnojsture  comes  up 
from  the  subsoil  by  capillary  action,  and  as  soon  as  it  evaporates  it  again 
condenses  on  the  blades  of  grass  and  the  leaves  of  plants  above. 

Various  estimates  and  measurements  have  been  made  of  the  amount 
of  dew.  The  figures  usually  given  are  these  :  in  a  well-watered  region, 
The  amount  during  a  single  night,  the  dew  would,  perhaps,  amount  to 
of  dew.  0  01  Of  an  mch  of  water,  and  the  total  amount  of  dew  which 
occurs  in  the  course  of  a  year  might  amount  to  an  inch.  In  desert 
regions,  of  course,  the  amount  is  practically  nothing. 

204.  The   part   played   by  latent   heat.  —  When  evaporation   takes 
place,  energy  is  required  to  separate  the  molecules,  and  this  energy 

is  called  latent  heat.     Conversely,  when  condensation  takes 

Condensa-  . 

tion  liber-  place,  this  latent  heat  again  makes  itself  felt.  It  may  either 
ates  latent  heat  the  surface  upon  which  the  condensation  takes  place, 

or  retard  its  rate  of  cooling  by  adding  to  its  supply  of  heat. 

The  small  daily  range  of  temperature  in  a  well-watered  region  is  an 

illustration  of  the  part  played  by  latent  heat.     During  the  day,  high 

Latent  heat    temperatures    are   prevented    because   so   much   energy   is 

lessens  daily  used  up  as  latent  heat  in  evaporation  and  not  in  heating  the 

air.  At  night,  cooling  is  retarded  by  the  liberation  of  so 
much  latent  heat. 


THE  MOISTURE   IN  THE  ATMOSPHERE  213 

205.    Conditions  for  the  formation  of  dew.  —  There  are  two  condi- 
tions for  the  formation  of  dew ;  a  clear  sky  and  absence  of  wind.     The 
reason  for  this  is  because  a  large  drop  in  temperature  during 
the  night  is  essential  for  the  formation  of  dew,  and  both 
clouds  and  wind  prevent  this.     Clouds  prevent  it  by  hinder-  forThTfor- 
ing  that  free  radiation  of  heat  to  space  which  is  essential  for  ™*™°SB 
a  large  drop  in  temperature.     Wind  prevents  it  by  mixing  clear  sky 
the  layer  of  air  in  contact  with  the  ground  which  has  be- 
come cold   by  conduction  with  the  warmer  layers  above. 
When  dew  is  deposited,  an  inversion  of  temperature  nearly  always  occurs. 

More  dew  forms  in  valleys  than  on  hilltops,  and  there  are  two  reasons 
for  this.     It  is  in  the  valleys  that  the  ponds,  lakes,  streams,  and  water 
courses  are  usually  located,  and  these  add  a  large  amount 
of  moisture  by  evaporation  to  the  atmosphere.     Further-  for°ms  in W 
more,  at  night  the  cold  and  thus  denser  air  drains  into  the  valleys  than 
valleys  from  the  surrounding  hilltops.       In  riding  over  a 
hilly  road  at  night,  the  increase  in  moisture  and  the  lower  temperatures 
on  descending  into  the  valleys  is  always  plainly  evident. 


FROST 

206.  Frost.  —  Frost  is  dew  which  has  formed  with  the  temperature 
below  32°  F.  It  consists  of  small,  translucent,  frozen  drops  placed 
side  by  side,  and  also  of  feathery,  spinelike  forms.  It  is  The  appear- 
sometimes  thought  that  the  translucent,  frozen  drops  are  ance  of  frost< 
due  to  the  moisture  which  has  exuded  from  the  leaves  of  plants  and 
frozen  there.  It  may  be  the  dew  which  had  formed  before  the  tem- 
perature went  below  32°  F.  The  moisture  which  comes  from  the  atmos- 
phere and  from  the  ground  usually  has  a  feathery,  spinelike  form.  In 
winter,  the  moisture  which  comes  up  from  the  subsoil  usually  forms  ice 
crystals" just  beneath  the  surface  of  the  ground,  raising  the  surface  and 
giving  to  the  ground  a  spongy  character. 

The  word  frost  is  really  used  in  two  different  senses.     It  is  used  to 
designate  the  deposit  of  frozen  dew  which  may  form  at  any  time  of  year, 
provided  the  temperature  is  below  32°  F.  and  the  dew  point  Light  and 
has  been  passed.     It  is  also  used  to  designate  the  occurrence  killing 
of  those  temperatures,  particularly  m  the  late  spring  and 

y  aufcann,  which  are  destructive  to  vegetation.     When  used  in  this 

"t  sedjf  two  kinds  of  frosts  may  be  distinguished ;  light  frosts  or 
white  frosts,  and  killing  frosts.  If  the  temperature  does  not  fall  much 


214  METEOROLOGY 

lower  than  4°  below  the  freezing  point,  only  the  very  tenderest  vegeta- 
tion is  killed  and  the  frost  is  spoken  of  as  a  light  or  white  frost.  If  the 
temperature  goes  below  28°  F.,  even  the  hardier  forms  of  vegetation  will 
be  killed,  and  the  frost  is  then  spoken  of  as  a  killing  frost. 

207.  Two  processes  operate  to  produce  the  cooling  which  may  result 
in  the  destructive  frosts  of  late  spring  and  early  autumn.     These  are, 

first,  the  importation  of  colder  air,  and  secondly,  the  radia- 
of  the  de-  tion  of  heat  from  the  ground  and  the  cooling  of  the  air  next 
structive  ^o  ft  by  conduction  and  radiation.  'On  some  occasions,  a 
late  spring  strong  northwest  wind  will  import  cold,  dry  air,  thus  hold- 
and  early  jng  down  the  maximum  temperature  during  the  day,  and 

causing  a  low  temperature.  If  the  wind  dies  down  during 
the  night  and  the  sky  becomes  clear,  it  takes  but  little  radiation  to 
cause  sufficient  cooling  to  produce  a  frost.  On  other  occasions, 
there  seems  to  be  but  little  importation  of  cool,  dry  air.  The  sky  is 
quite  clear  and  there  is  almost  no  cloud.  Radiation  is  excessive,  and 
the  resulting  large  drop  in  temperature  may  cause  a  frost.  While  both 
of  the  above  processes  are  usually  active,  it  is  generally  easy  to  see 
which  predominates.  As  in  the  case  of  dew,  a  clear,  still  night  is  neces- 
sary for  frost  formation.  These  conditions  are  most  likely  to  be  fulfilled 
on  the  first  or  second  night  following  the  passage  of  an  area  of  low  pres- 
sure and  the  transition  of  the  weather  control  to  an  area  of  high  pressure. 
As  will  be  seen  later,  this  facilitates  both  the  importation  of  colder  air 
during  the  day  and  the  radiation  at  night,  —  the  two  processes  which 
cause  the  low  temperatures  required  for  a  frost. 

208.  Prediction  of  frost.  —  A  frost  is  predicted  by  the  U.  S.  Weather 
Bureau  in  exactly  the  same  way  as  any  other  temperature.     The  ob- 
The  u  s.       servations  of  the  various  meteorological  elements  are  made 
Weather        at  many  stations  at  8  A.M.  and  8  P.M.,  and  these  are  distrib- 
method  of      u^ec^  ^y   telegraph   to   all   stations   where   predictions    are 
predicting       made.     From  the  weather  map  which  is  prepared  from  these 

observations,  and  from  the  sky  appearance,  the  probable 
clearness  of  the  sky  during  the  following  night,  and  the  probable 
wind  velocity  must  be  estimated.  From  these,  chiefly,  the  probable 
drop  in  temperature  is  determined.  In  this  way,  the  probable  mini- 
mum temperature  of  the  following  morning  and  thus  the  likelihood 
of  the  frost  is  determined. 

209.  There  is  a  widespread  opinion  that  if  the  frost  po^ct 
delayed  until  the  maximum  temperature  of  the  preceding  dl^knd  Uie 
dew  point  at  the  time  of  the  maximum  have  been  determined,  that 


THE  .MOISTURE   IN  THE  ATMOSPHERE  215 

a  much  more  accurate  estimation  of  the  probable  minimum  temperature 
of  the  following  morning  can  be  made.     It  is  generally  supposed  that 
the  temperature  will  drop  rapidly  until  the  dew  point  is 
reached  and  then  any  further  drop  will  be  much  retarded  SSion  from 
by  the  liberation  of  latent  heat.     In  other  words,  the  dew  the  maxi- 
point  will   serve   as   a  guide   in   determining   the   probable  ^"aturTand 
amount  of  drop.     As  a  matter  of  fact,  in  the  northeast  por-  dew  point 
tion  of  the  United  States  the  air  is  so  dry  and  the  dew  point  before^7 
lies  so  low  on  all  those  days  when  a  frost  is  imminent,  that 
the  dew  point  is  seldom  reached  and  plays  practically  no  part  in  deter- 
mining the  minimum  temperature.     The  drop  is,  also,  far  from  constant, 
and  must  be  carefully  estimated  for  each  individual  case,  taking  into 
account  the  probable  characteristics  of  the  afternoon  and  night. 

210.  If,  after  the  probable  minimum  temperature  in  the  thermometer 
shelter  has  been  estimated,  it  is  desired  to  determine  the  probable  tem- 
perature of  low-growing  vegetation  in  the  open,  at  various 

points  in  a  limited  area  surrounding  the  station  in  question,  ferenCe~De_ 
three  things  must  be  taken  into  account :  First,  that  plant  tween  the 
temperatures  go  below  the  real  air  temperature  because  the  temperature 
plants  are  in  the  open,  without  such  a  hindrance  to  radiation  in  a  ther- 
as  is  the  shelter  about  a  thermometer.     On  the  average,  Jheite^and 
vegetation  in  the  open  will  go  about  2°  F.  below  the  real  air  the  temper- 
temperature  and  in  extreme  cases  3°  or  4°  below.     Second,  vegetation 
vegetation  is  located  near  the  ground  and  not  at  the  height 
of  the  instruments  in  the  thermometer  shelter.     The  difference  in  tem- 
perature between  the  surface  of  the  ground  and  the  thermometer  shel- 
ter, say  from  5}  to  6  feet  above  the  surface  of  the  ground,  will  average 
perhaps  3°  or  4°,  and  in  extreme  cases  might  amount  to  6°  or  7°. 
Third,  the  variation  in  temperature  over  a  limited  area  may  amount  to 
several  degrees.     In  fact,  for  a  limited  area  it  may  easily  amount  to  6° 
or  7°  and  in  certain  cases  may  be  much  more  than  that.      Thus,  the 
temperature  of  vegetation  in  the  open,  near  the  ground,  in  the  coldest 
portion  of  a  limited  area,  may  be  expected  to  average  from  10°  to  14° 
lower  than  the  estimated  minimum  in  the  thermometer  shelter,  and 
under  extreme  circumstances  may  differ  by  from  5°  to  10°  more. 

211.  Protection  from  frost.  -—  Tender  vegetation  is  usually  protected 
from  frost  by  covering  it  with  cloth  or  paper  or  some  such  Ordinar 
covering.     The  radiating  surface  is  thus  transferred  from  protection 
the  ground  or  the  leaves  of  the  plants  to  the  covering,  and 

it  is  thus  protected  against  frost. 


216  METEOROLOGY 

It  has  often  been  proposed  to  cover  a  whole  village  or  even  a  portion 
of  a  state  with  a  smoke  layer  by  building  smudge  fires.  If  concentrated 
The  use  of  action  of  this  kind  could  be  secured,  and  a  smoke  layer  of 
a  smoke  sufficient  thickness  could  be  produced,  the  radiation  would 
be  transferred  from  the  surface  of  the  ground  to  the  surface 
of  the  smoke  layer,  and  temperatures  from  5°  to  6°  higher  than  would 
otherwise  occur  might  be  maintained  underneath  the  smoke  layer. 
Experiments  are  just  beginning  to  be  made  on  such  a  large  scale 
as  to  make  possible  definite  conclusions. 

212.  Frost  observations,  frost  data,  and  charts.  —  The  observations 
'made  of  frosts  are  the  dates  of  the  first  light  frost  and  the  first  killing 

frost  in  the  autumn,  and  the  last  light  and  the  last  killing 
vations86  frost  in  the  spring.  From  these  observations,  the  normal 
taken  and  dates  of  the  first  and  last  light  and  killing  frost  may  be  deter- 
niined.  If  these  dates  are  known  for  a  whole  country  or  for 
the  whole  world,  they  may  be  indicated  on  charts.  The 
departure  from  normal,  however,  in  the  case  of  these  dates,  may 
amount  to  three  weeks  in  either  direction.  Charts  XVII  and  XVIII 
depict,  for  the  United  States,  the  normal  dates  of  the  first  killing  frost 
of  the  autumn  and  the  last  killing  frost  of  the  spring. 

FOG 

213.  The  nature  of  fog.  —  If  a  layer  of  air  of  considerable  thickness 
falls  below  the  temperature  of  saturation,  the  moisture  is  unable  to  come 
The  nature     out  as  dew  or  frost  at  the  bottom  of  the  layer,  but  condenses 
of  fog.  as  fOg      Thjg  condensation  takes  place  on  the  dust  and  other 
particles  in  the  air.     A  fog  particle,  therefore,  is  a  minute  water  drop, 
about  0.001  inch  in  diameter,  with  a  dust  particle  or  some  other  particle 
for  its  nucleus  (see  section  227).     Whenever  fog  occurs,  the  relative 
humidity  is  usually  very  high,  at  least  90  per  cent.     There  are  a  few  cases 
on  record,  however,  when  a  dense  fog  has  existed  with  a  relative  humidity 
even  below  60  per  cent.     In  these  very  exceptional  cases  there  must 
have  been  something  which  prevented  the  evaporation  of  the  fog  parti- 
cles, as  a  coating  of  oil  or  the  like. 

The  time  of  year  when  the  maximum  number  of  foggy  days  occurs  is 
very  different  in  different  parts  of  the  world.  On  the  Atlantic  coast  of 
The  time  of  North  America,  from  Maine  northward,  fogs  occur  chiefly 
maximum  during  the  summer.  In  the  southern  part  of  the  United 
fogginess.  states,  particularly  inland,  fogs  occur  chiefly  during  the 
winter.  In  New  England  and  the  middle  Atlantic  States,  particularly 


THE  MOISTURE   IN  THE  ATMOSPHERE  217 

inland,  fogs  are  of  two  entirely  different  kinds.     One  is  most  prevalent 
during  the  late  winter  and  early  spring,  while  the  other  occurs  chiefly 
during  the  late  summer  and  early  autumn.     One  is  caused  by  the  trans- 
portation of  air  and  the  other  by  radiation.     In  fact,  all  fogs  are  caused 
in  one  or  the  other  of  these  two  ways.     In  the  late  winter,     t/ 
warm  moisture-laden  air  may  come  up  from  the  south  over  p0rtationS" 
the  cold  snow-covered  regions  at  the  north.     As  a  result,  a  fogf  of 
layer  of  air  of  considerable  thickness  is  cooled  below  its  dew  th"radia- 


point,  and  condensation  'in  the  form  of  fog  takes  place, 
Again,  due  to  rain  or  a  continued  thaw,  the  air  may  have 
become  warmed  and  moisture-laden.  The  wind  suddenly  changes 
to  the  northwest,  the  temperature  drops  rapidly,  and  a  layer  of  air  of 
considerable  thickness  may  go  below  the  dew  point  and  the  moisture 
come  out  in  the.  form  of  fog.  In  both  cases,  the  fog  has  been  caused 
by  the  transportation  of  air,  and  this  transportation  is  brought  about 
by  the  presence  of  storms.  The  formation  of  fog  in  the  autumn  is  en- 
tirely different.  During  the  day,  the  temperature  rises  to  considerable 
heights  and  evaporation  is  considerable.  The  nights  are  growing  longer 
and  the  drop  in  temperature  is  considerable.  At  night  the  temperature 
often  goes  below  the  dew  point  for  a  considerable  thickness  of  air,  and 
the  moisture  comes  out  in  the  form  of  fog.  These  fogs  always  commence 
during  the  night  and  are  thickest  at  sunrise.  They  usually  disappear  dur- 
ing the  forenoon.  The  disappearance  of  these  fogs,  and  in  fact  all  fogs,  is 
brought  about  by  the  wind  or  a  rise  in  temperature.  The  wind  may  mix 
the  fog-laden  layer  of  air  with  a  dryer  layer,  thus  causing  its  disappearance. 
A  rise  in  temperature  may  so  increase  the  capacity  of  the  air  for  vapor 
that  the  fog  particles  evaporate  and  become  invisible  water  vapor. 

It  has  been  sometimes  stated  that  the  city  fogs  are  entirely  different 
from  the  country  fogs.     This  is  only  true  in  the  sense  that  combustion 
products  play  a  larger  part  in  their  formation,  thickness,  and         fo  g 
continuance.     The  number  of  fogs  in  London  has  steadily 
increased  with  the  growth  of  the  city  and  the  increase  in  the  combustion 
products.     This  is  shown  in  the  accompanying  table.     The  increase 

YEAR  NORMAL  ANNUAL  NUMBER  OF  FOGGY  DATS 

1871-1875  50.8 

1876-1880  58.4 

1881-1885  62.2 

1886-1890  74.2 

Very  recent  data  would  seem  to  show  that  the  number  of  foggy  days  has 
reached  its  maximum  and  is  now  decreasing.  x 

1  See  M  eteorologusche  Zeitschrift,  March,  1911,  p.  135. 

f 


218  METEOROLOGY 

in  the  dust  and  smoke,  due  to  condensation,  furnishes  more  nuclei  for 
the  condensation  of  water  vapor  and  also  facilitates  the  radiation  of 
heat  during  the  night,  and  thus  the  formation  of  fog.  Furthermore, 
after  the  fog  particle  has  once  formed,  it  becomes  coated  with  oil,  and  it 
is  thus  much  harder  for  it  to  be  evaporated  when  the  temperature  rises. 
In  fact,  the  fog  forms  to  such  a  thickness  that  the  heating  during  the 
day  is  no  longer  sufficient  to  disperse  the  fog  entirely,  and  it  continues 
to  grow  thicker  and  thicker  on  successive  days  and  only  disperses  when 
a  marked  change  in  the  weather  occurs. 

214.  Fog  observations,  fog  data,  and  charts. — The  only  observa- 
tions made  of  fog  are  the  day  on  which  it  occurs.     From  these  obser- 
The  obser-      vations,  the  normal  number  of  foggy  days  for  the  various 
vations          months  and  for  the  year  may   be   determined.      If  these 
ttuTresuits      normals  are  known  for  many  stations,  they  may  be  charted 
computed.      for  a  given  country  or  the  world. 

C.    CLOUDS 

THE  CLASSIFICATION  OF  CLOUDS 

215.  Early  History.  —  Although   the   varying   forms   of   the   clouds 
must  have  been  observed  by  the  earliest  peoples,  no  cloud  names  or 
Howard's       cloud  classifications  have  corne  down  to  us  from  antiquity, 
classifica-       The  first  classification  was  proposed  by  Lamarck,  a  French 

naturalist,  in  1801.  He  distinguished  six  kinds  of  clouds,  to 
which  French  names  were  given.  Luke  Howard  in  1803  proposed  the 
scheme  from  which  the  one  in  general  use  at  the  present  time  was  des- 
tined to  develop.  His  classification  included  seven  forms  in  all,  four 
type  forms  and  three  combinations  of  the  type  forms  to  indicate  inter- 
mediate varieties.  To  the  four  type  forms  of  cloud  he  gave  the  Latin 
names  Cirrus,  Cumulus,  Stratus,  and  Nj/mbu_s,  meaning  respectively 
lock  of  hair,  pile,  layer,  and  storm  cloud.  The  name  in  each  case  indi- 
cates the  most  striking  characteristic  of  the  cloud.  The  cirrus  is  the 
delicate,  fibrous,  hairlike  cloud.  The  cumulus  is  the  lumpy,  piled-up 
form  of  cloud.  The  stratus  is  the  level  sheet  or  layer.  The  nimbus  is 
the  cloud  from  which  precipitation  is  falling.  After  Howard's  time 
many  other  systems  of  classification  and  nomenclature  were  proposed, 
some  being  'mere  modifications  or  extensions  of  the  Howard  system. 
By  1891  nearly  twenty  systems  had  been  proposed  and  had  been  used 
more  or  less,  so  that  there  was  great  uncertainty,  inexactness,  and  in- 
definiteness  in  the  use  of  names. 


THE  MOISTURE  IN  THE  ATMOSPHERE  219 

216.  The  international  system.  —  In  1891,  Hildebrandsson  of  Upsala 
and  Abereromby,  an  English  meteorologist,  presented  a  report  on  cloud 
classification  to  the  International  Meteorological  Conference 

...  .  .  The  adop- 

wmch   was   then   in  session   at   Munich.     This   report   was  tion  of  the 

adopted,  and  the  system  thus  inaugurated  has  since  found  international 
almost  universal  acceptance.  In  this  system  thirteen  names 
are  used  to  designate  the  different  kinds  of  clouds.  Howard's  four  type 
forms  were  retained,  four  names  for  intermediate  varieties  were  formed 
by  combining  the  names  for  the  type  forms,  two  names  were  The  thirteen 
formed  by  prefixing  alto,  meaning  high,  and  three  by  prefixing  cloud  names. 
fracto,  meaning  broken  or  windblown,  to  the  names  for  type  forms,  thus 
making  in  all  thirteen  names.  The  following  is  an  arrangement  of  the 
four  type  forms  in  order  of  height  above  the  earth's  surface : 

Cirrus 

Cumulus  Stratus l 

Nimbus  Stratus 

The  thirteen  cloud  names  are : 

Type  forms  :  cirrus,  stratus,  cumulus,  nimbus 

Intermediate  varieties :     cirro-stratus,    cirro-cumulus,    strato-cumu- 

lus,  cumulo-nimbus 

Alto  forms  :  alto-stratus,  alto-cumulus 

Fracto  forms  :  fracto-stratus,  fracto-cumulus,  fracto-nimbus 

The  underlying  principle  of  the  classification  is  evident  from  the 
arrangement  as  regards  height.  Other  possible  names  would  have  been 
alto-cirrus,  alto-nimbus,  fracto-cirrus,  cumulo-cirrus,  strato- 
cirrus,  cumulo-stratus,  strato-nimbus,  nimbo-stratus,  cirro- 
nimbu,,  nimbo-cirrus,  nimbo-cumulus.  But  these  are  all  cipie  of 
impossible  combinations,  redundant,  or  unnecessary.  For  Ration!81 
example,  alto-cirrus  is  unnecessary  because  cirrus  is  always 
high ;  fracto-cirrus  is  unnecessary  because  cirrus  is  always  a  detached 
broken  cloud.  Strato-nimbus  is  unnecessary  because  a  nimbus  cloud  is 
in  the  form  of  a  layer.  Strato-cirrus  is  considered  the  same  as  cirro- 
stratus,  cumulo-stratus  the  same  as  strato-cumulus,  etc.  Cirro-nimbus 
is  impossible  because  the  two  kinds  of  clouds  never  occur  at  the  same 
altitude,  and  alto-nimbus  is  impossible  because  a  nimbus  cloud  is  always 
low. 

1  Stratus  is  here  considered  simply  as  a  cloud  in  a  horizontal  sheet  or  layer.  When 
not  near  the  earth's  surface  it  is  always  spoken  of  as  alto-stratus. 


220  METEOROLOGY 

These  cloud  names  may  also  be  grouped  as  follows : 

Cirrus  type  :         cirrus,  cirro-stratus,  cirro-cumulus 
methods  of  Stratus  type  :       cirro-stratus,  alto-stratus,  stratus,  fracto- 

grouping  stratus,  strato-cumulus 

doud^orms.  Cumulus  type :     cirro-cumulus,    alto-cumulus,    cumulus, 

fracto-cumulus,          strato-cumulus, 
cumulo-nimbus. 

Nimbus  type :      cumulo-nimbus,  \l   nimbus,     fracto-nim- 
-  bus 

The  International  Committee  grouped  them  as  follows : 

Upper  clouds  :  cirrus,  cirro-stratus 

Intermediate  clouds  :  cirro-cumulus,  alto-stratus,  alto-cumulus 

Lower  clouds  :  strato-cumulus,  nimbus,  fracto-nimbus 

Clouds  formed  by  di- 
urnal ascending 

currents  :-  cumulus,  fracto-cumulus,  cumulo-nimbus 

High  fogs  :  stratus,  fracto-stratus 

In  order  to  give  more  exact  names  to  the  cloud  forms  the  international 
system  is  sometimes  subdivided.  This  is  usually  done  by  adding 
Subdivision  Latin  adjectives  or  nouns  to  the  thirteen  names  of  the  inter- 
of  the  sys-  national  system.  An  exact  description  of  the  variety  to 
which  the  name  is  to  apply  should  then  be  given  or  a  refer- 
ence given  to  some  place  in  the  literature  on  the  subject  where  such  a 
description  can  be  found. 

217.    The  thirteen  cloud  forms. — 

Cirrus  (abbreviation  Ci.) 

Description:  The  name  Cirrus  is  applied  to  detached  clouds  gener- 
ally of  white  color  made  up  of  slender,  delicate,  irregularly  curling  fibers 
or  wisps  of  cloud.  This  kind  of  cloud  is  variously  described 
as  made  up  of  wavy  sprays  of  cloud,  irregularly  branching 
structures,  bundles  of  drawn-out  filaments,  long  curving  threads  of 
clouds,  or  fluffy  heads  with  long  streamers.  It  sometimes  takes  the 
form  of  belts  which  cross  the  sky  and  converge  in  perspective  towards 
one  or  two  points  of  the  horizon.  It  also  takes  the  form  of  feathers  or 
ribbons  or  delicate  fibrous  bands  and  is  sometimes  striated  or  rippled. 
It  is  often  called  popularly  "  cats'  whiskers  "  or  "  mares'  tails."  In  its 
less  typical  form  it  may  be  composed  of  gauzy,  clotted  masses,  or  almost 


CIRRUS 
CIRRO-CUMULUS 


CIRRUS 
CIRRO-STRATUS 


CUMULUS 
CUMULO-NIMBUS 


F  RA  CTO-CUMU  LUS 
STRATO-CUMULUS 


CUMULUS 
CUMULUS . 


THE  MOISTURE   IN  THE  ATMOSPHERE  221 

with  absent  of  structure.     It  very  seldom  causes  a  halo.     Tho  direc- 
tion of  motion  is  usually  from  the  west,  and  no  shadows  are  cause  i  by  it. 

Transition  forms:    Thickening:  Ci.  Cu.  or  Ci.  St. 

Thinning:   Disappears. 

Height:  Mean  28,000  ft.,  max.  46,000  ft.,  min.  9000  ft. 
Methods  of  formation:  (4),    (5);    perhaps    (6),    (3),    (8);    possibly 
(7),  (9).1 

Stratus  (abbreviation  St.) 

Description :    Stratus  was  originally  defined  as  lifted  fog  in  a  hori- 
zontal stratum.     It  is  now  applied  to  any  low-lying  horizontal  cloud 
sheet   of   approximately   uniform   thickness.     It   is   a   flat, 
structureless  cloud,  usually  of  wide  extent  and  causes  what 
is  called  "  gray  weather."     It  is  sometimes  thin  in  places  so  that  the 
under  surface  appears  as  parallel  lines  or  rolls  of  clouds  all  around  the 
horizon.     In  breaking  up  it  sometimes  appears  in  lenticular  patches. 

Transition  forms :  Thickening  :  Fr.  St.  or  Nb. 
Thinning  :  St.  Cu.  or  A.  St. 

Height:  Mean  2100  ft.,  max.  6000  ft.,  min.  400  ft. 
Methods  of  formation:   (1),  (3);  perhaps  (7). 

Cumulus  (abbreviation  Cu.) 

Description:  Cumulus  clouds  are  thick,  rounded  lumps,  whose  sum- 
mits- are  domes  or  turrets  with  protuberances  and  whose  bases  are  flat. 
When  viewed  opposite  the  sun,  they  are  white  with  dark 
centers.     When  viewed  near  the  sun,  they  are  dark  with 
brilliant  dazzling  white  edges.     They  cast  dense  shadows,  appear  in 
greatest  abundance  during  the  warm  part  of  the  day,  and    look  like 
exploded   cotton  bales.     They  often  appear  to  be  arranged  in  rows 
parallel  to  the  horizon,  in  which  case  the  flatness  of  the  base  is  particu- 
larly noticeable. 

Transition  forms :  Thickening  :  St.  Cu.  or  Cu.  Nb. 

Thinning :  A.  Cu. 

Height:  Top  :          Mean  6000  ft.,  max.  15,000  ft.,  min.  2400  ft. 
Bottom  :   Mean  4400  ft.,  max.  12,000  ft.,  min.  1600 

Methods  of  formation:   (2),  (3), 

1  These  refer  to  the  nine  methods  of  cloud  formation  to  be  discussed  later 
231-239). 


222  METEOROLOGY 


Nimbus  (abbreviation  Nb.) 

Description :  The  Nimbus  is  the  rain  cloud  and  is  a  dense,  dark  sheet 
^          of  formless  cloud  from  which  precipitation  is  falling.     It  is 
widely  extended,  and  when  breaks  occur  an  upper  cloud  area 
is  usually  seen. 

Transition  forms :  Thickening  :  Heavier  nimbus 

Thinning :  St.,  Fr.  St.,  or  St.  Cu. 
Height:  Mean  3600  ft.,  max.  18,000  ft.,  min.  200  ft. 
Methods  of  formation:  (1),  (2),  (3). 


Cirro-stratus  (abbreviation  Ci.  St.) 

Description:  Cirro-stratus  is  a  thin,  whitish  sheet  or  veil  of  cloud 
which  gives  the  whole  sky  a  milky  appearance.  It  is  widely  extended, 
Cirro-  always  distinctly  fibrous  and  streaky  in  appearance,  and 

stratus.  looks  like  a  tangled  web  of  short,  curling  fibers  matted  to- 
gether. It  often  produces  halos  around  the  sun  and  moon,  is  grayish 
or  white  in  color,  and  may  be  flocculent,  granular,  or  banded  at  times'. 

Transition  forms :  Thickening  :  A.  St. 

Thinning :  Ci. 

Height :  Mean  22,000  ft.,  max.  42,000  ft.,  min.  7000  ft. 
Methods  of  formation:  (1),  (3),  (8);  perhaps  (6),  (7),  (9). 


Cirro-cumulus  (abbreviation  Ci.  Cu.) 

Description :  Cirro-cumulus  consists  of  small  white  balls  or  flakes  of 
semi-transparent  clouds  without  shadows  or  with  very  faint  ones. 
Cirro-  These  are  arranged  in  groups  and  often  in  rows.  These  have 

been  likened  to  a  thin  sheet  broken  in  many  places  and  curled 
at  the  edges,  to  a  flock  of  sheep  lying  down,  to  the  foam  in  the  eddying 
wake  of  a  steamer,  or  to  floating  ice  cakes  on  the  surface  of  a  river.  The 
popular  name  is  "  mackerel  sky." 

Transition  forms :  Thickening  :  A.  Cu. 

Thinning  :  Disappears  or  Ci. 

Height:  Mean  20,000  ft.,  max.  35,000  ft.,  min.  7000  ft. 
Methods  of  formation:   (2),  (3),  (4) ;  possibly  (8),  (9). 


THE  MOISTURE   IN  THE   ATMOSPHERE  223 

Alto-stratus  (abbreviation  A.  St.) 

Description:    Alto-stratus  is  a  thick  veil    of   gray  or  bluish  color, 
brighter  near  the  sun,  without  fibrous  structure.      The  sun  and  moon 
are  sometimes  faintly  visible  through  it,  and  coronas  but  not 
halos  are  produced  by  it.     When  breaking  up  it  sometimes 
appears  in  lenticular  patches.       It  appears  most  often  in  the  early 
morning  and  in  winter. 

Transition  forms :     Thickening  :  St. 

Thinning  :  Disappears  or  Ci. 

Height:  Mean  15,000  ft.,  max.  36,000  ft.,  min.  3800  ft. 
Methods  of  formation:   (1),  (3),  (4),  (6);  possibly  (9). 

Alto-cumulus  (abbreviation  A.  Cu.) 

Description:  Alto-cumulus  is   the  name  given   to  white   or  grayish 
balls  of  dense  fleecy  cloud  with  shaded  portions  which  are  detached,  but 
frequently  lie   close   together.     The   cloud   balls   are   often  Alto- 
grouped  in  flocks  or  arranged  in  rows,  sometimes  in  two  cumulus- 
directions,  and  the  balls  in  the  center  of  a  group  are  generally  the  largest. 
Alto-cumulus  is  more  flattened  and  disklike  than  the  typical  cumulus. 

Transition  forms :  Thickening  :  St.  Cu. 

Thinning :  Ci.  Cu. 

Height:  Mean  12,000  ft.,  max.  27,000  ft.,  min.  2700  ft. 
Methods  of  formation:   (2),  (3),  (5). 

Fracto-stratus  (abbreviation  Fr.  St.) 

Description:   When   a  widely  extended    cloud    sheet  which   would 
ordinarily  be  called  stratus  is  torn  by  the  wind  or  mountain  Fracto- 
summits  into  irregular  fragments,  it  is  called  fracto-stratus.  stratus- 

(The  remaining  facts  are  the  same  as  for  stratus.) 

Fracto-cumulus  (abbreviation  Fr.  Cu.) 

Description  :  Fracto-cumulus  is  the  name  given  to  a  cumulus  cloud 
which  has  been  torn  by  high  winds  so  that  its  margins  are  Fracto- 
raggoi  and  irregular.     It  is  a  flat,  tattered,  broken  cumulus.  cumulus- 
Sometimes  the  cloud  appears  to  have  been  rolled  instead  of  torn. 

(The  remaining  facts  are  the  same  as  for  cumulus.) 


224  METEOROLOGY 

Strato-cumulus  (abbreviation  St.  Cu.) 

Description  :  Strato-cumulus  consists  of  large  balls  or  rolls  of  dark 
cloud,  flat  at  the  base,  often  covering  the  whole  sky  and  leaving  only  a 
strato-  little  blue  sky  here  and  there  in  the  breaks.  It  is  some- 

cumulus,        times  defined  as  stratus  thickened  here  and  there  into  cumulus 
or  cumulus  joined  together  with  a  common,  flat  base  to  make  a  layer. 

Transition  forms  :  Thickening:  Fr.  S.,  N.,  or  Fr.  N. 

Thinning:  Fr.  Cu.,  or  Cu. 

Height :  Mean  7000  ft.,  max.  14,000  ft.,  min.  1000  ft, 
Methods  of  formation  :  (2),  (3)  ;  possibly  (8). 

Cumulo-nimbus  (abbreviation  Cu.  Nb.) 

Description  :  Cumulo-nimbus  is  the  thundercloud.  It  consists  of 
heavy  masses  of  cloud  rising  like  mountains,  towers,  or  anvils.  Below  is 
Cumuio-  the  dark,  formless  nimbus,  or  fracto-nimbus,  cloud  from 
nimbus.  which  rain  is  falling.  At  the  top  a  veil  or  cap  of  fibrous 
texture  called  false  cirrus  is  often  seen.  It  is  a  detached  cloud,  but 
usually  covers  large  areas. 

Transition  forms  :  Thickening :  Large  Cu.  Nb. 

Thinning:  Cu. 
Height :  Top  :  Mean  28,000  ft.,  max.  42,000  ft.,  min.  16,000  ft. 

Bottom :  Mean  4400  ft.,  max.  8000  ft.,  min.  600  ft. 
Methods  of  formation  :  (2),  (3). 

Fracto-nimbus  (abbreviation  Fr.  Nb.) 

Description  :  When  nimbus  is  torn  into  small  patches,  or  if  low, 
Fracto-  ragged,  detached  fragments  of  cloud  (scud)  move  rapidly 
nimbus.  below  the  main  mass,  it  is  called  fracto-nimbus. 

(The  remaining  facts  are  the  same  as  for  nimbus.) 

218.  The  sequence  of  cloud  forms.  —  Many  observations  have  been 
made  to  determine  the  probability  that  fair  weather  or  precipitation  will 
The  usual  follow  the  appearance  of  a  certain  cloud*  form.  With  the 
sequence  of  exception  of  nimbus,  however,  which  by  definition  is  a  cloud 
ms*  from  which  precipitation  is  falling,  the  preponderance  of 
probability  is  so  small  that  it  is  of  no  value  in  forecasting. 

There  is,  however,  a  fairly  regular  sequence  of  cloud  forms  in  pass- 
ing from  a  clear  sky  to  stormy  weather.  First  comes  the  cirrus ;  this 


THE  MOISTURE  IN  THE  ATMOSPHERE  225 

thickens  to  cirro-stratus  or  cirro-cumulus  and  then  becomes  alto-stratus 
or  alto-cumulus ;  next  the  nimbus  appears  which  often  becomes  fracto- 
nimbus.  In  clearing  off,  the  sequence  is  generally  this :  the  nimbus 
usually  breaks  up  into  fracto-nimbus,  disclosing  often  an  upper  cloud 
area  of  cirrus  or  cirro-cumulus.  The  upper  cloud  layer  gradually  dis- 
appears and  the  lower  fracto-nimbus  becomes  strato-cumulus,  fracto- 
cumulus,  and  finally  cumulus. 

THE  OBSERVATIONS  OF  CLOUDS  AND  CLOUDINESS  AND  THE 
RESULTS  OF  OBSERVATION 

219.    Height  of  clouds.  —  Simultaneous  observations  of  the  altitude 
(angular  distance  above  the  horizon)  and  the  azimuth  (direction)  of  a 
cloud  obtained  from  two  stations  a  mile  or  two  apart  and 
connected  by  telephone,  permit  the  determination  of  the  methods  of 
height  of  the  cloud  above  the  earth's  surface  and  its  distance  determining 
from  the  two  stations.     It  is  necessary  that  the  two  stations  ^clouds* 
be  connected  by  a  telephone  in  order  to  agree  upon  the 
portion  of  the  cloud  which  is  to  be  observed  at  a  given  moment.     Cloud 
heights  may  also  be  determined  photographically  by  taking  simultaneous 
photographs  from  two  stations,  as  before.    Cloud  heights  may  also  be 
determined  by  a  single  observer  if  he  is  provided  with  an  accurate  map 
of  the  surrounding  country.     The  necessary  observations  in  this  case 
are  the  altitude  and  azimuth  of  the  cloud,  the  location  of  its  shadow  on 
the  surrounding  country,  and  the  date  and  time  of  observation.     The 
student  of  trigonometry  will  readily  see  how  these  observations  can  be 
utilized  in  computing  the  height  and  distance  of  the  cloud.     These  three 
do  not  exhaust  the  possible  methods  of  determining  a  cloud  height,  for 
nearly  a  dozen  have  been  proposed. 

Long  series  of  observations  of  cloud  heights  have  been  made  at  the 
Blue  Hill  observatory,  near  Boston,  at  Upsala  in  Sweden,  and  at  Berlin. 
The  results  of  these  measurements  show  that  the  height  of 
clouds  varies  all  the  way  from  zerot  which  is  actual  contact  many  Obser- 
with  the  earth's  surface,  to  about  ten  miles  in  the  case  of  the  vations  have 
lofty  cirrus  clouds.     The  greatest,  least,  and  mean  height  and  the 
of  the  various  kinds  of  clouds  have  already  been  given  in  results. 

section  217.  Clouds  are 

It  has  also  been  found  from  these  observations  that  the  higher  in 
average  height  of  clouds  in  summer  is  larger  than  that  in  ^nhT 

winter.  winter. 


226 


METEOROLOGY 


by  estima- 
tion. 


A  nepho- 
scope  and 
its  use. 


220.  Direction  and  velocity  of  motion.  —  The  direction  and  velocity 
of  motion  of  a  cloud  is  usually  determined  by  estimation.  A  cloud 
Obtained  *s  sP°ken  of,  for  example,  as  moving  rapidly  or  slowly  from 
ordinarily  the  northwest.  If  the  cloud  is  near  the  horizon,  however, 
reliable  estimations  cannot  be  made. 

Exact  determinations  of  the  direction  of  motion  and  the 
velocity  of  motion  of  a  cloud  may  be  made  at  the  same  time  that  its 
height  is  determined,  if  the  same  cloud  is  observed  twice  at  an 
interval  of  twenty  or  thirty  minutes. 

Ordinarily,  an  instrument  which  is  called  a  nephoscope,1  and  is  pic- 
tured in  Fig.  101,  is  used  for  determining  the  direction  and  velocity 
of  motion  of  clouds.  This  consists  essentially  of  a  horizon- 
tal mirror  of  black  glass,  provided  with  a  divided  circle.  A 
small  compass  box  is  usually  attached  to  the  mirror  so  that 
the  zero  of  the  scale  may  be  set  at  either  magnetic  or  true  north.  This 

is  provided  with  an  eyepiece  held 
in  a  curved  framework  so  that  it 
can  be  moved  up  and  down  and 
at  the  same  time  remain  at  an 
unchanging  distance  from  the 
center  of  the  mirror.  The  mirror 
is  usually  provided  with  a  series 
of  concentric  circles  at  equal  dis- 
tances from  each  other  and  with 
a  scale  etched  across  its  face. 
When  a  cloud  is  to  be  observed, 

the  mirror  is  turned  and  the  eyepiece  placed  at  such  a  height  that  the 
image  of  the  cloud  is  observed  to  follow  the  divided  scale  across  the 
mirror.  The  direction  of  the  cloud  motion  can  then  be  determined  by 
reading  the  horizontal  divided  scale,  and  if  its  height  is  estimated,  its 
velocity  of  motion  may  be  determined  by  observing  the  distance  on 
the  mirror  passed  over  in  a  given  interval  of  time. 

The  results  of  such  observations  show  that  the  clouds  move  much 
more  rapidly  than  the  surface  winds.  When  surface  winds 
are  moving  with  a  velocity  of  50  miles  an  hour,  it  is  not  at 
all  unusual  for  the  cloud  velocity  to  be  150  or  even  200  mites  per  hour. 
Definition  of  22i.  Cloudiness.  —  The  term  cloudiness  refers  to  the 
cloudiness,  amount  of  the  sky  covered  by  clouds,  and  has  nothing  to 
do  with  the  kind  of  clouds.  It  is  usually  determined  by  a  naked-eye 

=  cloud;  ffKoirtw  =  I  look  at. 


FIG.  101.  —  A  Nephoscope. 


Results. 


THE  MOISTURE   IN  THE  ATMOSPHERE 


227 


estimation,  and  for  this  estimation  the  sky  is  divided  into  two  portions 
by  a  small  circle  at  an  altitude  of  45°.    The  amount  of  the  upper  clouds 
and  of  the  lower  clouds  is  estimated  separately  and  the  HOW  esti- 
average    determined.      It    is    customary    to    estimate    the  mated- 
amount  in  fifths,  since  the  sum  of  these  estimations  will  in  this  case 
give  at  once  the  cloudiness  in  tenths  for  the  whole  sky. 

There  are  five  adjectives  used  to  express  the  degree  of  cloudiness. 
Cloudless  is  used  when  no  clouds  at  all  are  visible.     The  sky  is  said  to 
be  clear  if  it  is  from  0  to  TBU  covered  with  cloud.     The  term 
partly  cloudy  is  used  to  designate  an  amount  of  cloudiness  a(jjectives 
between  T%  and  T7^  inclusive.     The  word  cloudy  designates  used  to 
a  sky  from  ^  to  all  covered  with  cloud.     If  the  sky  is  com-  cioudTness 
pletely  cloud-covered,  it  is  usually  spoken  of  as  overcast. 
Very  often  but  three  of  these  terms  are  used  in  connection  with  cloudi- 
ness :  clear,  partly  cloudy,  and  cloudy.    In  this  case,  cloudless  is  included 
in  clear,  and  overcast  in  cloudy.     The  term  fair  is  also  sometimes  used. 
It    is   synonymous   with 
partly  cloudy  and  allows 
T^-O   of  an  inch   of  pre- 
cipitation, but  no  more. 

222.  Sunshine  rec- 
ords. —  There  are  vari- 
ous instru-  „ 

The  three 
ments    for  kinds  of 

determining  sunshine 
°   recorders, 
the  amount  of 

sunshine,  which  is  the 
converse  of  cloudiness. 
The  three  in  most  general 
use  are  called  the  burnt 
paper,  the  photographic, 
and  the  electrical  con- 
tact sunshine  recorder. 

The  burnt  paper  sun- 
shine recorder,  often 

called  the  Campbell-Stokes  sunshine  recorder,  is   illustrated   in  Fig. 
102,  and  consists  of  a  sphere  of  glass  which  focuses   the  The  burnt 
rays  of   the  sun  on  a  piece  of   paper    held   in  a  curved  ghTne81 
framework  at  the  back.     This  faces  south  and  is  exposed  recorder, 
so  that  the  sun  may  shine  upon  it  from  sunrise  until  sunset.     When- 


FIG.   102.  —  The  Burnt  Paper  Sunshine  Recorder. 


228 


METEOROLOGY 


ever    the    sun    is    shining,   the    glass    bulb  focuses   its   rays   on   the 
paper  and  chars  a  track.     If  the  sun  goes  under  a  cloud,  it  is  evident 

on    the    chart  by  the  absence 
of     charring.       The     strip     of 
paper    is    renewed    every    day, 
way 


FIG. 


103.  —  The  Photographic  Sunshine 
Recorder. 


and    in    this 
rate     record    of 
of  sunshine  is 
kept. 

The  photo- 
graphic sun- 
shine recorder, 
often  called 
the  Jordan 
recorder,  is 
illustrated  in 
Fig.  103.  It  - 
consists  of  a 

light-tight  cylindrical  box  (sometimes  two)  contain- 
ing a  piece  of  sensitized  photographic  paper.    The 
.  sun's  rays  are  allowed  to  fall  upon  this 

The  photo-  f 

graphic          paper  through  a  very  small  opening,  and 

as  the  sun  passes  from  rising  to  setting, 
a  track  is  left  on  this  photographic  paper. 
Any  interruption  in  the  sunshine  makes  itself  mani- 
fest as  an  interruption  in  this  track.  By  changing 
the  position  of  the  small  opening,  records  for  a  whole 
month  may  be  kept  on  a  single  piece  of  paper. 

223.  The  sunshine  recorder  used  by  the  U.  S. 
Weather  Bureau  is  the  electrical  contact  recorder. 
It  is  pictured  in  Fig.  104,  and  consists 
essentially  of  a  black  bulb  thermom- 
eter surrounded  by  a  glass  jacket  from 
which  the  air  has  been  extracted.  Two 
wires  have  been  sealed  in  the  stem  of 
the  thermometer.  When  the  sun  shines,  the  black 
bulb  quickly  absorbs  its  radiant  energy,  the  mer- 
cury rises  in  the  stem,  contact  is  made,  and  in 
the  office  below  the  pen  makes  a  step  every  minute 
in  the  line  it  is  tracing  on  a  revolving  drum.  If 


an     accu- 
the    duration 


sunshine 
recorder. 


The 

Weather 

Bureau 

sunshine 

recorder. 


FIG.  104.  — The  U.  S. 
Weather  Bureau  Sun- 
shine Recorder. 


THE  MOISTURE   IN  THE   ATMOSPHERE  229 

the  sun  ceases  to  shine,  the  black  bulb  thermometer  radiates  its  heat, 
the  mercury  drops,  contact  is  broken,  and  the  pen  makes  a  straight 
line  upon  the  revolving  drum.  In  this  way  a  continuous  record  of 
sunshine  is  kept. 

The  amount  of  sunshine  may  be  expressed  in  two  ways :  either  as  the 
absolute  duration  in  hours  and  decimals  of  an  hour,  or  as  nowtne 
a  percentage.     The  percentage  is  the  ratio  of  the  actual  amount  is 
duration  of  the  sunshine  to  the  total  possible. 

224.  Observations   of   clouds,   cloudiness,   and   sunshine.  —  At  the 
regular  stations  of  the  U.  S.  Weather  Bureau,  the  kind  of  cloud  and  the 
cloudiness  are  observed  at  eight  in  the  morning  and  eight  in  Theobserva- 
the  evening.     A  continuous  record  of  the  sunshine  is  also  kept  tions  made- 
by  means  of  an  electrical  contact  sunshine  recorder.     The  black  bulb 
thermometer,  with  its  vacuum  jacket,  is  exposed  on  the  roof  of  the  build- 
ing.    The  record  is  made  on  a  revolving  drum  in  the  office,  on  which  is 
recorded  wind  direction,  wind  velocity,  and  rainfall,  as  well  as  sunshine. 
At  the  cooperative  stations,  the  average  cloudiness  of  the  day  as  a  whole 
is  the  only  thing  which  is  recorded. 

225.  Normal  values,   data,  and  charts.  —  From  these  observations 
of  cloudiness  and  sunshine,  the  following  average  and  normal  values 
may  be  computed :    the  normal   cloudiness  at  8  A.M.  and  Tfae 

8  P.M.  for  the  various  months  and  for  the  year,  the  normal  values 

hourly  sunshine,   the  normal   monthly  sunshine,    and  the  whichafe 
*  computed, 

normal   yearly   sunshine.     For   these   normals   the    annual 

variation  in  cloudiness  and  the  diurnal  and  annual  variation  in  sunshine 
may  be  determined  in  the  regular  way.  In  the  case  of  the  regular  sta- 
tions of  the  U.  S.  Weather  Bureau,  the  records  which  are  kept  constantly 
computed  to  date  are :  hours  of  sunshine  and  percentage  of  possible  ; 
average  cloudiness  at  8  A.M.  and  8  P.M.  ;  number  of  days  clear,  partly 
cloudy,  and  cloudy;  hourly  sunshine  in  percentages. 

226.  For  the  northeastern  part  of  the  United  States  and  for  the 
larger  part  of  the  earth's  surface,  the  amount  of  cloudiness  is  a  maximum 
during  the  day  and  a  minimum  during  the  night.     The  The  charac_ 
reason  for  this  is  convection,  which  causes  an  increase  in  teristks  of 
the  amount  of  cumulus  clouds  by  day.     The  annual  varia-  2^  Annual 
tion   in   cloudiness   depends   more   upon   the   locality.     In  variation  in 
general,  the  summer  time  is  freer  from  clouds  than  the 

winter  time.-  The  following  table,  which  gives  the  normal  values  of 
cloudiness  for  the  various  months  and  for  the  year  for  various  stations 
in  the  United  States,  will  make  clear  the  characteristics  of  the  variation. 


230 


METEOROLOGY 


NORMAL  VALUES  OF  SUNSHINE  (PERCENTAGE  OP  POSSIBLE)  FOR  THE 
VARIOUS  MONTHS  AND  FOR  THE  YEAR 


h 

0  Q 

j 

1 

LENGTH 
RECOR 

2 

•< 

>-9 

1 

a 
•< 

% 

K 
E 
3 

>H 
•< 

s 

1 
P 

1-3 

| 

| 

i 

! 

1 

0 

s 

ANNUA 

Albany,  N.Y.  .  .  . 

7 

43 

59 

55 

60 

60 

65 

61 

68 

64 

55 

39 

34 

54 

Bismarck,  N.  Dak.  . 

10 

51 

56 

53 

56 

59 

60 

69 

66 

63 

60 

49 

46 

57 

Boise,  Idaho  . 

5 

33 

45 

53 

55 

64 

76 

87 

82 

79 

63 

45 

45 

61 

Boston,  Mass. 

10 

51 

57 

53 

53 

57 

60 

60 

60 

62 

54 

45 

52 

55 

Buffalo,  N.Y. 

14 

31 

42 

49 

55 

55 

66 

65 

66 

60 

48 

27 

24 

49 

Charleston,  S.C. 

51 

48 

53 

63 

66 

61 

54 

51 

57 

55 

58 

50 

56 

Chicago,  111. 

10 

46 

53 

51 

62 

63 

68 

71 

68 

63 

61 

42 

38 

57 

Columbus,  Ohio 

10 

35 

44 

44 

59 

61 

65 

71 

70 

65 

60 

39 

32 

54 

Denver,  Col.  . 

14 

73 

67 

67 

68 

61 

69 

67 

66 

75 

76 

71 

68 

69 

Galveston,  Tex. 

14 

47 

44 

47 

58 

66 

72 

70 

67 

68 

73 

59 

51 

60 

Helena,  Mont. 

10 

43 

48 

53 

57 

54 

60 

73 

74 

61 

62 

42 

41 

56 

Indianapolis,  Ind. 

7 

43 

47 

40 

52 

57 

61 

72 

64 

67 

63 

46 

39 

54 

Los  Angeles,  Cal. 

7 

68 

73 

69 

70 

60 

67 

78 

76 

76 

75 

79 

80 

73 

New  Orleans,  La. 

14 

46 

44 

50 

54 

65 

55 

50 

51 

64 

64 

51 

47 

53 

New  York,  N.Y.   . 

10 

53 

58 

54 

57 

56 

60 

57 

60 

60 

52 

51 

52- 

56 

Omaha,  Neb.  .  .  . 

7 

60 

56 

54 

60 

60 

65 

74 

68 

67 

59 

54 

49 

60 

Philadelphia,  Pa. 

50 

53 

51 

55 

58 

61 

63 

63 

66 

60 

53 

55 

57 

Phoenix,  Ariz.   .  . 

8 

72 

79 

80 

85 

89 

94 

83 

82 

87 

87 

84 

85 

84 

Portland,  Me. 

57 

60 

57 

60 

68 

61 

65 

66 

64 

54 

46 

53 

58 

Portland,  Ore.   .  . 

14 

31 

36 

41 

43 

49 

69 

61 

49 

44 

23 

20 

22 

41 

St.  Louis,  Mo.   .  . 

13 

53 

51 

52 

58 

63 

67 

70 

71 

71 

70 

52 

49 

60 

St.  Paul,  Minn.  .  . 

8 

49 

55 

49 

58 

55 

59 

66 

59 

61 

52 

44 

44 

54 

Salt  Lake  City,  Utah 

14 

43 

44 

52 

59 

64 

79 

81 

77 

79 

69 

55 

44 

62 

San  Francisco,  Cal. 

9 

53 

56 

60 

70 

67 

76 

71 

60 

67 

67 

57 

56 

63 

Seattle,  Wash.   .  . 

9 

23 

39 

46 

50 

49 

52 

62 

57 

48 

35 

16 

18 

41 

Topeka,  Kan.  .  .  . 

60 

56 

57 

59 

61 

63 

72 

73 

64 

66 

51 

54 

61 

Washington,  D.C.  . 

13 

48 

50 

48 

53 

55 

63 

64 

55 

68 

60 

51 

54 

57 

The  last  year  included  in  these  normals  is  1903. 

If  these  normal  values  of  cloudiness  and  of  sunshine  have  been  deter- 
mined for  many  stations,  over  the  earth's  surface,  charts  may  be  pre- 
pared which  will  show  these  values.  Charts  XIX  and  XX 
show  the  normal  sunshine  in  per  cent  for  January  and  for 
July  for  the  United  States.  Chart  XXI  gives  the  normal  an- 
nual cloudiness  for  the  whole  earth.  Lines  connecting  those 
places  which  have  the  same  amount  of  cloudiness  are  called 
isonephs,  and  these  were  first  drawn  for  the  world  by  Teisserenc  de  Bort 
in  1884.  It  will  be  seen  that  the  geographical  variation  of  cloudiness  is 
somewhat  correlated  with  the  general  wind  system.  The  average  cloudi- 
ness at  the  equator  is  between"  50  per  cent  and  60  per  cent.  This  drops 
down  to  a  minimum  of  about  40  per  cent  at  latitude  25°  or  30°  N. 
It  then  increases  with  increasing  latitude  to  a  maximum  at  about  65°  N. 


The  cloudi- 
ness of  the 
United 
States  and 
the  world. 


THE  MOISTURE  IN  THE  ATMOSPHERE  231 

with  a  value  of  between  60  per  cent  and  70  per  cent,  and  then  drops 
somewhat  as  the  pole  is  approached.  Cloudiness  varies  from  place  to 
place  because  of  altitude,  temperature,  inclosure  by  mountains,  near- 
ness to  bodies  of  water,  and  the  character  of  the  storms. 

THE  NATURE  OF  CLOUDS  » 

227.    Nuclei  of  condensation.  —  There  are  several  laboratory  experi- 
ments which  would  seem  to  show  that  the  condensation  of  water  vapor 
always  takes  place  on  some  material  object  or  on  the  dust  iniabora. 
or  water  particles  which   are  present  in  the  atmosphere,  tory  experi- 
If  a  vessel  contains  a  quantity  of  air  from  which  all  dust 
particles  have  been  removed,  the  temperature  may  be  lowered  tion  are 
many  degrees  below  the  dew  point  without  causing  any  r 
condensation  in  the  form  of  fog  or  cloud.     The  condensation  takes 
place  slowly  and  entirely  on  the  sides  and  bottom  of  the  containing 
vessel.     The  reduction  in  temperature  is  ordinarily  brought  about  by 
quickly  rarefying  the  air  and  thus  cooling  it  by  expansion.     If,  however, 
ordinary  air  which  contains  a  large  amount  of  dust  is  used  instead  of  the 
dust-free  air,  as  soon  as  the  temperature  is  lowered  below  the  dew  point, 
it  immediately  becomes  filled  with  a  foglike  condensation.     It  is  accord- 
ingly supposed  that  a  dust  or  moisture  particle  serves  as  the  nucleus 
or  center  of  condensation  of  the  water  vapor  which  forms  clouds  or  fog. 

There  are  certain  facts,  however,  which  are  not  in  accord  with  this 
supposition.     The  amount  of  sediment  in  rain  water  is  extremely  slight, 
yet  each  raindrop  is  composed  of  myriads  of  cloud  particles,  There  . 
each  one  of  which  is  supposed  to  have  a  dust  particle  for  its  but  little 
nucleus.     This  seeming  contradiction  is  explained  by  two  sediment  in 

rain  water. 

considerations.     In  the  first  place,  the  dust  particles  in  the 
atmosphere  are  probably  extremely  minute.     It  has  also  been  found 
that  ionized  air  permits  condensation  as  well  as  dusty  air.     It  has  been 
furthermore  found  that  the  air  is  always  in  a  more  or  less  _ 

Ionized  air 

ionized  condition.  This  condition  can  be  brought  about  acts  the 
by  electrical  discharges,  by  ultra-violet  light,  by  cathode 
rays,  or  even  by  the  impact  of  the  air  against  obstacles. 
Condensation,  however,  does  not  readily  take  place  on  ions.  It  has 
been  found  that  the  air  must  contain  several  times  as  much  moisture  as 
will  saturate  it,  before  condensation  on  the  ions  can  be  forced.  It 
would  thus  seem  that  dust  particles  must  play  by  far  the  larger  part  in 
serving  as  nuclei  of  condensation  in  the  atmosphere. 


232  METEOROLOGY 

228.  Size  and  constitution  of  cloud  particles.  —  Cloud  particles  vary 
in  size  from  ^TF  to  T1IVo  °f  an  inch  m  diameter.     These  results  have 
Size  of  keen  obtained  by  direct  measurement  and  also  by  deduc- 
doud            tions  from  certain  optical  phenomena  in  which  the  cloud 

particles  play  a  part.  It  was  formerly  supposed  that  the 
cloud  particles  were  hollow  spheres.  The  reasons  for  this  assumption 

wSe  the  fact  that  cloud  particles  remained  suspended  for 
cieTare  such  long  times  in  the  atmosphere,  and  furthermore  certain 
solid,  not  optical  phenomena  were  better  explained  on  this  assumption. 

Recent  observations,  however,  have  shown  conclusively  that 
cloud  particles  are  solid  throughout.  The  reason  for  the  suspension 

in  the  atmosphere  lies  in  their  small  size.     It  can  be  shown 

^at  a  water  drop  YO^TF  °f  an  mc^  m  diameter  would  ordi- 
suspension  narily  fall  at  the  rate  of  less  than  two  inches  per  second  in 
current?  "*  a"*  a^  a^  ordinary  pressures.  It  would  thus  require  but  a  very 

feeble  ascending  air  current  to  keep  a  cloud  particle  in  sus- 
pension or  even  to  cause  it  to  rise. 

229.  Haze.  —  Haze  occurs  both  high  up  in  the  atmosphere  and  near 
Two  kinds      the  earth's  surface.     The  high  haze  pales  out  the  blue  color 
of  haze.         of  ^he  sky  ^y  ^ay  an(j  gives  to  it  a  whitish  or  washed-out 
appearance.     At  night  it  makes  itself  manifest  by  dimming  the  light 

.  .  of  the  stars  and  rendering  the  fainter  ones  invisible.  In 
and  expiana-  this  it  acts  as  a  very  thin  cirrus  cloud,  and  thus  thick  haze 
tion  of  the  js  often  spoken  of  as  cirrus  haze.  It  is  caused  by  minute 

high  haze. 

ice  particles  in  the  upper  atmosphere,  and  is  thus  a  conden-. 
sation  product. 

Haze  near  the  earth's  surface  makes  itself  manifest  by  obscuring  and 
rendering  indistinct  the  outlines  of  distant  objects.  It  is  well  known 
Description  that  dust  plays  a  large  part  in  this,  and  some  authorities 
and  expiana-  credit  the  cause  of  haze  to  the  presence  of  an  abnormal 
haze'ne'ar8  amount  of  dust  in  the  atmosphere.  The  fact  that  haze 
the  earth's  occurs  chiefly  in  September  and  October,  when  the  dry 

leaves  are  falling  from  the  trees  and  being  blown  about 
by  the  increasing  winds,  would  seem  to  lend  color  to  this  ex- 
planation. It  has  also  been  thought  that  haze  is  due  to  the  con- 
densation of  water  vapor  on  these  large  dust  particles  near  the 
earth's  surface,  but  as  haze  often  occurs  when  the  air  is  far  above 
the  dew  point,  it  would  hardly  seem  that  condensation  on  any  such 
scale  is  possible.  A  good  part  of  the  indistinctness  of  distant  objects 
is  probably  optical  and  not  mechanical.  That  is,  it  is  due  to  the 


THE  MOISTURE   IN  THE  ATMOSPHERE  233 

mixing  of  layers  of  air,  of  different  temperature  or  containing  very 
different  amounts  of  invisible  water  vapor,  and  thus  having  different 
refractive  indices. 


THE  FORMATION  OF  CLOUDS 

230.  Introduction.  —  The  clouds   are  formed  by  the  condensation 
of  water  vapor  in  a  quantity  of  air  which  has  become  supersaturated; 
that  is,  which  has  gone  below  its  dew  point  and  is  containing  HOW  clouds 
more  moisture  than  the  given  quantity  of  air  at  the  tempera-  are  formed- 
ture  in  question  can  contain.     This  condition  may  be  produced  by  the 
addition  of  water  vapor  to  a  quantity  of  air  already  near  the  point  of 
saturation,  or  by  lowering  the  temperature  so  that  the  air  can  no  longer 
hold  the  water  vapor  which  it  formerly  contained.     It  must  be  held  in 
mind  that  when  condensation  occurs  in  the  free  atmosphere,  the  condi- 
tions are  very  different  than  they  are  in  the  usual  laboratory  experi- 
ments.    When  the  amount  of  water  vapor  which  saturates  a  given  quan- 
tity of  air  is  being  determined  in  a  laboratory,  large  objects  and  parti- 
cles are  present  upon  which  the  condensation  may  take  place.     In  the 
free  atmosphere  the  condensation  must  take  place  upon  the  extremely 
minute  particles  or  ions.     This  explains  why  air  has  often  been  found 
which  contains  more  than  the  saturating  amount  of  moisture,  and  yet 
without  condensation.     Valuable  information  could  be  gained  by  deter- 
mining the  amount  of  water  vapor  which  saturates  air  when  condensa- 
tion must  take  place  on  minute  particles.     The  amount  would  be  much 
greater  than  that  given  in  ordinary  tables,  and  would  probably  be  dif- 
ferent for  particles  of  different  sizes. 

There  are  nine  processes  which  may  lead  to  cloud  formation.     Eight 
of  these  produce  condensation  by  cooling  the  air,  and  one  by  adding 
water  vapor.     These  nine  processes  may  be  divided  into  three  The  ldne 
groups.     The  first  three  are  the  major  processes  in  cloud  processes  of 
formation.     The  second  group  of  three  are  those  processes  ^on  nm^be" 
which  are  common,  but  yet  are  not  such  powerful  cloud  pro-  divided  into 
ducers   as  those  in  the  first  group.     The  three  processes 
which    form    the    third    group    are   ordinarily   unimportant    and    in- 
significant.    These  nine  processes  are  numbered  one  to  nine  inclusive, 
and  in  the  previous   sections   they   have  been   referred   to   by  these 
numbers. 

231.  Condensation   in    warm    winds    blowing    over    cold    surfaces 
(Method  1).  —  If  warm,  moisture-laden  air  blows  over  a  cold  surface, 


234  METEOROLOGY 

f 

its  temperature  will  be  reduced,   and  it  may  be  lowered  below  the 

dew  point,  in  which  case  condensation  must  take  place.  There  are  four 
minor  illustrations  of  this  method  of  cloud  or  fog  formation, 
condensa-  The  first  one  is  the  spring  fogs  while  it  is  thawing.  The 
tion  is  pro-  ground  is  perhaps  snow-covered  and  frozen,  and  the  tem- 
peratures are  well  beloW  the  freezing  point.  The  wind 
begins  to  blow  from  the  south,  and  warm  air  is  brought  by  transporta- 
tion and  is  carried  over  these  snow-covered,  frozen  surfaces, 
minor  iiius-  The  snow  begins  to  melt  and  the  air  goes  below  its  dew 
tnitions  of  ^  point,  and  a  fog  is  the  result.  A  second  example  of  this  pro- 
cess is  the  formation  of  fog  near  Newfoundland.  A  warm, 
moisture-laden  wind  from  the  south  blows  over  the  Gulf  Stream  against 
the  icebergs  and  cold  water  brought  down  from  the  arctic  regions.  The 
result  is  a  cooling  of  the  warmer  air  below  the  dew  point  and  the  forma- 
tion of  the  many  fogs  which  are  prevalent  in  this  region.  This  is 
especially  true  in  summer,  when  the  temperature  contrasts  are  particu- 
larly marked.  A  third  example  is  the  so-called  mountain  cloud.  This 
is  particularly  noticeable  if  the  mountain  is  snow-capped.  The  air  mov- 
ing over  the  summit  is  cooled  below  the  dew  point,  and  a  cloud  banner 
streams  out  from  the  mountain  top  in  a  direction  opposite  to  the  wind 
direction.  This  cloud  usually  has  a  certain  definite  length.  It  disap- 
pears, due  to  the  fact  that  the  air  which  had  become  cooled  in  passing 
the  mountain  top  has  become  warmed  again  by  mixture  or  by  absorbing 
the  sun's  r.adiant  energy  sufficiently  to  go  above  its  dew  point  and  con- 
tain the  moisture  in  the  form  of  invisible  water  vapor.  A  fourth  example 
of  this  process  is  the  formation  of  the  so-called  frost  work  in  winter.  A 
warm  wind  blows  against  objects  which  are  colder, than  the  air.  The 
air  in  contact  with  them  is  cooled  below  the  dew  point,  and  the  moisture 
is  deposited  on  these  objects  in  the  form  of  frost  work.  This  frost  work 
grows  out  to  windward  against  the  air  current.  These  forms  are  partic- 
ularly prevalent  on  the  iron  work  of  towers  or  buildings  which  are 
located  on  high  mountains. 

The  major  example  of  this  cloud-forming  process  is  the  formation  of 

clouds  when  the  wind  is  south.     A  south  wind  transports  large  quantities 

of  warm,  moisture-laden  air  from  southern  countries  and 

exa^ieTof     carries  it  over  colder  surfaces  farther  north.     As  a  result, 

this  cloud-      immense  quantities  of  air  are  cooled  below  the  dew  point, 

process          an(^  cl°uds  result.     Due  to  irregularities  in  the  surface  and 

friction,  there  is  probably  a  certain  amount  of  mixing  on  the 

part  of  the  air,  and  also  rising  and  falling.    Other  cloud-forming  processes 


THE  MOISTURE  IN  THE  ATMOSPHERE  235 

would  thus  probably  be  operative  at  the  same  time.  These  clouds  are 
chiefly  of  the  stratus  and  nimbus  variety,  and  occur  ordinarily  about 
one  half  mile  to  two  miles  above  the  earth's  surface.  The  v-  A  A 

Jvina  ana 
reason  for  this  particular  height  is  twofold :   the  air  near  the  height  of 

earth's  surface  is  retarded  by  friction  so  that  the  greater  the  the  cloud' 
height  above  the  earth's  surface,  the  more  vigorous  the  transportation. 
Due  to  this  cause,  the  most  vigorous  transportation,  and  thus  condensa- 
tion, would  take  place  at  the  greatest  heights.  But  the  amount  of  mois- 
ture in  the  atmosphere  decreases  with  altitude.  Thus,  two  causes 
are  counterbalanced  to  produce  a  maximum  of  moisture  transportation, 
and  thus  cloudiness  at  the  height  stated  above. 

232.    Condensation     in     ascending     currents     due     to     convection 
(Method  2).  —  When  air  rises,  due  to  convection,  it  comes  into  regions 
of  less  barometric  pressure,  and  consequently  it  expands. 
The  expansion  causes  cooling  at  the  rate  of  1.6°  F.  for  300  ft.  condensa- 
(0.993°  C.  for  100  meters).     As  a  result  the  dew  point  is 
often  passed,  and  the  excess  moisture  must  then  condense  in 
the  form  of  cloud.     Since  convection  occurs  usually  during  the  day, 
clouds  formed  by  this  process  are  generally  daytime  clouds,  and  the  kind 
is  some  one  of  the  cumulus  forms. 

Two  constants  make  possible  the  computation  of  the  height  at  which 
a  convection-formed  cloud  ought  to  appear.  These  two  constants  are  the 
adiabatic  rate  of  cooling,  that  is,  the  cooling  due  to  the  ex-  The  compu_ 
pansion  as  the  air  rises,  and  the  lowering  of  the  dew  point  due  tation  of  the 
to  expansion.  The  value  of  the  first  constant  is  1.6°  F.  for  ^hfchthe 
300  feet.  The  other  constant  is  introduced  here  for  the  first  cloud  will 
time.  Suppose  that  a  cubic  foot  of  air  contains  a  certain 
amount  of  moisture.  If  this  air  rises  a  certain  height,  it  will  have 
expanded  and  become  two  cubic  feet ;  and  since  the  amount  of  moisture 
has  remained  unchanged,  each  cubic  foot  will  now  contain  one  half  the 
moisture  which  it  contained  before.  As  a  result,  the  dew  point  will 
have  become  lowered.  This  dropping  back  of  the  dew  point,  due  to 
expansion,  amounts  to  .33°  F.  for  300  feet,  or  .2°  C.  for  100  meters.  Thus, 
as  air  rises,  its  temperature  lessens  1.6°  F.  for  each  300  feet,  and  the 
dew  point  is  lowered  .33°  F.  for  each  300  feet.  The  result  is  that  the 
temperature  of  the  air  approaches  the  dew  point  at  the  rate  of  the  dif- 
ference, namely  1.27°  F.  for  each  300  feet.  The  height  at  which  a  con- 
vection-caused cloud  should  be  formed  can  thus  be  computed  by  divid- 
ing the  difference  between  the  temperature  and  the  dew  point  by  1.27, 
and  multiplying  it  by  300.  For  example,  suppose  on  a  summer  after- 


236  METEOROLOGY 

noon  the  temperature  is  85°  F.  and  the  dew  point  has  been  found  to  be 
70°  F.     The  height  of  a  convection-caused  cloud  would  be  -        -  300 

or  3543  feet. 

After  a  cloud  begins  to  be  formed,  the  air  continues  to  rise,  and  it 
will  continue  to  rise  until  its  temperature  has  fallen  to  the  temperature 
of  its  surroundings.  In  order  to  determine  this,  the  vertical 
temperature  gradient  must  be  known.  There  are  three 
clouds  are  things  which  favor  further  rise  of  the  air  after  it  has  com- 
^J^y  menced  to  become  cloudy.  These  are  the  absorption  of 
radiant  energy,  the  latent  heat  of  condensation,  and  the 
latent  heat  of  fusion.  As  soon  as  the  rising  air  becomes  cloudy,  it  ab- 
sorbs the  radiant  energy  of  the  sun  and  earth.  Furthermore,  the  latent 
heat  of  condensation  is  supplied  to  the  rising  air,  thus  lessening  the  rate 
of  cooling,  and  causing  further  rise.  As  soon  as  the  freezing  point  is 
passed,  the  latent  heat  of  fusion  is  added  to  the  latent  heat  of  conden- 
sation. As  a  result,  all  of  these  convection-caused  clouds  are  usually 
extremely  thick,  because  the  rising  air  supplied  with  heat  in  these  three 
ways  must  rise  to  considerable  heights  before  it  has  cooled  by  expansion 
to  the  temperature  of  the  surrounding  air.  In  the  case  of  a  thunder- 
cloud, the  thickness  may  amount  to  as  much  as  five  or  even  six  miles. 

There  are  several  illustrations  of  clouds  formed  in  this  way.  The 
cumulus  clouds,  which  occur  particularly  in  summer,  and  generally 
during  the  hotter  parts  of  the  day,  are  usually  convection- 
ofconvec-  formed.  This  can  be  decided  by  noting  whether  they 
tion-formed  disappear  in  the  late  afternoon,  as  soon  as  convection  stops. 
A  thunderstorm,  as  will  be  seen  later,  is  convection-caused, 
and  is  simply  an  overgrown  cumulus  cloud.  The  clouds  formed  over 
certain  islands  are  also  good  illustrations  of  this  process.  The  ground 
warms  more  rapidly  during  the  day  than  the  ocean.  This  causes  con- 
vection, the  warm  air  being  forced  to  rise  by  the  cool  air  which  comes  in 
from  the  surrounding  ocean.  Thus  certain  islands  are  cloud-covered 
during  the  day,  and  the  sky  becomes  clear  again  at  night  when  convec- 
tion ceases.  In  some  cases  this  process  becomes  vigorous  enough  to 
cause  a  thundershower  each  day. 

d  as  233>    C°nclensation  *n  forced  ascending  currents  (Method 

cent  may  be  3).  — When  air  is  forced  to  rise,  it  will  expand  and  become 
caused  in       cooler,  and  perhaps  cloudy,  in  just  the  same  way  as  in  con- 
vection. .  There  are  two  ways  in  which  air  may  be  forced 
to  ascend ;   one  is  by  passing  over  a  mountain  or  hill  or  other  barrier 


THE  MOISTURE  IN  THE  ATMOSPHERE  237 

which  forces  it  to  rise,  and  the  second  is  by  being  forced  to  rise  in  a 
storm  center,  that  is,  in  an  area  of  low  barometric  pressure. 

If  air  is  forced  to  rise  by  a  barrier,  a  bank  of  cloud  parallel  to  the 
barrier  ordinarily  forms  on  the  leeward  side.     This  is  a  fairly  common 
cloud  in  mountainous  regions,  and  the  position  of  the  cloud 
is  determined  by  the  wind  direction.     Air  forced  to  rise  by  tenstics  of 
storm  centers  is  one  of  the  chief  causes  of  the  stratus  and  the  clouds 
nimbus  clouds  which  often  cover  large  sections  of  the  country. 
The  formation  of  these  immense  cloud  areas  will  be  fully  discussed  in 
the  chapter  on  storms. 

234.  Condensation    caused    by     diminishing     barometric    pressure 
(Method  4).  —  If,  for  any  reason,  the  barometric  pressure  diminishes, 
the  air  will  at  once  expand  and  become  cooler.     If  the  air 

is  nearly  saturated  with  moisture,  that  is,  if  it  is  near  its  fo£JJeJ  by 

dew  point,  it  will  require  but  little  diminution  in  pressure  diminishing 

to  cause  the  air  to  become  supersaturated,  and  condensation 

in  the  form  of  cloud  will  then  take  place.     This  is  a  process 

of  intermediate  importance  in  cloud  formations,  and  gives  rise  chiefly 

to  clouds  of  the  cirrus  or  stratus  variety. 

235.  Condensation    in    atmospheric    waves   (Method    5).  —  When- 
ever a  fluid  flows  over  obstacles,  waves  are  usually  formed  in  it.     There 
are  many  illustrations  of  waves  formed  in  this  way  in  nature.  How 

The  surface  of  a  field  of  drifting  snow  usually  becomes  wavy,  atmospheric 
Sea  sand  underneath  the  water  also  becomes  wavy,  due  to  the 
passing  backward   and   forward   of  the  water.     A  stream 
flowing  over  an  irregular  bed  usually  has  waves  formed  on  its  surface. 
In  just  the  same  way,  when  air  flows  over  a  rough  surface,  waves  are 
formed  in  it.     These  waves  may  have  a  height  of  only  a  few  hundred 
feet  from  trough  to  crest,  or  at  times  they  may  attain  a  height  of  half  a 
mile  or  even  more.     When  the  air  rises,  due  to  the  passage  of  one  of 
these  waves,  it  expands,  becomes  cooler,  and  may  go  below  its  dew 
point.     When  it  falls   it   is  compressed,  rises  in  tempera-  The  charac. 
ture,  and  usually  is  cloudless.     The  waves  in  the  atmos-  tenstics  of 
phere  would  thus  give  rise  to  a  series  of  clouds  in  the  form  of  formecTby 
parallel  bars.     This  is  of  very  common  occurrence,  and  all  atmospheric 
the  rippled  or  striated  clouds  are  due  to  atmospheric  waves. 
This   appearance  is  particularly  noticeable  in  connection  with  cirrus 
or  stratus  clouds. 

236.  Condensation  caused  by  radiation  (Method  6).  —  A  layer   of 
air  near  its  dew  point  may  radiate  its  heat  to  space  or  the  cold 


238  METEOROLOGY 

ground,  and  go  below  its  dew  point  and  become  cloudy.  This  is 
often  noticed  in  the  northeast  portion  of  the  United  States 
on  ^ne  s^>  clear,  cold  winter  mornings.  Excessive  radia- 

a  radiation-    tion/  has  taken  place  from  the  upper  layers  of  the  atmosphere, 

cloud*  ^nev  nave  cooled  below  the  dew  point,  and  a  thin  overcast- 

ing in  the  form  of  an  alto-stratus  cloud,  is  the  result.  It  is 

visible  in  the  early  morning,  and  with  the  rise  in  temperature  it  soon 

disappears. 

237.  Condensation    due    to    conduction    (Method  7). — This    is    a 
very  minor  factor  in  cloud  production.     A  layer  of  air  could  lose  suffi- 

cient  heat  by  conduction  to  adjoining  layers  to  go  below  its 
auction  dew  point  and  become  cloudy.  Suppose  a  layer  of  air  is  at 
could  pro-  a  temperature  of  40°  and  is  saturated  with  moisture,  and 

duce  a  cloud. 

suppose,  furthermore,  that  the  layer  of  air  above  or  below  it 
has  a  temperature  of  30°.  By  conduction,  this  layer  of  air  might  lose 
its  heat  to  the  adjoining  layer,  go  below  the  dew  point,  and  become 
cloudy.  This  process  of  cloud  formation  would  probably  produce 
some  cirrus  or  stratus  cloud  form. 

238.  Condensation  by  mixing  air  (Method  8).  —  If  two  quantities 
of  saturated  air  of  different  temperatures  are  mixed,  the  result  is  always 
How  mixing  condensation.     A    numerical    example    will    illustrate    the 
saturated  air  truth  of  this.     Suppose  a  cubic  foot  of  saturated  air  with  a 
tempera-*1      temperature  of  70°  F.,  and  another  cubic  foot  of  saturated 
tures  pro-       air  with  a  temperature  of  50°  F.  are  mixed.    The  cubic  foot 
duces  cloud.   ^  air  ^^  ^  temperature  of  70°  F.  contains  7.99  grains 
of  water  vapor  per  cubic   foot,  while  the  cubic   foot  of  air  with  the 
temperature  of   50°  F.  contains  4.09  grains.      These  values    are  ob- 
tained from  the  table  in  section  181.     The  resulting  air  must  contain 
the  average  of  these  two  values,  namely  6.04.     The  resulting  tempera- 
ture will  be  60°  F.,  and  it  will  be  seen  from  the  table  that  air  at  60°  F. 
can  contain  but  5.76  grains  per  cubic  foot.     The  excess  must  condense 
in  the  form  of  cloud.     As  a  matter  of  fact,  only  a  fraction  of  this  excess 
would  actually  condense  in  the  free  atmosphere,  for  as  soon  as  condensa- 
tion started,  latent  heat  would  cause  a  rise  of  temperature  and  thus 
enable  the  air  to  hold   more  moisture.     The  variety  of   cloud  formed 
in  this  way  would  be  of  the  stratus,  perhaps  cirrus,  type. 

Historically  this  process  of  cloud  formation  was  one  of  the  first  to  be 
Historical  considered,  and  in  fact  it  was  thought  that  it  was  one  of 
importance.  fae  major  processes  in  cloud  formation.  It  is  now  known, 
however,  that  it  plays  a  very  minor  part  in  the  formation  of  cloud. 


THE  MOISTURE  IN.  THE  ATMOSPHERE  239 

239.  Condensation    by    diffusion    of    water    vapor    (Method    9). — 
Suppose  a  given  quantity  of  air  saturated  with  water  vapor  is  situated 
between  two  layers  of  air  containing  a  larger  amount  of 
moisture.     By  diffusion  some  of  this  moisture  may  pass  to  sionmay 
the  layer  of  air  in  question,   thus  supersaturating  it  and  cause  <ron~ 
causing    cloudy    condensation.     Cirrus    or    stratus    would 
probably  be  the  cloud  form  produced. 

240.  Conditions  that  favor  a  clear  sky.  —  The  converse  or  opposite 
working  of  all  the  methods  of  cloud  formation  just  mentioned  would 
favor  a  clear  sky.     One  of  the  nine  processes,  however,  has  _ 

,   ,,  .     .  x-  mi  -       Thecondi- 

no  opposite  or  converse,  and  this  is  convection.     The  onus-,,  tions  that 
sion  of  this  one  would  leave  two  major  processes  of  cloud  favor  a  clear 
formation.     There  are  thus  two  major  processes  which  favor 
a  clear  sky.     These  would  be  cold  winds  blowing  over  a  warm  surface 
and  forced  descending  air  currents.     It  is  a  well  known  fact  of  observa- 
tion that  clear  skies  are  very  prevalent  when  the  wind  is  north  or  north- 
west, and  when  the  station  is  covered  by  an  area  of  high  barometric 
pressure,  which  causes  descending  and  outflowing  air  currents. 


D.     PRECIPITATION 

THE  KINDS  OF  PRECIPITATION 

241.   Rain. —  Four  of  the  seven  forms  of  condensation  have  already 
been  fully  considered.     These  are  dew,  frost,  and  fog,  which  occur  near 
the  earth's  surface,  and  clouds,  which  form  high  up  in  the  Precipita- 
atmosphere.     The  remaining  three,  rain,  snow,  and  hail,  are  tion- 
all  included  under  the  general  term  of  precipitation,  and  they  require 
the  most  vigorous  condensation  for  their  production. 

Raindrops  are  formed  from  the  cloud  particles.      These  are  not  all 
of  the  same  size,  and  the  larger  ones  will  fall  faster  than  the  smaller 
ones,  or,  if  they  are  being  carried  up  by  ascending  air  currents,  The  forma_ 
it  will  be  the  larger  particles  which  are  carried  up  less  rapidly,  tion  of  a 
Due  to  these  motions  many  collisions  must  occur,  and  when- 
ever two  cloud  particles  collide,  they  coalesce  into  a  diminutive  rain- 
drop.     As  soon  as  a  raindrop  begins  to  fall  at  all  rapidly,  it  v^11  soon 
come  into  layers  of  air  warmer  than  itself,  and  condensation  of  water 
vapor  will  then  take  place  on  the  cold  drop.     A  raindrop  thus  increases 
in  size,  due  to  collision  and  condensation,  until  it  reaches  the  base  of 
the  cloud  and  begins  its  final  fall  to  the  surface  of  the  earth. 


240  METEOROLOGY 

But  for  two  circumstances,  all  clouds  would  yield  precipitation.  In 
the  first  place,  the  velocity  of  the  ascending  air  currents  is  often  suffi- 
cient  to  hold  stationary  or  even  carry  up  drops  of  consider- 
cioud  does  able  size.  Then  again,  after  a  raindrop  leaves  the  base  of  a 
not  yield  cloud,  it  at  once  begins  to  evaporate,  and  often  disappears 
long  before  the  earth's  surface  is  reached.  It  is  not  at  all 
uncommon  to  see  a  dark  trail  of  rain  depending  from  a  cloud  while  no 
trace  of  precipitation  reaches  the  earth  below.  It  was  formerly  thought 
that  electricity  played  a  large  part  in  preventing  the  formation  of  rain- 
drops by  a  cloud.  It  was  known  that  all  clouds  are  highly  electrified, 
and  it  was,  supposed  that  the  small  cloud  particles,  being  charged  with 
like  electricity,  would  be  kept  from  coalescing  by  electrical  repulsion. 
It  was  accordingly  supposed  that  raindrops  could  form  only  when  the 
cloud  particles  had  been  discharged  by  a  lightning  flash  or  in  some 
other  way.  Later  experiments  seem  to  show  that  in  most  cases  elec- 
tricity helps  rather  than  hinders,  if  it  plays  any  part  at  all. 

In  size,  raindrops  vary  from  very  small  to  perhaps  yV  of  an  inch  in 
the  case  of  large  pattering  raindrops.  A  raindrop  two  or  three  times 
The  size  of  larger  than  this  could  not  exist,  as  it  would  separate  into 
raindrops.  parts  due  to  the  rapidity  of  its  fall  through  the  air.  The 
size  of  raindrops  is  usually  determined  by  allowing  them  to  fall  into  a 
layer  of  flour.  By  allowing  drops  of  measured  sizes  to  fall  into  the  same 
flour  and  noting  the  size  of  the  dough  balls  formed,  the  size  of  the  rain- 
drops can  be  determined.  Raindrops  are  largest  at  the  base  of  the 
cloud,  and  diminish  in  size,  due  to  the  evaporation,  as  the  earth's  sur- 
face is  approached. 

Of  the  nine  processes  which  may  lead  to  cloud  formation,  only  the 
The  three  three  major  processes  are  vigorous  enough  to  cause  suffi- 
cioud-form-  cient  condensation  to  produce  precipitation.  These  are 
cess^which  warm  winds  over  cold  surfaces,  convection,  and  forced 
may  lead  to  ascent  by  a  barrier  or  storm.  Thus  whenever  precipitation 
on>  falls  from  a  nimbus  cloud,  it  has  been  formed  by  one  or  more 
of  these  three  processes. 

\  242.  Snow.  —  When  condensation  is  sufficiently  vigorous  to  cause 
precipitation  while  the  temperature  is  below  the  freezing  point,  snow- 
The  struc-  flakes  instead  of  raindrops  are  formed.  The  structure  of 
ture  of  snowflakes  may  be  carefully  studied  by  catching  them  on  a 

snowflakes.  piece  of  bJack  ^^  and  observing  them  through  a  magnify- 
ing glass.  They  have  also  been  observed  and  photographed  through 
a  microscope.  Many  observers  have  sketched  the  varied  forms  of  snow- 


THE  MOISTURE  IN  THE  ATMOSPHERE  241 

flakes,  but  the  most  complete  study  of  them  has  been  made  by  Mr.  Wilson 
A.  Bentley,  of  Nashville,  Vt.,  who  has  observed  them  for  more  than 
twenty  years,  and  whose  microphotographs  have  been  reproduced  in 
the  Monthly  Weather  Review  for  1901  and  1902.  TJje  six  examples 
pictured  in  Fig.  105  are  taken  from  this  collectio#^Tf  the  tempera- 
tures are  low,  the  snowflakes  are  always  small,  flat,  and  regular.  They 
always  have  angles  of  60°  or  120°,  which  are  characteristic  of  crystal- 
lized water.  They  bear  every  evidence  of  having  been  formed  about  a 
single  center  by  continuous  condensation  or  by  the  addition  of  small 
cloud  particles.  Attempts  have  been  made  to  correlate  their  varied 
appearance  with  the  temperature,  the  type  of  storm,  the  rapidity  of 
condensation,  etc.,  but  the  effect  of  each  factor  which  enters 
in  has  not  yet  been  determined.  If  the  temperature  is 
very  low  —  at  least  below  zero  Fahrenheit  —  fine  ice  needles  ture  on  the 


are  formed  instead  of    snowflakes.     If  the  temperature  is          of 


near  the  freezing  point,  particularly  in  the  lower  layers  of 
the  atmosphere,  the  snowflakes  often  mat  together  and  form  large  clots. 
If  the  temperature  is  still   higher,  the  snowflakes  sometimes  partially 
melt.     Much  of  the  rain  which  falls  in  the  winter  time  probably  left  the 
cloud  as  snow  and  melted  during  the  descent. 

243.    Hail.  —  There  are  three  kinds  of  hail,  and  each  occurs  at  a  dif- 
ferent time  of  year  and  is  formed  in  a  different  way.  Three  kinds 

The    hail  which   occurs   during    the  winter    consists    of  ofhail- 
small,  clear  pellets  of  ice  of  about  the  size  of  large  raindrops  —  in 
fact,  they  are  frozen  raindrops.     Hail  of  this  kind  occurs 
when  the  temperature  of    the    cloud   where    the   raindrop 
is   formed  is  above  32°  F.  while  the  lower  layers  of  the  air  are  still 
below  the  freezing   point.     As   a  result,  the  raindrop  freezes  during 
its  fall,  and  reaches  the  ground  as  a  hailstone.     The  quantity  is  usually 
small,  although  on  rare  occasions  the  ground  may  be  covered  to  a  depth 
of  three  or  four  inches  by  hail  of  this  kind. 

The  so-called  soft  hail  consists  of  small  white  pellets  of  what  looks 
like  compacted  snow'.     It  occurs  usually  in  very  small  quantities  during 
March  and  April,  and  occasionally  during  the  autumn.     It  goft  ^ 
falls  nearly  always  from  an  overgrown  cumulus  cloud.     It 
is  supposed  to  be  formed  from  frozen  cloud  particles  mixed  with  rain- 
drops and  compacted  by  a  high  wind. 

In  the  summer  time  hail  never  occurs  except  during  a  thundershower. 
The  hailstones  are  usually  large,  in  some  cases  several  inches  Summer 
in  diameter,  and  they  consist  of  concentric  layers  of  compact  hail. 


242  METEOROLOGY 

snow  and  ice.  Hail  nearly  always  falls  at  the  beginning  of  a  shower, 
and  sometimes  great  damage  is  done.  There  are  records  when  the  hail- 
stones during  a  single  shower  have  covered  the  ground  to  a  depth  of 
more  than  a  foot.  The  full  consideration  of  the  formation  of  this  kind 
of  hail  will  be  deferred  until  the  mechanism  of  a  thundershower  has  been 
studied,  but  the  structure  of  the  hailstones  would  seem  to  show  that 
they  had  been  formed  in  a  cloud  whirling  about  a  horizontal  axis.  The 
nucleus  is  carried  up  and  coated  with  snow  ;  it  then  falls  and  is  coated 
with  water  ;  it  is  then  carried  up  again,  the  water  freezes,  and  it  is  once 
more  coated  with  snow.  This  process  continues,  adding  coat  after 
coat,  until  the  hailstone  becomes  too  heavy  to  be  longer  sustained,  and 
it  falls  to  the  ground.  As  will  be  seen  later,  it  is  in  the  squall  cloud 
at  the  front  of  a  thundershower  that  these  conditions  are  actually 
realized. 

Section  119  of  the  instructions  for  preparing  meteorological  forms 
of  the  U.  S.  Weather  Bureau  says  :    "  Care  should  be  taken  in  deter- 

mining  the  character  of  precipitation  when  in  the  form  of 
Bureau  sleet  or  hail.  Only  the  precipitation  that  occurs  in  the  form 
definition  of  of  frozen  rain  should  be  called  sleet.  Hail  is  formed  by 

accretions  consisting  of  concentric  layers  of  ice,  or  of  alternate 
layers  of  ice  and  snow.  It  frequently  happens  that  snow  falls  in  the 
form  of  small,  round  pellets,  which  are  opaque,  having  the  same  appear- 
ance as  snow  when  packed.  This  should  never  be  recorded  as  sleet." 
The  above  is  perfectly  definite,  and  in  use  at  all  Weather  Bureau  stations. 
According  to  it,  the  winter  hail  or  frozen  raindrops  should  be  called 
sleet.  The  soft  hail  should  be  called  snow,  and  only  the  summer  hail 
should  be  recognized  as  hail.  Unfortunately,  in  most  dictionaries, 
books  on  meteorology,  and  the  popular  mind,  snow  mixed  with  rain  is 
considered  sleet,  and  several  kinds  of  hail  are  recognized. 

244.   Ice  storms.  —  It  sometimes  happens  that  it  rains  very  soon 
after  a  continued  period  of  cold  while  the  temperature  of  the  ground 

and  the  layer  of  air  next  it  is  still  considerably  below  the 
tenstics  of  freezing  point.  As  a  result,  the  rain  freezes  to  everything 


storm  ^at  ^  toucnes>  and  trees,  shrubs,  vines,  and  the  ground  itself 

become  covered  with  a  layer  of  ice.  This  is  known  as  an 
ice  storm,  and  considerable  damage  is  sometimes  done  by  breaking  down 
trees  and  vines  on  account  of  the  weight  of  the  ice  which  forms  on  them. 
These  storms  are  particularly  prevalent  in  New  England,  and  Fig. 
106  illustrates  such  a  storm.1 


1  See  Monthly  Weather  Review,  December,  1900. 


THE  MOISTURE   IN  THE  ATMOSPHERE  243 

245.  Rain-making.  —  It  has  been  a  favorite  popular  belief  that  rain 
can  be  produced  by  cannonading  and  heavy  artillery  fire.     The  frequent 
occurrence  of  thundershowers  on  July  4  has  L.^n  used   as  Rain  on 

an  argument  in  favor  of  this  belief,  and  also  the  fact  that  rain  July  4  and 
has  followed  so  many  of  the  great  battles.  As  regards  after  battles- 
July  4,  statistics  at  many  stations  for  many  years  show  that  there  are 
no  more  thundershowers  on  the  Fourth  than  on  the  third  or  fifth.  In 
connection  with  rain  following  battles,  it  has  been  pointed  out  that  the 
fact  was  mentioned  long  before  gunpowder  was  invented.  There  are 
two  possible  reasons  why  the  fact  has  been  so  often  noted.  In  the  first 
place,  the  discomfort  and  suffering  brought  about  by  the  rain  would 
surely  cause  it  to  be  mentioned.  And  again,  an  army  usually  gets  into 
position  during  good  weather  while  the  roads  are  good,  so  that  by  the 
time  the  battle  begins  a  rain  period  would  be  due. 

Influenced,  perhaps,  by  this  popular  belief,  many  attempts  have  been 
made  by  the  so-called  rain-makers  to  cause  rain  by  artificial  means, 
and  in  fact  considerable  money  has  been  expended.  The  Rain- 
methods  employed  are  either  to  cause  violent  explosions  in  makinS' 
the  upper  air  or  to  liberate  a  large  amount  of  gas  in  the  upper  air.  The 
only  way  in  which  an  explosion  could  cause  rain  would  seem  to  be  by 
furnishing  through  the  smoke  particles  numerous  nuclei  of  condensa- 
tions, or  by  causing  cloud  particles  to  coalesce,  due  to  the  wave  motion 
caused  by  the  explosion.  The  liberation  of  gas  might  cause  convection. 
But  all  these  effects  would  seem  to  be  on  too  diminutive  a  scale  to  influ- 
ence the  immense  masses  of  air  which  must  be  affected  in  order  to 
produce  rain  over  a  considerable  area.  As  far  as  results 

The  results. 

are  concerned,  it  has  never  been  proven  that  any  ram  has 

fallen  which  would  not  have  fallen  if  the  experiments  had  not  been 

tried. 

246.  Cooling  produced   by  precipitation.  —  As  soon  as  it  begins  to 
rain,  particularly  in  summer,  the  air  is  usually  much  cooler.     One  reason 
for  this  is  the  fact  that  the  raindrops  are  from  3°  to  15°  F.  Raindrops 
colder  than  the  air  at  the  earth's  surface.     It  is  to  be  noted  in  are  colder 
this  connection  that  a  raindrop  in  falling  from  a  cloud  is  not 
compressed  and  heated  by  the  compression,  as  air  would  be,  but  retains 
its  temperature  during  the  descent.     There  are  other  causes,  Other 
however,  of  the  cooling  caused  by  precipitation,  such  as  causes  of 
the  cutting  off   of   insolation    by  the    clouds,  the  coming  c 

down  of  cooler  air  from  above,  and  the  cooling  due  to  evaporation  from 
the  wet  ground. 


244 


METEOROLOGY 


Front  View. 


Vertical  Section. 


Receive* 


THE  DETERMINATION  OF  PRECIPITATION  AND  THE  RESULTS  OF 

OBSERVATION 

247.    The  measurement  of  a  rainfall.  —  The  only  measurement  made 

in  connection  with  a  rainfall  is  the  amount,  that  is,  the  thickness  of  the 

layer  of  water  which  the  rainfall  would  produce  on  a  level 

1  he  amount 

is  measured  surface,  provided  none  were  lost.     The  instrument  for  deter- 
mining this  is  called  a  rain  gauge,  and  consists  in  its  simplest 
form  of  a  vessel  for  catching  the  rain  and  a  measuring  rod 
for  determining  its  depth.     Since  the  rain  gauge  is  so  simple  in  con- 
struction and  rain  is  one  of  the  most  important  of  the  meteorological 

elements,  it  is  no  wonder 
that  rain  gauges  of  one 
form  or  another  have  been 
in  use  for  nearly  three 
hundred  years. 

The  U.  S.  Weather  Bureau 
standard  rain  gauge,  as  illus- 
trated   in     Fig. 
107,  consists  of  a 
galvanized    iron 
cylindrical     can 
eight    inches    in 
diameter  and  about  twenty 
inches  high.     It  is  provided 
with  a  funnel-shaped  cover 

FIG.   107. -The  U.  S.  Weather  Bureau  Rain  Gauge.     °r  receiver>  with   a  beveled 

rim  sharp  on  the  inside  and 

accurately  circular  in  order  to  catch  the  amount  of  rain  which  falls  on 
a  definite  area.  The  shape  is  that  of  a  funnel  with  a  small  opening, 
in  order  to  prevent  evaporation.  The  funnel  opens  into  an  inside 
brass  cylinder  which  has  just  one  tenth  the  area  of  the  outer  can.  The 
depth  of  the  water  in  this  cylinder  is  determined  by  inserting  a  measur- 
ing rod  and  noting  the  height  to  which  it  is  wetted.  The  measure- 
ments are  made  to  the  tenth  of  an  inch,  and  thus  the  amount  of  the 
precipitation  determined  to  the  hundredth. 

It  is  ordinarily  stated  that  the  rain  gauge  should  be  exposed  in  the 
Exposure  of  open  from  three  to  six  feet  above  the  ground,  and  at  a  con- 
a  ram  gauge,  giderable  distance  from  trees,  buildings,  or  any  obstruction. 
The  disadvantage  of  this  exposure  is  that  there  is  no  protection  against 


fforitontal  Section,  JTJT 


Description 
of  the  U.  S. 
Weather 
Bureau  rain 
gauge. 


THE  MOISTURE   IN  THE  ATMOSPHERE  245 

the  wind,  and  the  eddy  caused  by  the  wind  passing  over  and  around  the 
rain  gauge  itself  lessens  the  amount  of  precipitation  which  is  received. 
If  the  rain  gauge  is  exposed  at  a  greater  height,  wind  velocities  are  larger, 
and  the  loss  would  be  still  greater.  A  rain  gauge  should  not  be  exposed 
on  a  roof,  as  it  is  impossible  to  know  what  effect  the  eddies  caused  by  the 
building  will  have  on  the  amount  collected.  Ordinarily  a  roof  exposure 
increases  the  amount  collected  somewhat.  The  rain  gauge  of  the  U.  S. 
Weather  Bureau  stations  are  usually  located  on  the  flat  roofs  of  build- 
ings in  large  cities,  and  thus,  on  account  of  their  location,  the  indica- 
tions of  the  gauges  may  differ  5  or  even  10  per  cent  from  the  correct 
amount. 

Recording  rain  gauges  have  also  been  devised,  and  these  work  either 
on  the  float  or  tipping  bucket  principle.  In  the  case  of  the  float  instru- 
ments, as  the  water  rises  the  float  is  carried  up  and  makes  a  Recording 
record  on  a  revolving  drum.  In  the  case  of  the  tipping  rain  gauges, 
bucket  instruments,  the  bucket  becomes  filled  whenever  a  hundredth 
of  an  inch  of  precipitation  has  fallen.  It  then  tips  over,  brings  another 
bucket  into  place,  empties  itself,  and  makes  a  mark  on  a  revolving 
drum. 

In  addition  to  the  amount,  the  time  of  beginning  and  ending  is  also 
noted  in  connection  with  a  rainfall.     If  the  amount  is  too  other  ot>- 
small  to  measure,  it  is  recorded  as  T  (trace)  in  the  records,  stations. 

248.    The  measurement  of  a  snowfall.  —  Two  measurements  are  made 
in  connection  with  a  snowfall :   one  is  the  depth  of  the  snow  which  has 
fallen,  and  the  other  is  the  water  equivalent  of  the  snow-  Two  meas_ 
fall.     The   depth   is   determined   simply   by   measuring  it  urements 
with  a  measuring  rod.     The  only  difficulty  is  to  find  some  E 
place  where  the  depth  has  not  been  changed  by  drifting.     For  this 
reason    it    is    customary  to  measure    the    depth    at    three    or    four 
different  places  which  seem  to    be  as  free  from  drifting  as  possible, 
and  take  the  average.     The  depth  is  ordinarily  recorded  to  the  tenth 
of  an  inch. 

In  order  to  determine  the  water  equivalent,  the  snow  is  sometimes 
caught  in  the  outer  can  of  the  rain  gauge  when  the  funnel  and  inner 
cylinder  have  been  removed.     It  is  then  melted  down  by  How  thg 
placing  it  in  a  warm  room  or  by  adding  a  known  quantity  water  equiv- 
of  warm  water,  which  is  deducted  as  soon  as  it  has  melted.  Jent  1S. 

determined. 

The  water  is  then  poured  into  the  inner  cylinder  and  meas- 
ured, and  the  water  equivalent  thus  determined.     If  it  is  thought  that 
too  much  snow  has  been  blown  out  of  the  rain  gauge,  another  method 


246  METEOROLOGY 

may  be  used.  A  sample  may  be  taken  by  inverting  the  can  and  pressing 
it  down  in  the  soft  snow  in  some  place  free  from  drifting.  By  passing 
a  piece  of  tin  or  a  shingle  underneath,  a  sample  may  be  secured  and 
melted  down  as  before.  It  would,  of  course,  be  more  accurate  to  repeat 
this  several  times. 

It  has  been  found  that  the  number  of  inches  of  snow  which  corre- 
sponds to  an  inch  of  water  is  by  no  means  constant.     It  requires  all 
Ratio  of         ^ne  way  from  6  to  30  inches  of  snow  to  make  an  inch  of 
snow  to         water,  depending  on  the  lightness  of  the  snow.     The  aver- 
age value,  however,  is  about  ten. 

249.  Observations   of  precipitation.  —  The  observations  which    are 
made  of  precipitation  at   both  the  regular  and  cooperative  stations 
The  obser-     °^  ^ne  ^-  ®-  Weather  Bureau  are  the  same.     These  are  :  the 
vations          kind  of  precipitation;    the  time  of  beginning  and  ending; 

the  amount  of  a  rainfall;    the  depth  and  water  equivalent 
of  a  snowfall ;   the  amount  of  snow  on  the  ground  each  day. 

There  are  also  many  special  stations  which  observe  practically  noth- 
ing else  except  rain  and  snow. 

250.  Normal  values  and  precipitation  data.  —  From  these  observa- 
tions of  precipitation,  three  sets  of  normals  may  be  computed.     These 
The  various    are  •  ^ne  normal  hourly,  daily,  monthly,  and  annual  amount 
normals  of      of  precipitation  (rain  and  melted  snow) ;  the  normal  monthly 

on'  and  annual  amount  of  snowfall ;  the  normal  monthly  and 
annual  number  of  days  with  precipitation.  Since  all  of  these  quanti- 
ties are  mere  numbers,  these  normals  are  computed  in  the  regular 
way. 

The  graph  which  represents  the  daily  variation  in  the  amount  of  pre- 
The  daily  cipitation  is  determined  by  plotting  to  scale  the  normal 
variation.  hourly  precipitations.  Its  form  is  usually  quite  irregular, 
and  is  very  different  for  different  places  and  different  times  of 
the  year. 

251.  The  accompanying  table  gives  the  amount  of  the  precipitation 
(rain  and  melted  snow)  for  the  various  months  and  for  the  year  for 
several  years,  and  also  the  normal  values  for  Albany,  N.Y. 


THE  MOISTURE  IN  THE  ATMOSPHERE 


247 


THE  AMOUNT  OF  PRECIPITATION  FOR  THE  VARIOUS  MONTHS  AND  FOR  THE 
YEAR  FOR  SEVERAL  YEARS,  AND  THE  NORMAL  VALUES,  FOR  ALBANY,  N.Y. 


JAN. 

FEB. 

MAR. 

APR. 

MAY 

JUNE 

JULY 

AUG. 

SEPT. 

OCT. 

Nov. 

DEC. 

YEAR 

1874 

3.61 

2.90 

1.97 

4:97 

2.32 

4.71 

6.78 

1.94 

4.01 

1.77 

2.19 

.76 

37.93 

1875 

2.14 

1.65 

3.27 

3.63 

2.57 

3.98 

2.46 

6.55 

2.63 

5.97 

2.29 

1.11 

38.25 

1876 

1.57 

4.09 

4.28 

3.51 

2.96 

4.40 

4.97 

.53 

5.17 

1.64 

2.65 

2.42 

38.19 

1877 

1.95 

.36 

3.33 

1.42 

2.77 

4.60 

4.00 

4.57 

1.82 

7,86 

2.70 

.71 

36.09 

1878 

4.45 

4.12 

2.18 

3.99 

3.65 

4.54 

5.52 

3.76 

3.20 

3.37 

4.43 

6.16 

49.37 

1879 

2.54 

2.80 

3.79 

3.17 

.89 

4.62 

5.10 

4.25 

3.47 

1.24 

2.56 

4.23 

38.66 

1880 

2.96 

2.67 

2.17 

2.75 

3.35 

2.21 

3.78 

2.84 

2.86 

2.45 

2.49 

2.01 

32.54 

1881 

2.86 

2.50 

3.80 

1.34 

3.90 

3.76 

2.22 

2.07 

2.38 

3.19 

3.44 

4.88 

36.34 

1882 

2.64 

3.31 

1.79 

1.27 

4.15 

3.98 

3.Q7 

1.38 

7.79 

.27 

.97 

2.24 

33.76 

1883 

2.43 

3.00 

1.77 

2.65 

3.20 

6.30 

5.96 

3.69 

3.19 

3.49 

1.14 

2.55 

39.37 

1884 

2.98 

3.85 

4.00 

2.09 

2.79 

1.80 

5.04 

5.27 

1.80 

2.64 

3.44 

3.20 

38.90 

1885 

3.09 

1.38 

.62 

2.89 

1.92 

1.98 

1.98 

7.58 

2.00 

5.54 

3.90 

1.51 

34.39 

1886 

3.66 

1.40 

2.73 

3.67 

3.40 

3.19 

2.56 

.87 

2.51 

2.43 

5.40 

2.19 

34.01 

1887 

3.02 

2.86 

2.90 

2.49 

2.27 

2.99 

4.61 

4.61 

1.94 

2.22 

4.36 

5.43 

39.70 

1888 

3.04 

2.07 

5.62 

1.95 

2.98 

3.18 

2.52 

4.74 

4.68 

6.10 

4.48 

3.30 

44.66 

1889 

2.83 

1.81 

1.76 

1.25 

3.32 

6.43 

4.19 

3.63 

3.68 

3.48 

5.00 

2.14 

39.51 

1890 

2.28 

2.52 

3.72 

1.64 

5.19 

2.72 

2.37 

5.66 

8.91 

5.76 

1.18 

2.94 

44.89 

1891 

6.12 

4.14 

3.12 

2.27 

1.69 

2.65 

6.11 

5.88 

1.94 

2.13 

2.40 

3.23 

41.68 

1892 

4.08 

2.13 

1.64 

.56 

5.30 

4.41 

4.22 

6.70 

2.08 

.60 

2.29 

.82 

34.83 

1893 

1.31 

4.63 

2.00 

2.10 

5.08 

2.92 

1.82 

7.21 

3.20 

1.67 

.91 

2.54 

35.39 

1894 

2.54 

2.61 

.85 

2.02 

4.64 

3.29 

2.96 

2.26 

4.18 

4.62 

1.96 

3.18 

35.11 

1895 

1.65 

1.63 

1.31 

3.09 

1.72 

1.72 

4.02 

3.14 

1.80 

2.35 

4.78 

2.59 

29.80 

1896 

.98 

4.03 

4.66 

.98 

1.55 

2.49 

3.57 

2.25 

3.31 

1.53 

1.80 

.73 

27.88 

1897 

1.62 

2.05 

1.85 

3.12 

4.69 

4.45 

6.67 

4.43 

1.87 

1.01 

4.65 

4.38 

40.79 

1898 

2.96 

3.57 

1.08 

2.63 

4.07 

5.58 

1.07 

6.67 

1.61 

4.29 

4.22 

.02 

38.77 

1899 

2.50 

2.92 

3.97 

1.03 

2.23 

1.61 

2.69 

1.77 

6.33 

.85 

1.47 

.55 

28.92 

1900 

2.33 

2.84 

4.62 

1.31 

1.36 

3.54 

3.41 

2.83 

.74 

1.83 

3.95 

.80 

30.56 

1901 

1.59 

.56 

4.14 

4.66 

4.79 

3.14 

4.26 

4.51 

2.96 

2.48 

2.02 

.42 

40.53 

1902 

.67 

3.04 

3.24 

2.33 

1.91 

3.91 

5.37 

3.98 

4.15 

2.80 

.95 

.13 

37.48 

1903 

1.83 

2.05 

3.55 

.79 

.15 

6.44 

3.51 

5.14 

1.30 

6.09 

1.65 

.59 

34.09 

1904 

2.51 

1.17 

1.94 

2.87 

2.16 

5.48 

2.96 

2.78 

3.88 

3.09 

.64 

.78 

31.26 

1905 

2.66 

.80 

2.43 

2.12 

.96 

3.58 

2.00 

3.83 

3.37 

2.38 

1.49 

.36 

26.98 

1906 

.97 

2.09 

2.54 

2.20 

3.90 

5.80 

3.91 

2.49 

1.57 

2.62 

2.46 

1.96 

32.51 

1907 

1.71 

1.15 

.87 

2.33 

3.21 

3.29 

4.14 

.74 

5.81 

3.71 

4.15 

2.52 

33.63 

1908 

1.36 

2.77 

1.43 

2.62 

4.26 

2.32 

5.33 

3.63 

.64 

2.07 

.40 

1.58 

28.41 

Sums 

87.43 

87.47 

94.94 

83.71 

105.30 

132.01 

136.05 

134.18 

112.78 

105.54 

94.81 

90.96 

1265.18 

Normals 

2.50 

2.50 

2.71 

2.39 

3.01 

3.77 

3.89 

3.83 

3.22 

3.02 

2.71 

2.60 

36.15 

It  will  be  seen  that  the  various  months  may  depart  widely  from  normal, 
as  the  amount  varies  all  the  way  from  almost  nothing  to  more  than 
twice  the  normal  amount.  In  the  case  of  the  annual 

The  annual 

amount  a  departure  of  20  per  cent  from  normal   is   not  variation  in 
uncommon.      A    departure    of    three    inches    is    common, 
of  six  inches  is  unusual,  and  of  eleven  inches  is  generally 
record    breaking.      The  maximum  occurs  in  July  and   the  minimum 
in  February.     The  reason  for  this  is  the  large  amount  of  rain  which 
falls  during  the  summer  thunder  showers.     Similar  data  can  be  com- 
puted for  every  station. 


248 


METEOROLOGY 


252.    The  accompanying  tables  give  the  normal  monthly  and  annual 
precipitation  for  various  stations  in  the  United  States  and  the  world. 

NORMAL  PRECIPITATION  IN  INCHES  FOR  THE  VARIOUS  MONTHS  AND  THE 

YEAR 


STATION 

-  K 

| 

1 

i 

| 

h 

S 

1 

£ 

1 

i 

1 

o 
fe 

Q 

ANNUAL 

Albany,  N.Y  

84 

2.61 

2.47 

2.75 

2.67 

3.50 

4.03 

4.12 

3.82 

3.37 

3.41 

2.97 

2.67 

38.39 

Atlanta  Ga 

44 

478 

5  05 

5  57 

407 

3  37 

3  96 

4.30 

4.47 

3.52 

2.31 

3.43 

4.64 

49  47 

Baltimore,  Md.    .     .     . 

39 

3.23 

3.54 

3.91 

3.15 

3.53 

3.79 

4.83 

4.20 

3iS6 

2.96 

43'.06 

Bismarck,  N.  Dak.  .    . 

34 

0.53 

0.53 

1.06 

1.79 

2.43 

3.35 

2.19 

1.95 

1.17 

1.04 

0^67 

0.59 

17.50 

Boise,  Idaho     .... 

32 

1.85 

1.51 

1.54 

1.16 

1.49 

0.96 

0.18 

0.18 

0.48 

1.20 

1.07 

1.64 

13.27 

Boston,  Mass  

91 

3.74 

3.51 

4.14 

3.81 

3.66 

8.14 

3.51 

4.15 

3.44 

3.71 

4.11 

3.78 

44.70 

Buffalo  NY         ... 

53 

3.07 

2.94 

2.90 

2.48 

3.18 

2  99 

3.20 

2  98 

3  34 

348 

3  25 

3  38 

37.19 

Charleston,  S.C.  .    .    . 

120 

3.08 

3.08 

3.31 

2.40 

3.41 

5.28 

6.22 

6.65 

5.22 

3.90 

2.70 

3.34 

48.59 

Chattanooga,  Tenn. 

31 

5.24 

5.06 

5.95 

4.36 

3.79 

4.05 

3.69 

3.88 

3.43 

2.72 

3.42 

4.43 

50.07 

Cheyenne,  Wyo.  .     .     . 

40 

0.38 

0.54 

0.98 

1.73 

2.48 

1.65 

2.06 

1.47 

1.00 

0.71 

0.41 

0.39 

13.80 

Chicago,  111  

39 

2.08 

2.30 

2.59 

2.72 

3.63 

3.52 

3.62 

3.02 

3.06 

2.43 

2.52 

2.05 

33.54 

Cincinnati,  Ohio  .    .    . 

75 

3.27 

3.18 

3.77 

3.20 

3.96 

4.20 

3.77 

3.61 

2.79 

2.59 

3.19 

3.38 

40.91 

Cleveland,  Ohio    .     .    . 

38 

2.53 

2.68 

2.84 

2.25 

3.27 

3.58 

3.64 

2.94 

3.28 

2.73 

2.59 

2.57 

34.90 

Columbus,  Ohio  .     .    . 

34 

2.97 

3.01 

3.49 

2.84 

3.80 

3.41 

3.65 

3.21 

2.41 

2.32 

2.91 

2.66 

36.68 

Davenport,  Iowa      .    . 

39 

1.66 

1.58 

2.24 

2.68 

4.26 

4.06 

3.63 

3.73 

3.15 

2.29 

1.83 

1.53 

32.64 

Denver,  Col.    .    ..    .    . 

37 

0.48 

0.47 

0.96 

2.09 

2.57 

1.46 

1.64 

1.35 

0.90 

0.97 

0.56 

0.60 

14.05 

El  Paso,  Tex.  .... 

55 

0.42 

0.47 

0.29 

0.19 

0.28 

0.59 

1.67 

1.88 

1.64 

0.87 

0.57 

0.44 

9.31 

Erie   Pa 

35 

2  99 

2  90 

2  68 

2  43 

3.54 

3.76 

3  10 

3.11 

3  54 

3.68 

3.50 

2.92 

38.15 

Flagstaff,  Ariz.     .    .     . 

21 

2.91 

2.62 

2.90 

1.33 

1.59 

0.47 

2.45 

2.89 

1.43 

1.18 

1.61 

2'.49 

Galveston,  Tex.    .     .    . 

37 

3.54 

3.05 

2.99 

3.11 

3.29 

4.32 

3.96 

4.79 

5.84 

4.35 

4.03 

3.75 

46.78 

H&VTG   IVIont/. 

30 

07S 

0  50 

0  54 

0  89 

2  04 

?  93 

1  89 

1  21 

1  06 

0  62 

0  68 

0  54 

13.63 

Helena,  Mont.      .    .    . 

29 

1.00 

0.66 

0.83 

1.06 

2.13 

2.26 

1.13 

0.69 

1.13 

0.77 

0.76 

0.79 

Indianapolis,  Ind.     .    . 

38 

2.96 

3.09 

4.04 

3.39 

4.00 

4.23 

4.06 

3.21 

2.99 

2.66 

3.48 

2.98 

4L09 

Jacksonville,  Fla.      .    . 

56 

2.82 

3.25 

3.39 

2.70 

3.93 

5.64 

6.37 

6.60 

8.16 

4.60 

8.81 

2.86 

52.53 

Kansas  Citv,  Mo.     .     . 

38 

1.23 

1.68 

2.40 

3.13 

4.59 

4.93 

4.47 

4.31 

3.71 

3.06 

1.96 

1.36 

36.94 

Key  West,  Fla.     .     .    . 

72 

2.02 

1.58 

1.63 

1.80 

3.06 

4.65 

3.56 

4.89 

6.49 

5.11 

2.13 

1.94 

38.26 

Lincoln,  Neb.       .     .    . 

34 

0.66 

0.95 

1.23 

2.60 

4.49 

4.61 

4.31 

3.48 

2.53 

2.08 

0.79 

0.70 

28.43 

Los  Angeles,  Cal.      .    . 

32 

3.03 

3.00 

3.05 

1.02 

0.48 

0.08 

0.01 

0.03 

0.10 

0.75 

1.33 

2.85 

15.75 

Madison,  Wis.      .    .    . 

51 

1.63 

1.50 

2.08 

2.54 

3.66 

4.01 

3.80 

3.15 

3.08 

2.32 

1.76 

1.72 

31.25 

Memphis,  Tenn.       .    . 

49 

4.99 

4.63 

5.17 

5.07 

4.34 

4.38 

3.39 

3.33 

2.94 

2.55 

4.44 

4.30 

50.22 

Milwaukee,  Wis.       .    . 

39 

1.63 

1.50 

2.08 

2.54 

3.66 

4.01 

3.80 

3.15 

3.08 

2.32 

1.76 

1.72 

31.25 

Minneapolis,  Minn. 

47 

1.04 

0.96 

1.69 

2.46 

3.63 

4.21 

3.34 

3.61 

3.56 

2.14 

1.40 

1.31 

29.35 

Mobile  Ala           .    . 

38 

480 

545 

7.17 

4.48 

4.24 

5.60 

6  68 

6.90 

5.12 

3.  15 

3  65 

456 

61  80 

New  Orleans,  La.     .    . 

64 

454 

4?8 

4  56 

4  53 

4  06 

*>  39 

6  53 

5  65 

449 

1  95 

381 

454 

55.63 

New  York,  N.Y.       .     . 

84 

3.29 

3.27 

3.45 

3.33 

3.55 

3.41 

4.08 

4.38 

3.44 

3.42 

3.55 

3.30 

42.47 

Omaha,  Neb.        .     .    . 

51 

0.68 

0.81 

1.36 

2.96 

4.35 

5.17 

4.43 

3.45 

2.95 

2.47 

1.00 

0.89 

30.46 

Philadelphia,  Pa.      .    . 

39 

3.23 

3.35 

3.43 

2.92 

3.30 

3.27 

4.14 

4.69 

3.36 

3.01 

3.11 

3.07 

40.88 

Phoenix,  Ariz.       .     .     . 

32 

0.85 

0.87 

0.61 

0.33 

0.11 

0.08 

0.89 

0.94 

0.69 

0.37 

0.67 

0.86 

7.27 

Portland,  Me.       .     .     . 

44 

3.76 

3.45 

3.89 

3.23 

3.56 

3.27 

3.47 

3.55 

3.31 

3.70 

3.67 

3.77 

42.63 

Portland,  Ore.      .     .    . 

60 

6.32 

4.96 

4.74 

2.94 

2.38 

1.77 

0.75 

4.58 

1.69 

3.11 

6.00 

7.21 

42.45 

St.  Louis,  Mo.      .    .     . 

75 

2.27 

2.64 

3.61 

3.61 

4.53 

4.83 

3.69 

3.48 

3.00 

2.83 

2.99 

2.62 

40.10 

St.  Paul,  Minn.    .    .    . 

73 

0.89 

0.79 

1.44 

2.41 

3.44 

4.10 

3.53 

3.47 

3.39 

1.99 

1.38 

0.97 

27.80 

Salt  Lake  City,  Utah  . 

36 

1.33 

1.48 

2.04 

2.07 

2.16 

0.77 

0.50 

0.850.91 

1.43 

1.39 

1.40 

16.33 

San  Francisco,  Cal. 

61 

4.82 

3.63 

3.32 

1.68 

0.74 

0.02 

0.02 

0.O20.31 

1.03 

2.62 

4.64 

22.96 

Seattle,  Wash.      .    .    . 

20 

4.42 

3.97 

3.19 

2.66 

2.22 

1.56 

0.68 

0.49  1.91 

2.70 

5.94 

5.94 

35.68 

Tampa,  Fla  

56 

2.56 

2.88 

2.76 

1.87 

2.73 

7.58 

9.36 

9.026.32 

2.41 

1.71 

2.29 

51.49 

Topeka,  Kan.       .    .    . 

22 

1.21 

1.50 

2.15 

2.53 

5.09 

4.78 

4.79 

4.57  3.33 

2.01 

1.27 

0.84 

34.07 

Washington,  D.C.    .    . 

73 

3.13 

3.09. 

3.47 

3.27 

3.71 

3.74 

4.34 

4.08  3.25 

3.12 

2.59 

3.01 

40.80 

Yuma,  Ariz  

38 

0.42 

0.53 

0.35 

0.08 

0.03 

T 

0.15 

0.49 

0.17 

0.21 

0.29 

0.41 

3.13 

The  year  1908  is  the  last  one  included  in  these  normals.    T  signifies  trace  —  less  than  one 
one  hundredth  of  an  inch. 


THE  MOISTURE   IN  THE  ATMOSPHERE 


249 


STATION 

1  LENGTH  OF 
RECORD 

5 

i 

a 
§ 

•5 

S 

"-S 

1 

1 

£ 

1 

1 

i 

1 

St.  Petersburg 

66 

0.87 

0.83 

0.91 

0.94 

1.69 

1.81 

2.68 

2.72 

2.01 

1.69 

1.42 

1.18 

16.77 

Stockholm  .     . 

35 

0.79 

0.71 

0.79 

0.91 

1.38 

1.65 

2.28 

2.44 

1.81 

1.97 

1.38 

1.10 

17.21 

London  .     .     . 

40 

2.01 

1.61 

1.69 

1.65 

1.93 

2.24 

2.40 

2.40 

2.40 

2.72 

2.28 

2.13 

25.47 

Berlin     .     .     . 

30 

1.54 

1.46 

1.85 

1.38 

1.73 

2.48 

2.72 

2.24 

1.65 

2.01 

1.85 

1.93 

22.84 

Vienna    .     .     . 

30 

1.34 

1.46 

2.01 

1.97 

2.83 

2.80 

2.64 

2.68 

1.65 

2.01 

1.81 

1.89 

25.08 

Constantinople 

48 

3.43 

2.72 

2.44 

1.65 

1.18 

1.34 

1.06 

1.65 

2.05 

2.52 

4.02 

4.80 

28.86 

Athens    .     .     . 

37 

2.20 

1.50 

1.46 

0.87 

0.83 

0.43 

0.32 

0.43 

0.55 

1.77 

2.99 

2.48 

15.83 

Jerusalem  .     . 

32 

6.38 

5.08 

3.54 

1.73 

0.28 

0.00 

0.00 

0.00 

0.04 

0.39 

2.28 

5.51 

25.24 

Paris       .     .     . 

30 

1.42 

1.30 

1.50 

1.69 

1.77 

2  13 

2.05 

2  13 

1  97 

2.40 

1.77 

1.81 

21  93 

Rome      .     .     . 

55 

2.87 

2.32 

2.48 

2.32 

2.17 

1.50 

0.63 

1.10 

2.72 

4.09 

4.46 

3.27 

29^92 

Capetown  .    . 

43 

0.67 

0.63 

0.95 

1.85 

3.90 

4.41 

3.50 

3.31 

2.17 

1.61 

1.10 

0.79 

24.88 

Adelaide      .     . 

50 

0.75 

0.67 

0.98 

1.85 

2.95 

2.99 

2.76 

2.48 

1.93 

1.73 

1.14 

0.91 

21.14 

Peking    .     .     . 

37 

0.12 

0.20 

0.24 

0.63 

1.42 

3.03 

9.45 

6.34 

2.56 

0.63 

0.28 

0.08 

24.96 

Hong-kong 

20 

1.34 

1.85 

2.64 

5.55 

13.43 

16.77 

13.31 

14.21 

8.19 

4.72 

1.69 

1.02 

84.72 

Tokyo    .     .     . 

2.17 

2.95 

4.37 

5.04 

5.91 

6.54 

5.16 

4.29 

7.99 

7.28  4.29 

*./£68.11 

Manila   .     .     . 

38 

1.14 

0.39 

0.75 

1.10 

4.02 

9.76 

15.00 

14.21 

14.76 

7.56 

5.35 

2.28 

76.34 

Bombay      .     . 

84 

0.12 

o.oo 

0.00 

0.04 

0.55 

20.55 

24.57 

14.88 

10.94 

1.77 

0.47 

0.04 

73.94 

Havana  .     .     . 

30 

2.72 

2.28 

1.81 

2.83 

4.49 

7.17 

5.04 

6.02 

6.69 

7.40 

3.07 

2.17 

51.69 

Rio  de  Janeiro 

40 

4.69 

4.33 

5.39 

4.57 

3.62 

1.85 

1.61 

1.85 

2.28 

3.07 

4.25 

5.43 

42.95 

Buenos  Aires  . 

40 

2.91 

2.60 

4.61 

2.83 

2.99 

2.80 

2.17 

2.32 

3.11 

3.62 

2.87 

3.90 

36.73 

New  York  42.47 


Seattle  35.68 


Chicago  33.54 


Bismarck  17.50 


n 


l. 


fc 


St.  Louis  40.10 


Washington  40.80 


Denver  14.05 


II 


San  Francisco 
22.96 


FIG.  108.  —  The  Annual  Variation  in  the  Amount  of  Precipitation  at  12  Stations  in  the  U.  S. 


250 


METEOROLOGY 


Rome  29.92 


London  25.47  St.Petersburg  16.77 


Jerusalem  25.24 


^11 


jLflllM  ?  i  s§l 


Bombay  73.94 


Manila  70.34 


Peking  24.96 


Tokio58.11 


10 


left! lils 


FIG.   109.  —  The  Annual  Variation  in  the  Amount  of  Precipitation  at  8  Foreign  Stations. 


In  Figs.  108  and  109  some  of  these  values  are  shown  graphically.      It 

will  be  seen  at  once  that  the  annual  variation  is  very  different 

monthly  and  at  different  places.     The  chief  factors  which  determine  the 

annual  pre-     amOunt  of  precipitation  and  its  distribution  throughout  the 

year  are  :  the  general  wind  system  ;  the  temperature  changes 

and  also  the  contrast  in  temperature  between  the  place  in  question  and 

these  regions  from  which  the  prevailing  winds  come ;  eleva- 

which  deter-  tion  and  inclosure  by  mountains ;     nearness  to  bodies  of 

mine  the        water;    the  characteristics  of  the  storms  which  occur.     At 

distribution      n  .    ..'     . 

ofprecipita-  San  Francisco,  for  example,  the  precipitation  occurs  nearly 
a^  ^urmS  tne  winter,  and  almost  no  rain  falls  during  the 
summer.  San  Francisco  is  located  in  the  region  of  prevail- 
ing westerlies,  and  furthermore,  a  continent  is  warmer  than  the 
San  Fran-  adjoining  ocean  during  the  summer  and  colder  in  winter. 
cisco.  Thus,  during  the  winter  we  have  the  moisture-laden 

prevailing    westerlies    blowing    from    the    warmer    ocean    over    the 
colder    land.      Thus,    condensation    occurs,     and    the    precipitation 


THE  MOISTURE   IN  THE  ATMOSPHERE 


251 


is  copious.  During  the  summer,  the  prevailing  westerlies  are  blowing 
from  a  colder  ocean  over  a  warmer  land.  As  a  result,  there  is  practi- 
cally no  precipitation.  At  New  York  the  precipitation  is 

,       ,  . .     ,       ,  .  New  York. 

caused  almost  entirely  by  storms,  extratropical  cyclones, 
and  thundershowers,  and  as  a  result,  the  distribution  throughout  the 
year  is  quite  uniform.  The  maximum  occurs  during  the  summer  when 
thundershowers  are  most  prevalent.  Thus  by  considering  these  dif- 
ferent factors,  the  amount  of  precipitation  and  its  distribution  through- 
out the  year  can  be  explained. 

253.   The  accompanying  table  gives  for  Albany,  N.Y.,  the  amount 
of  snowfall  for  the  various  months  and  for  the  year  for  several  , 

Snowfall  at 

years,  and  also  the  normal  values.  Albany. 

SNOWFALL  FOR  THE  VARIOUS  MONTHS  AND  FOR  THE  YEAR  FOR  SEVERAL 
YEARS,  AND  THE  NORMAL  VALUES,  FOR  ALBANY,  N.Y. 


JAN. 

FEB. 

MAR. 

APR. 

MAY 

JUNE 

JULY 

AUG. 

SEPT. 

OCT. 

Nov. 

DEC. 

YEAR 

1886 

19.7 

24 

8.0 

1.1 

0 

0 

0 

0 

0 

0 

22.3 

11.1 

64.6 

1887 

20.5 

15.4 

22.2 

3.5 

0 

0 

0 

0 

0 

0 

1.8 

27.2 

90.6 

1888 

20.9 

9.2 

50.9 

T 

0 

0 

0 

0 

0 

T 

9.5 

3.0 

93.5 

1889 

12.2 

4.0 

T 

0.2 

0 

0 

0 

0 

0 

0 

T 

6.0 

224 

1890 

5.0 

5.2 

11.0 

0 

0 

0 

0 

0 

0 

0 

0 

25.9 

47.1 

1891 

26.0 

10.0 

2.0 

11.0 

T 

0 

0 

0 

0 

T 

1.0 

1.0 

51.0 

1892 

10.0 

9.0 

5.2 

T 

0 

0 

0 

0 

0 

0 

T 

1.0 

25.2 

1893 

8.1 

40.7 

3.0 

4.8 

0 

0 

0 

0 

0 

T 

0.2 

12.4 

69.2 

1894 

14.9 

24.7 

1.0 

2.5 

0 

0 

0 

0 

0 

T 

8.6 

24.4 

76.1 

1895 

13.5 

15.6 

6.3 

T 

T 

0 

0 

0 

0 

T 

2.2 

T 

37.6 

1696 

7.1 

15.3 

23.8 

3.2 

0 

0 

0 

0 

0 

0 

0.1 

5.0 

54.5 

1897 

11.8 

9.4 

2.6 

T 

0 

0 

0 

0 

0 

0 

4.5 

9.6 

37.9 

1898 

19.8 

23.5 

2.0 

1.2 

0 

0 

0 

0 

0 

0 

10.3 

7.3 

64.1 

1899 

9.4 

27.5 

20.8 

T 

0 

0 

0 

0 

0 

T 

T 

'/.2 

'59.9 

1900 

7.9 

9.6 

22.8 

T 

T 

0 

0 

0 

0 

0 

14.4 

1.5 

56.2 

1901 

13.4 

4.2 

7.0 

T 

0 

0 

0 

0 

0 

0 

6.0 

21.6 

52.2 

1902 

2.1 

16.8 

9.1 

T 

T 

0 

0 

0 

0 

T 

1.8 

33.5 

63.3 

1903 

4.8 

9.3 

2.6 

1.0 

T 

0 

0 

0 

0 

0.3 

T 

12.5 

30.5 

1904 

20.8 

8.2 

11.1 

5.0 

0 

0 

0 

0 

0 

T 

6.2 

12.5 

63.8 

1905 

18.2 

7.8 

8.2 

2.0 

0 

0 

0 

0 

0 

T 

0.4 

2.1 

38.7 

1906 

2.5 

16.6 

15.8 

6.7 

0 

0 

0 

0 

0 

T 

10.4 

6.7 

58.7 

1907 

7.8 

10.8 

2.1 

8.5 

0.5 

0 

0 

0 

0 

0.1 

6.5 

10.6 

46.9 

1908 

5.1 

20.6 

5.6 

0.4 

0 

0 

0 

0 

0 

T 

1.7 

8.7 

42.1 

Suma 

281.5 

315.8 

243.1 

51.1 

0.5 

0 

0 

0 

0 

0.4 

107.9 

245.8 

1246.1 

Normals 

12.2 

13.7 

10.6 

2.2 

T 

0 

0 

0 

0 

T 

4.7 

10.7 

54.2 

It  will  be  seen  that  the  monthly  amounts  may  depart  widely  from  nor- 
mal, as  the  amount  varies  all  the  way  from  practically  nothing  Normal 
to  more  than  twice  the  normal  amount.     In  the  case  of  the  snowfa11- 
annual  amount,  a  departure  of  40  per  cent  from  normal  is  not  unusual. 


252 


METEOROLOGY 


In  the  accompanying  table  are  given  the  normal  monthly  and  annual 
amount  of  snowfall  for  several  stations  in  the  United  States. 


THE  NORMAL  AMOUNT  OF  SNOWFALL  IN  INCHES  FOR  THE  VARIOUS 
MONTHS  AND  FOR  THE  YEAR 


STATION 

LENGTH  OF 
RECORD 

fc 

n 

H 

K 
5 

i 

1.2 

2.5 
2.2 
2.7 
3.8 
0.0 
0.8 
1.3 
9.9 
0.0 
0.0 
2.0 
5.8 
1.0 
0.0 
0.9 
0.2 
0.3 
0.0 
4.0 
T 
0.8 
3.6 
2.5 
0.0 
0.8 
0.4 

(M 

§ 

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*  Albany,  N.Y.      .    .    . 
*  Bismarck,  N.  Dak.      . 
*  Boise,  Idaho    .... 
Boston,  Mass  
*  Buffalo,  N.Y  
*  Charleston,  S.C.      .    . 
*  Chicago  111          ... 

19 
29 
5 
36 
33 
19 
19 
24 
25 
20 
33 
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39 
19 
33 

8 
35 
37 
20 
24 
30 
32 
21 
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4.9 
6.4 
11.9 
16.4 
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11.5 
5.1 
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Columbus,  Ohio    .     .     . 
Denver,  Col  
*  El  Paso,  Tex.      .    .     . 
*  Galveston,  Tex.  .    .     . 
Havre   Mont     .... 

Helena,  Mont  
Indianapolis,  Ind.      .    . 
New  Orleans,  La.       .     . 
*  New  York,  N.Y.      .     . 

*  Omaha,  Neb.       .     .     . 
Philadelphia,  Pa.  .     .     . 
*  Phrenix,  Ariz.      .     .     . 
Portland   Me 

Portland,  Ore  
*  St.  Louis,  Mo.     .     .     . 
St.  Paul,  Minn.      .     .     . 
*  Salt  Lake  City,  Utah  . 
*  San  Francisco,  Cal. 
Topeka  Kan 

Washington,  D.C.      .     . 

T  indicates  a  trace  ;  less  than  one  tenth  of  an  inch.     Ordinarily  the  last  year  included  in 
the  normals  is  1908.    If  the  station  has  a  *  the  last  year  is  1903. 

254.   In  the  accompanying  table  will  be  found  the  normal  number 

of  days  with  precipitation  for  the  various  months  and  for 

Normal          the  year,  for  several  stations  in  the  United  States.     It  will  be 

daysbwith       seen  ^na^  ^or  ^ne  northeastern  part  of  the  country,  precipita- 

precipitatioa  tion  occurs  on  nearly  half  of  the  days  of  a  year. 


THE  MOISTURE  IN  THE  ATMOSPHERE 


253 


THE  NORMAL  NUMBER  OF  DATS  WITH  PRECIPITATION  FOR  THE  VARIOUS 
MONTHS  AND  FOR  THE  YEAR 


h 

0  D 

j 

STATION 

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JA 

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1 

t> 

H^ 

o 
p 

<J 

H 
CQ 

I 

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£ 

0 

5 

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t> 

z 

B 

<! 

Albany,  N.Y.       .     . 

31 

13 

12 

13 

11 

13 

13 

13 

11 

10 

10 

12 

12 

143 

Bismarck,  N.  Dak.  . 

34 

7 

8 

8 

8 

11 

12 

10 

8 

6 

6 

7 

7 

98 

Boise,  Idaho    .     .     . 

28 

11 

10 

10 

7 

8 

6 

2 

3 

7 

8 

11 

85 

Boston,  Mass.      .     . 

36 

12 

11 

13 

11 

11 

10 

11 

10 

9 

10 

11 

11 

130 

Buffalo,  N.Y.       .     . 

33 

19 

17 

16 

12 

13 

11 

11 

10 

11 

13 

19 

18 

170 

Charleston,  S.C.  .     . 

38 

10 

10 

10 

8 

9 

11 

12 

13 

10 

8 

8 

9 

118 

Chicago,  111.     .    .    . 

38 

11 

11 

12 

11 

12 

11 

10 

9 

9 

9 

10 

11 

126 

Columbus,  Ohio  .    . 

30 

15 

13 

14 

12 

13 

12 

11 

10 

9 

9 

11 

14 

142 

Denver,  Col.    .     .     . 

37 

4 

5 

7 

9 

11 

7 

9 

9 

5 

5 

4 

5 

80 

El  Paso,  Tex.  .    .    . 

25 

3 

3 

2 

1 

2 

4 

8 

8 

6 

4 

3 

3 

47 

Galveston,  Tex.  .    . 

33 

11 

10 

9 

7 

e 

7 

9 

10 

10 

7 

8 

11 

105 

Havre,  Mont.      .     . 

19 

8 

7 

7 

6 

9 

10 

7 

7 

6 

6 

6 

7 

85 

Helena,  Mont.     .     . 

28 

9 

7 

8 

8 

12 

12 

8 

5 

7 

6 

7 

8 

97 

Indianapolis,  Ind 

38 

13 

11 

13 

12 

13 

12 

10 

9 

8 

9 

11 

12 

133 

Key  West,  Fla.    .     . 

38 

4 

7 

5 

4 

8 

12 

13 

13 

16 

13 

8 

7 

110 

Los  Angeles  Cal. 

36 

New  Orleans,  La.     . 

39 

11 

10 

9 

7 

8 

13 

15 

14 

11 

6 

8 

10 

122 

New  York,  N.Y.      . 

33 

12 

11 

13 

11 

11 

11 

13 

10 

9 

10 

10 

11 

132 

Omaha,  Neb.  .     .    . 

39 

7 

7 

8 

10 

12 

11 

10 

8 

8 

7 

5 

7 

100 

Philadelphia,  Pa. 

12 

11 

13 

11 

12 

10 

12 

11 

9 

9 

9 

10 

129 

Phoenix,  Ariz.       .     . 

8 

4 

3 

3 

1 

1 

1 

5 

6 

4 

2 

2 

2 

34 

Portland,  Me.      .    . 

35 

12 

11 

13 

11 

12 

11 

12 

11 

10 

10 

11 

11 

135 

Portland,  Ore.      .     . 

35 

20 

17 

18 

15 

14 

11 

4 

4 

8 

13 

17 

20 

164 

St.  Louis,  Mo.     .    . 

38 

9 

9 

11 

11 

12 

11 

10 

8 

7 

7 

9 

10 

114 

St.  Paul,  Minn.   .     . 

38 

9 

8 

10 

10 

12 

12 

10 

10 

9 

9 

8 

9 

114 

Salt  Lake  City,  Utah 

36 

10 

9 

10 

9 

8 

5 

4 

6 

4 

7 

7 

10 

89 

San  Francisco,  Cal. 

12 

11 

11 

7 

4 

2 

0 

2 

5 

6 

10 

71 

Seattle,  Wash.     .    . 

19 

17 

16 

14 

14 

10 

5 

4 

9 

12 

17 

19 

156 

Topeka,  Kan.       .     . 

21 

6 

8 

8 

10 

12 

11 

9 

9 

8 

7 

6 

6 

100 

Washington,  D.C.    . 

38 

12 

11 

12 

11 

12 

11 

12 

11 

8 

9 

10 

10 

129 

The  year  1908  is  the  last  one  included  in  the  normals. 

255.   In  addition  to  the  various  normals  which  have  been  fully  dis- 
cussed, the  following  precipitation  data  may  be  computed  Precipita- 
from  the  observations  taken.  tion  data< 

(1)  Unusual  amounts  of  precipitation.  It  is  customary  at  various 
stations  to  note  the  largest  amount  of  precipitation  which  has  fallen 
during  a  given  time  interval  for  each  month,  or  for  the  year,  or 
for  the  whole  period  during  which  observations  have  been  taken. 
The  various  time  intervals  chosen  for  which  to  note  the  largest  amount 
of  precipitation  are  usually  five  minutes,  ten  minutes,  one  hour,  one 
day,  or  one  rainfall.  A  number  of  these  record  precipitations  are 
mentioned  in  GREELY'S  American  Weather. 


254  METEOROLOGY 

(2)  Number  of  consecutive  days  without  rain.     Unless  it  is  longer 
than  three  weeks,  it  is  usually  not  noted. 

(3)  Number  of  consecutive  days  with  rain  every  day. 

(4)  Duration  of  a  rainfall  —  long  continued  precipitation. 

256.  At  the  regular  stations  of  the  U.  S.  Weather  Bureau  the  follow- 
ing tables  are  kept  constantly  filled  out  and  computed  to  date  : 

us        ^  Precipitation  (inches  and  hundredths)  and  departure 
Weather     '  from  normal. 
Bureau  /2)  Greatest  in  24  hours ;  amount  and  date. 

stations. 

(3)  One  inch  an  hour  and  over ;  amount  and  date. 

(4)  Number  of  days  with  .01  inch  and  over ;  .04  inch  and  over. 

(5)  Number  of  days  with  .25  inch  or  over ;   1.00  inch  or  more. 

(6)  Total  snowfall  (inches  and  tenths) ;  number  of  days  with  snow. 

(7)  Greatest    snowfall  in  24  hours   (inches  and  tenths) ;    depth  on 
ground  at  end  of  month. 

(8)  Greatest  depth  of  snow  on  ground  and  date  (inches  and  tenths). 

(9)  Daily  precipitation  (inches  and  hundredths). 

(10)  Daily  snowfall  (inches  and  tenths). 

The  first  eight  are  kept  for  the  various  months  and  the  year ;  the  last 
two  are  daily. 

THE  DISTRIBUTION  AND  EFFECTS  OF  PRECIPITATION 

257.  Geographical  distribution  of  precipitation.  —  The  various  nor- 
mals of  precipitation  have  been  well  determined  at  many  stations  both 

in  this  country  and  other  parts  of  the  world.  There  are  still, 
annuai°pre-  however,  large  areas  for  which  only  scanty  material  is  avail- 
cipitation  able,  and,  in  the  case  of  the  oceans,  definite  observational 
world6  material  is  almost  entirely  lacking  except  at  island  stations. 

For  this  reason  more  than  ordinary  skill  is  necessary  in  pre- 
paring a  chart  which  will  exhibit  correctly  the  normal  annual  precipi- 
tation for  the  world.  Such  a  chart  has  been  recently  (1898)  prepared 
by  AlexandeDSupan  and  Chart  ^CXII  is  based  upon  this. 

258.  The  close  correlation  which  exists  between  the  general  wind 
system   and   the   normal   annual   precipitation   is   at   once   apparent. 
The  precipi-  The  equatorial  belt  of  low  pressure,  the  doldrums,  is  the 
tation  in  the   belt  of  largest  rainfall.     The  trade  winds  coming  into  this 

ns'  belt  from  either  side  bring  large  quantities  of  moisture- 
laden  air  at  fairly  high  temperatures.  Here  the  air  loiters,  and  finally 
rises  to  begin  its  poleward  journey  on  the  outside  of  the  atmosphere. 


THE  MOISTURE   IN  THE  ATMOSPHERE  255 

The  air  is  sultry,  the  sky  is  usually  cloud-covered,  and  local  convection 
causes  frequent  downpours  of  rain.  A  rainfall  far  above  100  inches  is 
common  in  this  belt. 

In  the  region  of  the  trade  winds  the  precipitation  is  much  less.     If 
they  blow  over  the  ocean,  they  gain  moisture  rapidly  as  the  doldrums 
are  approached,  but  the  air  also  rises  in  temperature,  and  \rade  wind 
its  increased  capacity  for  moisture  more  than  keeps  pace  with  pretipita- 
the  increase  in  moisture.     If  the  trade  winds  blow  from  the  tion* 
ocean  or  are  forced  to  rise  by  the  continental  elevations,  copious  pre- 
cipitation is  the  result.     On  account  of  the  direction  of  the  trade  winds 
(northeast  in  the  northern  hemisphere,  southeast  in  the  southern),  it  is 
the  eastern  coast  of  a  country  which  receives  the  copious  precipitations. 
Good  examples  of  this  are  to  be  found  in  the  relatively  large  precipita- 
tion on  the  coasts  of  Florida  and  Mexico  in  the  northern  hemisphere 
and  on  the  east  coast  of  southern  Brazil  and  the  east  coast  of  northern 
Australia  in  the  southern  hemisphere. 

The  horse  latitudes  (30°  to  35°  N.  and  30°  S.  latitude)  are  regions  of 
high  barometric  pressure  and  descending  air  currents.    The  precipitation 
in  these  two  belts  is  very  scant.     The  arid  portion  of  Central 
Siberia,  the  desert  of  Sahara  in  northern  Africa,  Nevada  and  cipitation  in 
Arizona  in  the  United  States,  and  the  dry  belt  which  stretches  *h®  h°rse 
across  the  lower  part  of  South  America  and  Australia,  all 
lie  in  or  near  the  horse  latitudes. 

In  the  region  of  the  prevailing  westerlies,  the  precipitation  is  moderate. 
It  is  caused  chiefly  by  the  west  winds  blowing  from  the  ocean  over  the 
land  or  by  the  many  storms  which  invade  the  prevailing         . 
westerlies.     When  these  winds  blow  from  the  ocean  over  the  in  the  pre- 
land  and  find  the  land  colder  or  are  forced  to  rise  by  conti-  **&*&  west~ 
nental  elevations,  it  is  the  west  coast  which  receives  the 
copious  precipitation.     Fine  examples  of  this  are  the  relatively  large 
values  of  precipitation  in  North  America  from  Oregon  to  Alaska,  in  the 
extreme  southern  part  of  South  America,  and  in  Scandinavia  in  Europe. 

259.  'The  chief  factors  which  determine  the  amount  of  precipitation 
received  at  any  station  and  its  distribution  throughout  the  The  factors 
year  are  of  sufficient  importance  to  bear  repetition.     They  which  deter- 
are :    the  general  wind  system ;   the  temperature  changes,  JJj^'JjJf®  f 
but  chiefly  the  contrast  in  temperature  between  the  place  precipitation 
in  question  and  those  regions  from  which  the  prevailing 
winds  come ;  elevation  and  inclosure  by  mountains ;  near- 
ness  to   bodies   of   water;    the   characteristics   of   the   storms   which 


256  METEOROLOGY 

occur.1  As  an  illustration  of  the  importance  of  these  factors  consider  a 
strip  across  North  America  from  the  Pacific  to  the  Atlantic  along  the 
northern  boundary  of  the  United  States.  The  most  copious  precipita- 
An  iiius-  tion,  about  100  inches,  is  found  on  the  Pacific  coast,  where  the 
tration.  moisture-laden  prevailing  westerlies  come  from  the  ocean 
over  the  ISnd  and  are  forced  to  rise  by  the  Rocky  Mountains.  Nearly 
all  of  the  precipitation  occurs  during  the  winter,  because  it  is  then  that 
the  land  is  colder  than  the  ocean.  After  the  Rocky  Mountains  are 
passed,  the  amount  of  precipitation  is  the  least  in  this  strip  —  only  ten 
to  twenty  inches.  The  reasons  are:  increasing  distance  from  the 
ocean,  elevation,  and  mountain  inclosure.  As  the  Atlantic  coast  is 
approached,  the  amount  steadily  increases,  and  reaches  about  forty 
inches  at  the  coast.  This  precipitation  is  nearly  all  caused  by  storms, 
extratropical  cyclones,  and  thundershowers,  and  thus  is  fairly  evenly 
distributed  throughout  the  year.  Near  the  coast  there  is  a  summer 
maximum  and  a  winter  minimum,  since  thundershowers  are  more 
prevalent  in  the  summer  time.  In  the  same  way  the  amount  of  precipi- 
tation at  any  station  and  its  distribution  throughout  the  year  can  be 
fully  explained  by  considering  these  determining  factors. 

260.  In  Charts  XXIII,  XXIV,  and  XXV  are  given  the  normal  an- 

nual precipitation  and  the  normal  precipitation  for  January, 
dotation"  February,  and  March,  and  for  July,  August,  and  September, 
charts  for  for  the  United  States.  All  of  the  varied  characteristics 
State^  °f  these  charts  can  be  readily  explained  on  the  fc^sis  of  the 

factors  discussed  above. 
Chart  XXVI  gives  the  normal  annual  snowfall  for  the  United  States. 

261.  Other,  precipitation   charts.  —  Since   precipitation   is   such   an 
important  climatic  or  meteorological  element,  nearly  all  of  the  normals 
The  more       °^  precipitation  and  precipitation  data  mentioned  in  sections 
important       255  and  256  have  been  determined  for  a  sufficient  number  of 

stations  to  make  possible  the  charting  of  them  for  various 
countries,  and  in  some  cases  for  the  world.  The  chief  ones  for  which 
charts  for  various  countries  and  sometimes  for  the  world  are  constructed 
are  :  the  normal  amount  of  precipitation  for  the  various  months  and  the 
year ;  the  normal  amount  of  snowfall  for  the  various  months  and  for  the 
year ;  the  normal  number  of  days  with  precipitation  for  the  various 
months  and  for  the  year ;  greatest  number  of  consecutive  days  without 
precipitation ;  greatest  number  of  consecutive  days  with  rain  every 
day. 

i  See  Monthly  Weather  Review,  April,  1902,  p.  204. 


THE  MOISTURE   IN  THE  ATMOSPHERE  257 

262.    Variation  in   the   amount   of   precipitation   with   altitude.  —  If 

one  rain  gauge  is  placed  in  the  open  a  few  feet  above  the  ground  and 

another  identical  instrument  placed  at  a  height  of,  say,  200 

feet,  as  much  in  the  open  as  possible,  it  has  been  found  that 

the  gauge  at  the  greater  height  will  catch  but  a  little  more  amount  with 

than  one  half  the  amount  caught  by  the  lower  gauge.     It  was  J^J  ****' 

formerly  thought  that  precipitation  decreased  with  altitude, 

but  careful  experimental  investigation  has  shown  that  this  discrepancy 

is  due  not  to  an  actual  difference  in  the  amount  of  precipitation,  but  to 

the  decided  increase  in  wind  velocity  with  elevation.     It  is  the  eddy 

formed  by  the  wind  in  passing  over  and  around  the  rain  gauge  which 

causes  the  deficit  in  the  amount  caught. 

There  is  probably,  however,  a  slight  increase  in  the  amount  of  precipi- 
tation with  altitude.     The  raindrops  are  largest  when  they  leave  the 
base  of  the  cloud,  and  decrease  in  size,  due  to  evaporation, 
during  their  fall  to  the  earth's  surface.     It  would  thus  be  increases 
expected  that  the  amount  of  precipitation  would  be  largest  sli«htly  withi 
at   the  average  height  of  the  rain-causing  clouds,  that  is, 
about  4000  feet  in  winter  and  considerably  higher  in  summer.     The  few 
different  and  uncertain  observations  which  have  been  made  would  seem 
to  confirm  this  expectation. 

In  a  mountainous  country,  the  regions  around  the  mountains  always 
have  the  largest  precipitation.     This  is  so  uniformly  true  that  it  is 
often  said  that  the  rainfall  map  of  a  mountainous  country  is 
almost  identical  with  the  topographic  map.     This  increase  Ous  regions 


is  not  due  to  any  great  extent  to  altitude,  but  to  the  fact  that  have 

precipitation. 

the  mountain  itself  by  its  presence  increases  the  amount  of 
precipitation.  When  the  air  is  forced  to  rise  in  going  over  the  moun- 
tain, the  windward  side  is  usually  deluged  with  rain,  while  the  leeward 
side  receives  but  little.  If  a  mountain  is  situated  in  a  country  where  the 
wind  direction  may  be  from  almost  any  point  of  the  compass  during 
precipitation,  then  the  whole  region  about  the  mountain  will  have  a 
large  amount  of  precipitation.  The  increase  in  precipitation  in  regions 
surrounding  mountains  is  thus  due  not  so  much  to  elevation,  but  pri- 
marily to  the  presence  of  the  mountain  itself. 

263.    Relation  of  rainfall  to  agriculture.  —  In  expressing  the  relation 
of  rainfall  to  agriculture,  it  is  often  stated  that  more  than  100  inches 
produces  vegetation  too  luxuriant  for  agriculture  *  that  from   Limiting 
100  to  18  inches  is  most  favorable  ;  that  from  18  to  21  inches  values- 
is  suitable  for  grazing  only  ;  that  below  12  inches  the  country  is  a  desert. 


258  METEOROLOGY 

Although  these  statements  are  in  the  main  true,  yet  certain  modifica- 
tions are  necessary.  In  the  first  place,  high  temperature  as  well  as 
excessive  precipitation  is  necessary  in  order  to  produce  too  luxuriant 
vegetation.  In  a  cold  region  where  a  good  part  of  the  precipitation 
TV  *VK  t  comes  in  winter  as  snow,  100  inches  would  not  be  unfa- 

Distn  button 

aisoim-  vorable.  The  distribution  of  the  precipitation  throughout 
the  year  is  also  quite  as  important  as  the  amount.  Forty 
inches  of  precipitation  which  comes  entirely  as  snow  during  three  months 
in  the  winter  would  not  be  favorable  for  agriculture,  even  if  the  summer 
were  sufficiently  warm. 

264.  Relation  of  rainfall  and  forests.  —  There  is  a  widespread  popu- 
lar belief  that  deforestation  decreases  rainfall  and  that  wholesale  tree- 
planting  will  increase  it.     The  results  of  observation,  how- 

Deforesta-  ,  .    .  .' 

tion  does  ever,  are  not  in  accord  with  this  opinion.  A  comparison  of 
not  reduce  ^he  earliest  records  of  rainfall  in  this  country  with  the  present 
observations  shows  that  the  amount  of  precipitation  and  its 
distribution  throughout  ttye  year  in  the  early  colonial  days,  when  most 
of  the  country  was  still  covered  with  virgin  forests,  was  practically  the 
same  as  at  present.  Differences  in  the  location  and  surroundings  of  the 
rain  gauge  probably  cause  far  greater  differences  in  the  amount  of 
precipitation  observed  than  the  presence  or  absence  of  forests.  There 
is  also  no  evidence  that  the  wholesale  tree  planting  in  Egypt,  now  nearly 
a  hundred  years  ago,  has  had  any  effect  whatever  on  the  rainfall  of  that 
country.  More  recent  observations  in  Mauritius  show  that  deforesta- 
tion may  reduce  the  number  of  rainy  days  slightly  without  changing 
appreciably  the  total  amount  of  precipitation.  The  factors  which  do 
determine  the  amount  of  precipitation  have  been  fully  discussed ;  and 
as  all  of  these  are  unchanging,  there  is  no  reason  to  believe  that  the 
destruction  of  forest  ought  to  influence  rainfall.  To  state  that  the 
presence  of  forest,  particularly  in  a  hilly  and  mountainous  country,  pre- 
it  does  affect  serves  the  fertility  of  the  valleys  is  an  entirely  different 
fertility.  matter.  The  forests  do  cause  a  large  part  of  the  rainfall  to 
be  retained  in  the  soil.  As  soon  as  the  forests  are  cut,  the  water  drains 
rapidly  into  the  valleys  after  every  storm,  and  the  floods  thus  caused 
wash  over  the  good  soil  and  often  destroy  its  fertility.  Forests  may 
also  retain  the  snows  of  winter,  and  thus  have  an  effect  on  irrigation 
or  agriculture;  but  this  is  entirely  apart  from  their  influence  on 
rainfall. 

265.  Effects  of  snowfall.  —  A  large  part  of  the  earth  is  covered  during 
the  winter  by  a  layer  of  snow,  varying  from  a  few  inches  to  perhaps 


THE  MOISTURE  IN  THE  ATMOSPHERE  259 

many  feet  in  thickness.  There  are  several  results  of  this  snow  layer 
which  deserve  mention  in  passing.  (1)  It  prevents  deep  freezing. 
Snow  is  a  very  poor  conductor  of  heat,  particularly  if  it  is 

j          A  e  e  11     The  four 

not  compacted.     As  a  result,  a  layer  ol  snow  ol  even  small  effects  of  a 
thickness  prevents  the  ground  from  freezing  to  the  depth  to  layer  of 
which  it  would,  if  it  were  bare.     (2)  Snow  also  lowers  the 
temperature  of  the  air.     It  is  a  good  reflector,  and  reflects  some  40  per 
cent  of  the  insolation  which  falls  upon  it.     As  a  result,  temperatures 
are  usually  lower  when  the  ground  is  snow-covered.     (3)  It  retards  the 
coming  of  spring,  for  the  layer  of  snow  must  all  be  melted  before  the 
temperature  can  go  much  above  32°  F.  or  the  frost  come  out  of  the 
ground.    (4)  It  causes  floods,  because  the  coming  of  a  sudden  thaw  or 
a  warm  rain  often  delivers  to  the  streams  the  precipitation  which  has 
fallen  during  several  storms  and  has  collected  on  the  ground  in  the  form 
of  snow.  * 

In  the  arctic  regions  and  in  high  mountains,  the  snow  drifts  or  slides 
into  the  ravines  and  valleys,  and  is  there  compacted  by  the  snow  forms 
wind,  surface  melting,  and  an  occasional  rainstorm  until  a  8laciers- 
glacier  is  produced.     Snow  thus  leads  to  the  formation  of  glaciers. 


QUESTIONS 

(1)  Define  evaporation  and  condensation.  (2)  State  the  sources  of  the  water 
vapor  of  the  atmosphere.  (3)  Which  is  the  heavier,  air  or  water  vapor?  (4)  Define 
latent  heat.  (5)  What  are  the  two  sources  of  latent  heat  ?  (6)  State  the  different 
ways  in  which  latent  heat  makes  itself  felt.  (7)  Upon  what  does  the  amount  of 
evaporation  depend  ?  (8)  How  is  the  amount  of  evaporation  determined  ?  (9)  De- 
scribe the  three  processes  by  which  water  vapor  is  distributed.  (10)  Upon  what 
does  the  capacity  of  the  air  for  water  vapor  depend  ?  (11)  When  is  air  said  to  be* 
saturated?  (12)  Distinguish  between  absolute  and  relative  humidity.  (13)  De- 
fine the  dew  point.  (14)  How  are  the  four  quantities,  temperature,  absolute 
humidity,  relative  humidity,  and  dew  point  related?  (15)  Describe  the  chem- 
ical hygrometer.  (16)  Describe  the  construction  and  action  of  a  hair  hygrom- 
eter. (17)  How  accurate  is  the  instrument,  and  how  may  it  be  standardized? 

(18)  What   other  apparatus   may   be   used   to   determine  relative   humidity? 

(19)  Describe  the  dew  point  hygrometer.     (20)  Describe  the  construction  and 
action  of  a  psychrometer.      (21)  How  is  the  effect  of  the  wind  obviated?     (22) 
What  moisture  observations  are  made  at  the  weather  bureau  stations?     (23) 
Describe   the   typical   daily   variation   in  absolute  humidity.     (24)  State  and 
explain  the  characteristics  of  the  annual  variation  in  absolute  humidity.     (25) 
How  are  the  general  wind  system  and  absolute  humidity  correlated  ?     (26)     De- 
scribe and  explain  the  characteristics  of  the  daily  variation  in  relative  humidity. 
(27)  Describe  and  explain  the  characteristics  of  the  annual  variation  in  relative 
humidity.     (28)  Describe    the    geographical    variation    in    relative    humidity. 
(29)     What  is  the  effect  of  water  vapor  on  the  general  circulation  of  the  atmos- 
phere?    (30)  What  are  the  seven  forms  of  condensation?     (31)  In  what  two 


260  METEOROLOGY 

ways  may  condensation  be  brought  about?  (32)  How  may  air  be  sufficiently 
cooled  to  cause  condensation?  (33)  Describe  and  explain  the  formation  of 
dew.  (34)  State  the  amount  and  sources  of  dew.  (35)  What  are  the  conditions 
for  the  formation  of  dew?  (36)  Describe  the  appearance  of  frost.  (37)  Dis- 
tinguish between  light  and  killing  frosts.  (38)  How  are  the  destructive  frosts 
of  spring  and  autumn  caused?  (39)  Describe  in  detail  the  methods  of  frost 
prediction.  (40)  What  protection  from  frost  may  be  used  ?  (41)  What  obser- 
vations of  frost  are  made?  (42)  How  is  fog  formed?  (43)  Distinguish  be- 
tween transportation  and  radiation  fogs?  (44)  How  do  city  fogs  and  country 
fogs  differ?  (45)  What  observations  of  fog  are  made?  (46)  State  the  early 
history  of  cloud  classification.  (47)  Describe  the  introduction  of  the  interna- 
tional system.  (48)  Treat  fully  the  international  system.  (49)  Describe 
in  detail  the  thirteen  cloud  forms.  (50)  What  is  the  usual  sequence  of  cloud 
forms?  (51)  Describe  the  methods  of  determining  the  height  of  clouds.  (52) 
How  is  the  direction  and  velocity  of  motion  of  clouds  determined?  (53)  De- 
scribe the  construction  and  action  of  a  nephoscope.  (54)  Define  cloudiness ; 
how  is  it  estimated?  (55)  Define  the  five  adjectives  used  to  designate  cloudi- 
ness. (56)  Describe  the  three  kinds  of  sunshine  recorders.  (57)  What 
observations  of  the  clouds,  cloudiness,  and  sunshine  are  made?  (58)  What 
are  the  characteristics  of  the  annual  and  daily  variation  in  cloudiness?  (59) 
Are  nuclei  of  condensation  necessary  for  the  formation  of  cloud  ?  (60)  How 
large  are  cloud  particles?  -(61)  What  holds  cloud  particles  in  suspension? 
(62)  Describe  the  two  kinds  of  haze.  (63)  Treat  in  detail  the  nine  processes 
in  cloud  formation.  (64)  What  conditions  favor  a  clear  sky?  (65)  How  is  a 
raindrop  formed?  (66)  Why  does  not  every  cloud  yield  rain?  (67)  What 
cloud-forming  processes  may  lead  to  precipitation?  (68)  Describe  the  struc- 
ture of  snowflakes.  (69)  Describe  in  detail  the  three  kinds  of  hail.  (70) 
What  are  the  characteristics  of  an  ice  storm?  (71)  What  rain-making  experi- 
ments have  been  tried,  and  are  they  successful?  (72)  Why  is  it  cooler  after  it 
has  rained?  (73)  How  is  rainfall  measured?  (74)  How  is  a  snowfall  meas- 
ured? (75)  What  is  the  water  equivalent  of  snow?  (76)  What  observations 
of  precipitation  are  made?  (77)  Describe  the  daily  and  annual  variation  in 
precipitation.  (78)  What  are  the  factors  which  determine  the  distribution 
throughout  the  year?  (79)  What  precipitation  data  are  usually  collected? 
(80)  Describe  in  detail  the  geographical  distribution  of  precipitation.  (81) 
Trace  the  correlation  between  the  general  wind  system  and  the  distribution  of 
precipitation.  (82)  How  does  the  amount  of  precipitation  vary  with  the 
altitude?  (83)  What  is  the  relation  of  rainfall  and  agriculture?  (84)  What  is 
the  relation  of  rainfall  and  forests?  (85)  What  are  the  effects  of  snowfall? 

TOPICS   FOR   INVESTIGATION 

(1)  The  methods  of  determining  the  amount  of  evaporation. 

(2)  Cobalt  chlorid  as  a  measurer  of  relative  humidity. 

(3)  The  theory  of  the  psychrometer. 

(4)  The  history  of  the  instruments  for  measuring  moisture. 

(5)  Frost  prediction. 

(6)  The  destructive  frosts  of  spring  and  autumn. 

(7)  Protection  from  frosts. 

(8)  Nuclei  of  condensation  for  fog  and  cloud. 

(9)  The  temperature  required  for  the  destruction  by  frost  of  different  kinds 

of  plants. 
(10)  History  of  cloud  classification. 


THE  MOISTURE   IN  THE   ATMOSPHERE  261 

(11)  The  methods  of  photographing  clouds. 

(12)  The  methods  of  determining  cloud  heights. 

(13)  Sunshine  recorders. 

(14)  Haze. 

(15)  Snow  crystals. 

(16)  The  structure  of  summer  hail. 

(17)  Rain-making  experiments. 

(18)  The  relation  of  rainfall  and  forests. 

PRACTICAL   EXERCISES 

(1)  Compare  the  amount  evaporated  by  a  Piche  evaporimeter  and  by  a  free 
water  surface  under  different  conditions. 

(2)  Determine  the  effect  of  the  character  of  the  surface  on  the  amount  evap- 
orated. 

(3)  Determine  the  moisture  in  the  atmosphere  by  all  the  different  methods, 
and  compare  the  results  under  different  conditions. 

(4)  Study  critically  the  behavior  of  a  hair  hygrometer. 

(5)  Study  the  amount  of  dew  deposited  and  the  characteristics  of  the  night. 

(6)  Whenever  a  fog  occurs,  determine  exactly  how  it  was  caused. 

(7)  Determine  by  the  shadow  method  the  height  of  a  few  cumulus  clouds. 

(8)  Observe  for  a  number  of  days  at  a  definite  time  the  kind  of  cloud,  and 
explain  exactly  how  it  was  formed. 

(9)  Determine  the  size  of  raindrops. 

(10)  Determine  the  temperature  of  the  raindrops  as  compared  with  the  air 
temperature. 

(11)  Work  out  the  effect  of  a  building  on  the  amount  of  rain  caught  by  a  rain 
gauge  in  different  locations. 

(12)  Determine  whether  July  4  is  more  rainy  than  July  3  or  5. 

In  connection  with  absolute  humidity,  relative  humidity,  frost,  fog,  cloudiness, 
sunshine,  and  precipitation,  the  various  normals  and  data  mentioned  in  this 
chapter  may  be  worked  out  for  one  or  more  stations.  The  graphs  representing 
the  diurnal  and  annual  variations  should  be  plotted  and  explained.  If  these 
determinations  have  been  made  for  many  stations  in  a  state,  charts  may  be 
prepared. 

REFERENCES 

For  the  description,  illustration,  construction,  and  use  of  apparatus  to  measure 
absolute  humidity,  relative  humidity,  dew  point,  the  altitude  of  clouds, 
direction  and  velocity  of  motion  of  clouds,  sunshine,  and  precipitation,  see : 

ABBE,  CLEVELAND,  Meteorological  Apparatus  and  Methods,  Washington,  1888. 
MARVIN,  C.  F.,  Circular   G  (instructions    for    the  care  and    management  of 

sunshine  recorders)   and  E    (measurement  of    precipitation),   Instrument 

Division,  U.  S.  Weather  Bureau. 

MOORE,  JOHN  W.,  Meteorology,  2d  ed.,  pp.  109-113,  175-189,  220-241. 
Report  of  the  Chief  of  the  Weather  Bureau,  1898-1899,  Vol.  II.     (Report  on 

the  International  Cloud  Observations  by  F.  H.  BIGELOW.) 
Sunshine   Recorders   and   their    Indications    by   R.    H.    CURTIS.      Quarterly 

Journal  of  the  Royal  Met.  Soc.,  XXIV,  1898. 
Apparatus  catalogues  of  various  firms.     (See  p.  110.) 
See  also  the  various  guides  to  observers  mentioned  in  Appendix  IX,  in  group 

(2)  B. 


262  METEOROLOGY 

For  the  observations  of  moisture,  frost,  fog,  cloudiness,  sunshine,  and  precipi- 
tation consult  the  publications  mentioned  on  pages  no  and  in. 

For  moisture,  cloud,  and  precipitation  normals  for  various  places,  see : 

HANN,  Handbuch  der  Klimatologie. 

VAN  BEBBER,   Handbuch  der  Meteorologie. 

BLODGET,  Climatology  of  the  United  States. 

Climatology  of  the  United  States.     Bulletin  Q  of  the  U.  S.  Weather  Bureau 

by  A.  J.  HENRY. 
Summary  of  the  Climatological   Data  for  the  United   States  by  Sections  (106 

are  to  be  issued). 

For  moisture  only : 

Report  of  the  Chief  of  the  Weather  Bureau,  1896-1897,  1901-1902. 

HENRY,  ALFRED  J.,  Report  on  the  Relative  Humidity  of  Southern  New 
England  and  other  Localities.  Bulletin  19  of  the  U.  S.  Weather  Bureau. 

Temperature  and  Relative  Humidity  Data.  Bulletin  O  of  the  U.  S.  Weather 
bureau  by  WILLIAM  B.  STOCKMAN. 

Report  on  the  Temperature  and  Vapor  Tensions  in  the  United  States.  Bul- 
letin S  of  the  U.  S.  Weather  Bureau  by  FRANK  H.  BIGELOW. 

For  clouds  only : 

Report  of  the  Chief  of  the  Weather  Bureau,  1896-1897. 

For  precipitation  only : 

FRITZSCHE,     RICHARD,      Niederschlag,     Abfluss,    und     Verdunstung     auf     den 

Landfldchen  der  Erde,  Halle,  1906. 
HEBERTSON,  ANDREW  J.,  The  Distribution  of  Rainfall  over   the    Land,  London, 

1901. 
SUPAN,    ALEXANDER,   Die    Verteilung   des    Niederschlags    auf  der  festen    Erd- 

oberfldche,  Gotha,  1898. 
Great  Britain,  Rainfall  Tables  for  the  British  Islands,  London,  1897.     (Official, 

No.  114.) 
HELLMANN,   G.,   Die    Niederschldge  in  den  norddeutschen  Stromgebieten,  Berlin, 

1906. 

SANDERRA,   MASO   MIGUEL,    The   Rainfall  in  the    Philippines,   Manila,    1907. 
Voss,  ERNST  LUDWIG,    Die   Niederschldgsverhdltnisse  von    Sudamerika,   Gotha, 

1907. 

WILD,  H.,  Die  Regenverhdltnisse  des  Russischen  Reiches,  St.  Petersburg,  1887. 
Tables  and  Results  of  the  Precipitation  in  Rain  and  Snow  in  the  United  States. 

(Smithsonian    Contributions    to    Knowledge,    353.)     4°,  2d  ed.,  xx  +  249 

pp.,  Washington,  1881. 

Report  of  the  Chief  of  the  Weather  Bureau,  1891-1892,  1896-1897. 
HARRINGTON,  MARK  W.,  Rainfall  and  Snow  of  the  United  States,  computed 

to  the  end  of  1891.     Bulletin  C  of  the  U.  S.  Weather  Bureau. 
HENRY,  ALFRED  J.,  Rainfall  of  the  United  States.     Bulletin  D  of  the  U.  S. 

Weather  Bureau. 
BIGELOW,  FRANK    H.,  The  daily  normal  Temperature  and  the  daily  normal 

Precipitation  in  the  United  States.    Bulletin  R  of  the  U.  S.  Weather  Bureau. 

For  charts  of  moisture,  cloud,  sunshine,  and  precipitation,  see : 
BARTHOLOMEW,  Physical  Atlas,  Vol.  Ill,  Atlas  of  Meteorology. 
HANN,  Atlas  der  Meteorologie,  1887. 
ELIOT,  SIR  JOHN,  Climatological  Atlas  of  India,  Edinburgh,  1906. 


THE  MOISTURE  IN  THE  ATMOSPHERE  263 

Russia,  Atlas  climatologique  de  I 'empire  de  Russie,  St.  Petersbourg,  1900. 
BLODGET,  LORIN,  Climatology  of  the  United  States,  Philadelphia,  1857. 
LOOMIS,  Contributions  to  Meteorology. 

GREELY,  GEN.  A.  W.,  American  Weather,  New  York,  1888. 
Climatic  charts  of  the  United  States,  Weather  Bureau,  Washington,  D.C.,  1904. 
Climatology  of  the  United  States.     Bulletin  Q,  by  A.  J.  HENRY  (W.  B.  361). 
CLARK,  KENNETH  McR.,  "A  new  set  of  Cloudiness  Charts  for  the  United 
States,"  Quart.  Jour.  Roy.  Met.  Soc.,  Vol.  37,  No.  158,  April,  1911. 

For  moisture  only : 

Report  of  the  Chief  of  the  Weather  Bureau,  1896-1897  and  1901-1902. 

For  precipitation  only : 

HERBERTSON,  ANDREW  J.,  The  Distribution  of  Rainfall  over  the  Land,  London, 

1901. 
SUPAN,  ALEXANDER,  Die  Verteilung  des   Niederschlags  auf  der  festen  Erdober- 

flache,  Gotha,  1898. 
HELLMANN,  G.,   Die    Niederschldge  in  den  norddeutschen  Stromgebieten,  Berlin, 

1906. 
Voss,  ERNST  LUDWIG,    Die   Niederschldgsverhdltnisse   von   Sudamerika,   Gotha, 

1907. 
DUNWOODY,  H.  H.   C.,   Geographical  Distribution  of  Rainfall  in  the  United 

States.     Professional  Papers  of  the  Signal  Service,  No.  9. 
HENRY,  ALFRED  J.,  Rainfall  of  the  United  States.     Bulletin  D  of  the  U.S. 

Weather  Bureau,  1897. 
Report  of  the  chief  of  the  Weather  Bureau,  1896-1897. 

For  information  about  clouds  and  cloud  classification,  see : 

Atlas  de  las  Nubes,  para  el  service  meteorologico  Republica  Mexicana,  1906. 
BARBER,  The  Cloud  World,  its  Features  and  Significance,  London,  1903. 
CLAYDEN,  Cloud  Studies,  E.  P.  Dutton  and  Co.,  New  York,  1905. 
CLAYTON,  Observations  made  at  the  Blue  Hill  Observatory.    (Published  in  the 

Annals  of  the  Harvard  College  Observatory.) 
Illustrative  Cloud  Forms.      (Issued  by  the  Weather  Bureau.) 
International  Met'l  Committee,  International  Cloud  Atlas,  Paris,  1896. 
LEY,  Cloudland,  London,  1894. 
VINCENT,  Atlas  des  nuages,  Bruxelles,  1907. 
See  also  other  books  mentioned  in  Appendix  IX. 

For  a  discussion  of  the  destructive  frosts  of  spring  and  autumn,  see : 

CLINE,  Irregularities  in    Frost  and    Temperature   in  Neighboring    Localities, 

Third  Convention  of  Weather  Bureau  Officials,  Proceedings,  Washington, 

1904,  p.  250. 
DAY,  Frost  Data  of  the  United  States  and  Length  of  the  Crop-Growing  Season, 

Bulletin  V  of  the  U.  S.  Weather  Bureau. 
GARRIOTT,  Notes  on  Frost,  Farmers  Bulletin  No.  104. 
HAMMON,  Frost,  Weather  Bureau  Publication  No.  186. 
McAoiE,  Frost  Fighting,  Weather  Bureau  Publication  No.  187. 
Monthly  Weather  Review,  August,  1908. 

For  recent  articles  on  evaporation,  see : 
BIGELOW,  Monthly  Weather  Review,  1907  on. 
KIMBALL,  Monthly  Weather  Review,  December,  1904. 


CHAPTER   VI 

THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE 

A.     TROPICAL   CYCLONES 

DEFINITION  AND  DESCRIPTION 

Definition  and  chief  characteristics,  266. 

Description  of  the  approach  and  passage  of  a  tropical  cyclone,  267. 

Distribution  of  the  meteorological  elements  about  a  tropical  cyclone,  268,  269. 

Some  special  tropical  cyclones,  270,  271. 

Rules  for  mariners,  272. 

THE  REGIONS  AND  TIME  OF  OCCURRENCE 

Regions  of  occurrence,  273. 
Tracks  of  tropical  cyclones,  274. 
Frequency  at  different  times  of  year,  275. 

THE  ORIGIN  OF  TROPICAL  CYCLONES 

The  convectional  theory,  276-281. 
Comparison  with  the  observed  facts,  282. 

THE  COMPARISON  OF  A  TROPICAL  CYCLONE  AND  THE  CIRCUMPOLAR 
WHIRL  IN  THE  GENERAL  WIND  SYSTEM,  283 

B.     EXTRATROPICAL   CYCLONES  AND  ANTICYCLONES 

DEFINITION  AND  DESCRIPTION  OF  AN  EXTRATROPICAL  CYCLONE 

Definition  and  chief  characteristics,  284. 

Distribution  of  the  meteorological  elements  about  a  typical  extratropical  cyclone, 

285-288. 

A  comparison  of  the  tropical  and  extratropical  cyclones,  289. 
Description  of  the  approach  and  passage  of  an  extratropical  cyclone,  290. 

ANTICYCLONES,  291-295 

THE  TRACKS  AND  VELOCITY  OF  MOTION  OF  EXTRATROPICAL  CYCLONES 

Tracks  in  the  northern  hemisphere,  296. 
Tracks  across  Europe,  297. 
Tracks  across  the  United  States,  298-301. 
Velocity  of  motion,  302. 

THE  TRACKS  AND  VELOCITY  OF  MOTION  OF  ANTICYCLONES,  303-305 
STATISTICS  ON  EXTRATROPICAL  CYCLONES  AND  ANTICYCLONES,  306 

264 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     265 

THE   ORIGIN  AND    GROWTH   OF    AN    EXTRATROPICAL   CYCLONE    AND 
ANTICYCLONE 

Theories  as  to  the  origin  of  extratropical  cyclones  and  anticyclones,  307-314. 
Growth  of  an  extratropical  cyclone,  315. 
Growth  of  an  anticyclone  316. 

The  characteristics  which  lows  and  highs  should  have  and  the  comparison  with  the 
observed  facts,  317. 

THE  EFFECT  OF  PROGRESSION  ON   THE  DIRECTION  AND  VELOCITY  OP 

CYCLONIC  WINDS,  318,  319 
THE  CORRELATION  OF  THE  METEOROLOGICAL  ELEMENTS,  320      -/ 

C.     THUNDERSHOWERS 

DEFINITION  AND  DESCRIPTION 

Definition  and  chief  characteristics,  321. 

Description  of  the  approach  and  passage  of  a  thundershower,  322. 

Distribution  of  the  meteorological  elements  about  a  thundershower,  323. 

Cross  section  of  a  thundershower,  324. 

The  observations  of  thunder  showers,  325. 

THE  REGIONS  AND  TIME  OF  OCCURRENCE 

Geographical  distribution,  326. 

Relation  to  extratropical  cyclones  and  V-shaped  depressions,  327. 

Path  across  a  country,  328. 

Direction  and  velocity  of  motion,  329. 

Time  of  day  and  season  of  occurrence,  330. 

Periodicity  of  thundershowers,  331. 

THE  ORIGIN  AND  GROWTH  OF  A  THUNDERSHOWER 

Three  classes  of  thundershowers,  332. 

Heat-  or  convection-caused  thundershowers,  333,  334. 

Hail,  335. 

Cyclonic  thundershowers,  336. 

Thundershowers  due  to  local  conditions,  337. 

THUNDER  AND  LIGHTNING,  338 

D.     TORNADOES 

DEFINITION  AND  DESCRIPTION 

Definition  and  chief  characteristics,  339. 
Description  of  the  approach  and  passage  of  a  tornado,  340. 
Distribution  of  the  meteorological  elements  about  a  tornado,  341. 
Observation  of  tornadoes,  342. 

THE  REGIONS  AND  TIME  OF  OCCURRENCE 

Geographical  distribution,  343, 

Relation  to  extratropical  cyclones,  344. 

Path  across  a  country,  345. 

Time  of  day  and  season  of  occurrence,  346. 

THE  ORIGIN  AND  GROWTH  OF  A  TORNADO 

The  origin  of  tornadoes,  347. 

Explanation  of  the  facts  of  observation,  348. 

PROTECTION  PROM  TORNADOES,  349 


266  METEOROLOGY 

E.     WATERSPOUTS   AND   WHIRLWINDS 

WATERSPOUTS,  350 
WHIRLWINDS,  351 

F.  CYCLONIC  AND  LOCAL  WINDS 

INTRODUCTION,  352 

THE  CYCLONIC  AND  LOCAL  WINDS  OF  THE  UNITED  STATES 

Warm  wave  (sirocco),  353. 
Cold  wave  (blizzard),  354. 
Chinook  (foehn),  355. 

THE  CYCLONIC  AND  LOCAL  WINDS  OF  OTHER  COUNTRIES,  356 

A.     TROPICAL    CYCLONES 

DEFINITION  AND  DESCRIPTION 

266.  Definition  and  chief  characteristics.  —  A  tropical  cyclone  is  a 
storm  which  can  be  best  denned  by  stating  its  chief  characteristics. 
Definition  of  ^  *s  a  vas^  atmospheric  whirl,  turning  counterclockwise  in 
a  tropical  the  northern  hemisphere,  clockwise  in  the  southern,  with 

spirally  inflowing  winds  which  nearly  always  attain  destruc- 
tive velocities.  The  pressure  is  low  in  the  center  and  it  is  attended  by 
a  large  cloud  area  from  which  rain  pours  in  torrents,  sometimes  with 
thunder  and  lightning.  The  whole  formation  is  from  300  to  600  miles 
in  diameter  and  is  not  stationary,  but  moves  with  a  very  moderate 
velocity  over  a  fairly  well-defined  course. 

These  storms,  technically  known  as  tropical  cyclones,  because  they 

are  whirling  storms  which  originate  within  the  tropics,  have 
names  for  various  names  in  different  parts  of  the  world.  They  are 

the  tropical     usually  called  hurricanes  in  the  West  Indies,  typhoons  in  the 
cyclone. 

China  Sea,  and  baguois  in  the  Philippine  Islands. 

The  whirling  nature  of  these  storms  was  fftst  recognized  by  Varenius 

in    his    Geographica   Generalis    in  1650.      The    famous    sea    captain, 

hi    ri     Dampier,  who  experienced  one  in  1687,  correctly  describes 

cai  rise  of       it  and  states  that  a  typhoon  is  a  kind  of  violent  whirl  which 

our  present     occurs  on  the  coasts  of  Tonkin  in  July,  August,  and  Septem- 

mformation. 

ber.  The  first  definite  and  exact  information  concerning  the 
nature  and  characteristics  of  these  storms  comes,  however,  from  Redfield 
in  this  country,  Reid  in  England,  and  Piddington  in  Calcutta,  in  the 
first  part  of  the  last  century.  Other  investigators  who  have  added 
much  to  our  knowledge  of  these  storms  are :  Dove,  Espy,  Ferrel, 
Vines,  Doberck,  Meldrum,  Blanford,  Poey,  Algue",  and  Eliot. 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     267 

267.  Description  of  the  approach  and  passage  of  a  tropical  cyclone. 
—  Tropical  cyclones  have  been  vividly  described  by  many  sea  captains 
who  have  passed  through  them,  and  observers  who*  have  The  coming 
experienced  them  on  land.     The  first  signs  of  the  approach  of  the 

of  the  storm  are  to  be  found  in  both  sea  and  sky.  The  sky  cyclone- 
is  covered  with  a  thin  cirrus  haze  which  causes  lurid  red  sunsets  and 
halos  or  rings  about  the  sun  by  day  and  the  moon  by  night.  The  air  is 
still,  moisture-laden,  sultry,  and  oppressive.  The  barometer  rises  unduly 
high  or  remains  stationary  when  the  daily  drop  is  expected.  The  wind 
disappears  and  the  long  rolling  swell  of  ominous  import  appears  on  the 
ocean.  Soon  the  barometer  begins  to  fall.  A  breeze  springs  up,  but 
the  air  is  still  sultry  and  oppressive.  The  cirrus  haze  becomes  true 
cirrus,  which  usually  stretches  in  bands  across  the  sky  and  begins  to 
thicken  into  cirro-stratus  or  sometimes  cirro-cumulus.  The  barometer 
begins  to  fall  more  rapidly,  the  wind  increases,  and  on  the  horizon  the 
dark  rain  cloud,  shieldlike,  has  appeared.  The  barometer  now  falls 
with  startling  rapidity ;  the  blue-black  rain  cloud  rushes  overhead ;  rain 
falls  in  torrents,  cooling  the  air ;  the  wind  has  increased  to  full  hurricane 
strength,  a  hundred  miles  an  hour  or  more ;  the  sea  is  lashed  into  fury. 
This  may  continue  many  hours,  when  suddenly  the  wind  ceases,  the 
clouds  break  through,  the  temperature  rises,  the  moisture  grows  less, 
and  the  barometer  is  at  its  lowest,  for  the  calm  central  eye  The  calm 
of  the  storm  has  been  reached.  The  respite  is  but  brief,  central  eye- 
perhaps  twenty  or  thirty  minutes  when  the  wind  changes  to  the  opposite 
direction  and  increases  to  full  hurricane  strength  as  suddenly  as  it 
ceased.  Rain  again  falls  in  torrents,  and  everything  is  as  before 
except  that  the  barometer  is  rising.  After  several  hours  the  The  disap- 
end  of  the  storm  is  reached,  the  wind  dies  down,  the  pearingof 
rain  ceases,  the  nimbus  clouds  break  through  and  give  t 
place  to  cirrus,  the  temperature  rises  somewhat.  A  while  later  and 
the  nimbus  clouds  sink,  shield  like,  below  the  horizon,  the  cirrus 
retreats  after  it,  the  wind  is  a  gentle  breeze,  the  barometer  has  reached 
its  accustomed  height;  and  but  for  the  wreckage  and  the  ominous 
heaving  of  the  ocean  one  would  not  know  that  a  storm  had  passed. 

268.  Distribution  of   the   meteorological   elements    about  The  distrj_ 
a  tropical   cyclone.  —  The  distribution  of  the  meteorological  bution  of  the 
elements  (temperature,  pressure,  wind,  moisture,  cloud,  and  Moated18 
precipitation)  about  a  tropical  cyclone  —  in  short  the  whole  by  two 
structure  of  the  storm  —  can  be  well  shown  by  means  of 

two  diagrams.     The  first,  Fig.  110,  contains  the  pressure,  wind,  cloud, 


268 


METEOROLOGY 


and  precipitation,  while  the  second,  Fig.  Ill,  shows  the  temperature 
and  moisture. 

269.  The  isobars  are  often  nearly  circular,  although  at  times  oval  in 
form.  The  longer  'axis  of  the  oval  usually  lies  in  the  direction  of  motion 
The  distri-  °^  ^ne  cvcl°ne>  although  it  may  make  any  angle  Avith  this 
button  of  the  direction.  The  central  pressure  averages  about  28.5  inches, 
elements.  although  pressures  as  low  as  27  inches  have  been  observed.1 
The  belt  of  slightly  higher  pressure  surrounding  the  cyclones  and  called 


600  MILES 


FIG.  110.  —  The  Distribution  of  the  Meteorological  Elements  about  a  Tropical  Cyclone. 

the  pericyclonic  ring  usually  has  a  pressure  of  about  30.1  inches.  It 
will  be  remembered  that  the  occurrence  of  this  high  pressure  was  one 
of  the  first  signs  of  the  coming  of  the  cyclone. 

The  wind    blows  spirally  inward,  turning    counterclockwise  in  the 
northern  hemisphere.     Its  direction  makes  an  angle  of  about  30°  with 

1  See  Meteor ologische  Zeitschrift,  1902,  p.  474,  for  a  barograph  trace  during  a  typhoon. 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     269 


the  isobaric  lines.  The  value  of  the  angle  is  nearly  the  same  all  the  way 
round,  although  somewhat  greater,  35°  to  40°,  in  the  northeast  quad- 
rant and  somewhat  less,  20°  to  25°,  in  the  southwe%  quadrant.  The 
wind  velocity  is  small  on  the  outside,  nothing  in  the  calm  central  eye, 
and  attains  its  greatest  velocity  in  the  middle  of  the  rain  area.  The 
maximum  velocity  nearly  always  reaches  100  miles  an  hour  and  some- 
times probably  nearly  200.  The  arrows  in  the  diagram  by  their  direction 
and  length  show  the  direction  and  velocity  of  the  wind. 

The  cloud  area  is  shaded  in  the  diagram  and  is  concentric  with  the 
isobars.     On  the  outside,  the  cirrus  radiates  in  all  directions,  while  the 


FIG.  111.  —  Temperature  and  Moisture  Changes  in  a  Tropical  Cyclone. 

nimbus  clouds  occupy  the  inside  portion.  The  transition  clouds  are 
usually  cirro-stratus,  perhaps  cirro-cumulus.  The  calm  central  eye  is 
sometimes  almost  cloudless.  The  precipitation  is  excessive,  often 
several  inches,  and  covers  the  same  area  as  the  nimbus  clouds.  It  is 
sometimes  accompanied  by  thunder  and  lightning,  although  this  is  most 
common  at  the  end  of  the  storm.  It  is  said  that  the  most  violent  cy- 
clones are  never  accompanied  by  thunder  and  lightning. 

The  temperature  and  moisture  are  practically  the  same  in  all  quad- 
rants, and  thus  would  be  represented  in  the  diagram  by  ovals  concentric 
with  the  isobars.  As  this  would  complicate  the  diagram  too  much, 
they  are  shown  in  section  in  Fig.  111.  The  temperature  is  high, 


270  METEOROLOGY 

perhaps  80°  F.,  before  the  coming  of  the  cyclone.  It  drops  to  perhaps 
70°  when  the  nimbus  cloud  comes,  due  to  the  cooling  caused  by  the 
precipitation.  I^the  calm  central  eye  it  rises  nearly  as  high  as  before 
the  coming  of  the  cyclone  and  drops  again  when  the  precipitation  begins 
afresh.  The  relative  humidity  is  high  at  the  beginning,  is  perhaps 
95  per  cent  during  the  precipitation,  and  drops  in  the  calm  central  eye 
to  perhaps  70  per  cent,  depending  on  the  rise  in  temperature. 

The  whole  formation  is  from  300  to  600  miles  in  diameter,  while  the 
central  eye  has  a  diameter  from  fifteen  to  twenty-five  miles. 

The  distribution  of  the  six  meteorological  elements  is  thus  completely 
portrayed  by  means  of  these  two  diagrams.  The  sequence  of  the 
changes  which  might  be  expected  in  the  meteorological  elements  due  to 
the  passage  of  a  cyclone  centrally  or  obliquely  pver  a  station  can  be 
determined  by  considering  these  diagrams  as  free  and  moving  them 
centrally  or  obliquely  over  a  given  point. 

270.  Some  special  tropical  cyclones.  —  Hundreds,  yes,  more  than  a 
thousand,  of  these  storms  have  been  more  or  less  carefully  observed  in 

different  parts  of  the  world  since  1400  A.D.  It  is  thus  im- 
Possible  to  write  up  the  life  history  of  all  of  them  or  even  the 
which  have  most  important  of  them.  Various  books  on  meteorology 
served"  often  give  partial  lists  of  the  most  destructive  of  them  and 
perhaps  a  more  detailed  description  of  one  or  two.  For  all 
the  known  facts  about  any  one,  the  reader  must  be  referred  to  the 
periodical  literature  of  the  subject.1  (See  Appendix  IX.) 

271.  The  last  very  destructive  hurricane  in  the  West  Indies  is  the 
one  which  caused  such  appalling  losses  at  Galveston,  September  8,  1900. 

It  is  estimated  that  here  above  6000  lives  were  lost  and 
S'hSri-68"  over  $30,000,000  worth  of  property  was  destroyed.  This 
cane  on  cyclone,  unfortunately,  is  not  very  typical  as  regards  its 
8'  behavior  or  the  path  which  it  followed.  It  appeared  first 
as  a  small  storm  on  the  morning  of  September  1  southeast  of 
the  island  of  Hayti.  It  passed  over  Cuba  on  the  5th  and  reached  south- 
western Florida  on  the  6th.  Here  it  turned  abruptly  to  the  left  and 
crossed  the  Gulf  of  Mexico,  reaching  Galveston  on  the  evening  of  the  8th 
with  its  destructive  energy  at  its  maximum.  It  was  then  about  500 
miles  in  diameter  with  a  central  pressure  of  less  than  28.48  inches,  and 
a  wind  velocity  of  more  than  a  hundred  miles  an  hour.  The  center 
passed  south  of  Galveston  but  within  less  than  40  miles  of  the  city. 

1  For  pictures  of  the  wreckage  caused  by  a  hurricane  see  Monthly  Weather  Review, 
Sept.,  1906. 


THE   SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     271 

At  Galveston,  the  lurid  sunsets  and  the  brick  dust  sky  as  heralds  of 
an  approaching  hurricane  were  entirely  lacking.  The  cirrus  clouds 
made  their  appearance  on  the  morning  of  the  7th,  coming  from  the  south- 
east, and  the  high  tides  and  the  heavy  ocean  swell  appeared  during  the 
afternoon  of  the  same  day.  The  cirrus  clouds  became  a  mixture  of 
cirrus,  alto-stratus,  and  cumulus  during  the  afternoon  of  the  7th,  and 
later  changed  to  strato-cumulus.  On  the  8th,  in  the  early  morning,  the 
cloud  forms  were  fracto-stratus  and  strato-cumulus  with  here  and  there 
a  little  blue  sky,  but  this  was  soon  followed  by  showery  weather.  The 
dense  rain  cloud  came  at  noon  and  continued  until  midnight.  The 
wind  went  to  the  northeast  on  the  7th  and  blew  with  constantly  increas- 
ing velocity.  On  the  afternoon  of  the  8th  it  was  still  northeast  and  blow- 
ing a  gale.  By  5  in  the  afternoon,  it  had  reached  full  hurricane  strength. 
Between  5  and  6.15  it  blew  at  the  rate  of  84  miles  an  hour  for  several 
five-minute  periods.  At  6.15  the  anemometer  and  other  meteorological 
instruments  blew  away  after  a  wind  velocity  of  100  miles  per  hour  had 
been  recorded  for  two  minutes.  By  estimation,  the  wind  velocity 
reached  120  miles  per  hour  between  6.15  and  8.30  P.M.  After  8.30 
the  wind  still  continued  of  hurricane  strength,  but  shifted  first  to  the 
east,  then  the  southeast,  and  finally  reached  the  south  by  11  P.M. 
From  that  time  on,  the  direction  continued  south  and  the  velocity 
steadily  decreased.  The  pressure  dropped  rapidly  all  during  the  8th 
and  reached  its  lowest,  28.48  inches,  at  8.30  P.M.  As  the  storm  passed 
a  little  south  of  Galveston,  the  central  pressure  must  have  been  a  little 
less  than  this.  After  8.30,  the  barometer  rose  rapidly.  That  all  this 
would  be  the  natural  sequence  of  events  can  be  seen  by  moving  the 
diagram  given  as  Fig.  110  from  right  to  left  below  a  given  point.  The 
destruction  of  life  and  property  at  Galveston  was  caused  by  a  storm  wave 
as  well  as  by  the  excessive  wind  velocity.  The  water  rose  steadily  all 
during  the  8th,  flooding  a  large  part  of  the  city,  and  at  7.30  P.M.  there 
was  a  sudden  rise  of  four  feet  in  a  few  minutes. 

After  passing  Galveston,  the  storm  turned  to  the  right,  passed  inland 
and  up  the  Mississippi  Valley  as  far  as  Nebraska,  where  it  then  again 
turned,  crossing  Lake  Michigan  and  Maine  and  passing  out  over  Nova 
Scotia  on  the  12th.  As  soon  as  it  went  inland  it  lost  its  violence  and 
many  of  the  characteristics  of  a  tropical  cyclone  and  was  gradually 
transformed  into  an  extratropical  cyclone.  It  also  grew  larger,  and 
by  the  time  it  reached  the  St.  Lawrence  Valley  it  was  more  than  a  thou- 
sand miles  in  diameter.  It  retained,  however,  a  low  central  pressure, 
rather  high  winds,  and  a  compact  form  all  through  its  life  history.  Figure 


272 


METEOROLOGY 


112  shows  its  path  in  detail,  and  Chart  XXVII  reproduces  the  8  A.M. 
weather  map  for  September  8,  1900,  when  it  was  near  Galveston. 


272.  Rules  for  mariners.  —  The  rules  for  mariners  which  are  ordi- 
The  rules  narily  given  in  connection  with  tropical  cyclones  are  :  first, 
for  mariners.  ^o  avoid  running  before  the  wind,  particularly  when  the 
center  of  the  cyclone  is  to  the  westward,  as  this  would  bring  the  vessel 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     273 

towards  the  center  and  across  the  track  of  the  coming  cyclone ;  secondly, 
to  so  direct  the  vessel  as  to  avoid  as  far  as  possible  the  so-called  "  dan- 
gerous half  "  of  the  cyclone. 

To  apply  these  rules  necessitates  the  locating  of  the  center  of  the 
cyclone,  and  this  can  be  done  from  the  wind  direction.     When  cyclones 
were  first  studied,  it  was  thought  that  the  winds  were  truly  L0catin  the 
circular,  that  is,  followed  the  isobars ;    and  if  this  were  the  storm 
case,  the  application  of  Buys  Ballot's  famous  law  :    "  Stand  centen 
with  your  back  to  the  wind  and  the  low  pressure  will  be  on  your  left 
hand,"  would  give  at  once  the  direction  of  the  storm  center.     But 
it  is  now  known  that  the  wind  direction  makes  an  angle  with  the  isobars, 
and  for  this  reason  the  storm  center  can  be 
more  readily  located  by  means  of  a  diagram. 
Let  A  (Fig.  113)  represent  the  wind  direction, 
then  B  must  be  the  position  of  the  isobaric  line 
and  the  line  C,  at  right  angles  to  B,  must  give 
the  direction  toward  the  center  of  the  cyclone. 
The  direction  of  the  center  is  thus  known,  and 
from  observations  of  the  sea  and  sky  a  close 
estimation  of  its  distance  can  be  made.     The    ISOBARIC, 
direction  of  the  cirrus  bands  across  the  sky,  the  WIND 

location  of  the  nimbus  cloud  on  the  horizon,  and 
the  direction  of  motion  of  the  ocean  waves  are      Center  "of 
also   guides  as  to   the   direction  in  which  the        Cyclone, 
storm  center  is  located. 

The  so-called  "  dangerous  half  "  of  a  tropical  cyclone  is  the  north 
and  northeast  portion  when  the  cyclone  is  moving  northwest  and  the 
south  and  southeast  portion  when  it  is  moving  northeast.  The  Danger- 
All  this  applies  to  the  northern  hemisphere.  These  portions  ous  half  of  a 
are  considered  the  more  dangerous  because  the  wind  ve-  cycc 
locities  are  somewhat  larger.  The  reason  for  this  is  because  in  these 
portions  the  velocity  of  the  wind  about  the  center  is  combined  with 
(added  to)  the  velocity  of  the  permanent  winds  in  the  region  through 
which  the  cyclone  is  moving.  On  the  other  side  of  the  storm,  these  two 
velocities  are  opposed.  (See  figure  115.) 


THE  REGIONS  AND  TIME  OF  OCCURRENCE 

273.   Regions  of  occurrence.  —  There  are  five  regions  of  the  earth 
where   tropical   cyclones   occur,   and   these  are   shown   in   Fig.   114. 


274  METEOROLOGY 

They  are :  (1)  The  West  Indies,  the  Gulf  of  Mexico,  and  the  coast  of 
The  five  Florida ;  (2)  The  China  sea,  Philippine  Islands,  and  Japan ; 
regions  of  (3)  each  side  of  India  in  the  Bay  of  Bengal  and  in  the 
Arabian  Sea;  (4)  east  of  Madagascar  near  the  islands 
of  Mauritius  and  Reunion ;  (5)  east  of  Australia,  near  Samoa.  The 


WESTERN     HEMISPHERE  EASTERN     HEMISPHERE 

FIG.  114.  —  The  Five  Regions  of  Occurrence  of  Tropical  Cyclones. 

tropical  cyclones  always  occur  on  the  west  side  of  an  ocean.     This  is 
true   of    the   north   Atlantic,    the   north    Pacific,   the   south    Pacific, 

and   the   south   Indian   oceans.      Tropical   cyclones   never 

originate  on  land,  and  if  they  run  ashore,  they  weaken, 
on  the  west  lose  their  destructive  violence,  and  are  soon  transformed 
ocean  never  *n*°  extratropical  cyclones.  A  range  of  mountains  3000 
on  land  or  feet  high  is  often  sufficient  to  completely  destroy  a  tropical 
Atlantic0"1  cyclone.  It  is  also  a  very  significant  fact  that  tropical 

cyclones  never  occur  in  the  south  Atlantic  Ocean. 
274.    Tracks  of  tropical  cyclones.  —  The  tropical  cyclones  originate 
in  the  doldrums,  not  directly  at^the  equator,  but  from  8  to  12°  from  it 

on  either  side.  Those  in  the  northern  hemisphere  move 
the  track  of  northwest  through  the  trade  wind  belt-ivith  a  velocity  of 

c  clone*1  ^rom  6  to  12  mi^es  an  k°ur<  They  curve  to  the  right  in 
about  30°  north  latitude,  moving  first  due  north  and  then 
northeast  through  the  region  of  the  prevailing  westerlies.  They  grow 
somewhat  larger,  and  the  velocity  of  motion  increases  to  20,  30,  or  even 
40  miles  per  hour.  The  path  has  somewhat  the  form  of  a  parabola  with 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     275 


its  vertex  in  20°  north  latitude.  In  the  southern  hemisphere  they  move 
southwest  at  first,  and  then  recurve  in  about  25°  south  latitude  and 
move  southeast  through  the  region  of  the  prevailing  westerlies.  Fig- 
ure 115  shows  the  direction  of  rotation  of  these  storms,  the  dangerous 
half  (shaded),  and  the  form  of  the  path  for  both  hemispheres. 


S    X* 


PREVAILING  WESTERLIES 


X 


/ 


NORTHEAST 
TRADE  WINDS 


EQUATOR 


FIG.  115.  —  The  Direction  of  Rotation,  Dangerous  Half,  and  Path  of  Tropical  Cyclones. 

The  characteristics  of  the  paths  followed  by  the  West  Indies  hurri- 
canes are  well  shown  in  Fig.  116,  which  gives  the    paths  The  tracks 
followed    by    all    recorded    hurricanes    during    September  ^ies  hurrL 
from  1878  to  1900.     The  mean  track  is  also  indicated.  canes. 


276 


METEOROLOGY 


The  tracks 
of  the  cy- 


In  the  case  of  the  tropical  cyclones  which  occur  each  side  of 
In^a  m  the  Bay  °f  Bengal  and  in  the  Arabian  Sea,  this 
characteristic  parabolic  path  is  lacking.  Here  the  course 
f°U°wed  is  more  irregular,  and  the  paths  are  much  shorter 
and  more  nearly  straight  lines. 


irregular 


FIG.  116.  — The  Hurricanes  of  the  West  Indies  during  September,  from  1878  to  1900. 
(After  GARRIOTT,  U.  S.  Weather  Bureau.) 

275.  Frequency  at  different  times  of  year.  —  The  frequency  of 
occurrence  at  different  times  of  year  is  exhibited  in  the  accompanying 
table  for  the  five  regions  of  the  earth  where  tropical  cy- 
clones occur.  This  table  gives  the  percentage  frequency 
for  the  different  months,  the  name  of  the  investigator  who 
collected  the  statistics,  and  in  some  cases  the  period  cov- 
ered. The  total  number  of  these  storms  is  also  added,  but 
these  numbers  are  not  comparable  for  the  different  regions,  because 
the  length  of  time  is  not  the  same  and  there  was  no  common  standard 
of  intensity  for  determining  how  violent  a  storm  must  have  been  to  be 
considered  a  tropical  cyclone. 

It  will  be  seen  that  in  connection  with  the  cyclones  in  the  West  Indies, 
and  in  the  China  Sea  and  Philippines,  the  greatest  number  occurs  dur- 


The  fre- 
quence at 
different 
times  a 
year. 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     277 


ing  the  months  of  July,  August, 
September,  and  October;  that  is, 
in    the    late    summer.-  The  maxi_ 
In  the  case  of  the  south  mum  num- 
Pacific  and  south   In- 
dian  oceans,  the  largest  late 

i  i      •         summer. 

number  occurs  during 
December,  January,  February,  and 
March,  again  in  the  late  summer 
of  that  hemisphere.     In  the  case 
of  the  cyclones  of  the 
Bay  of  Bengal  and  the  they  occur 
Arabian  Sea  there  are  between  the 

monsoons. 

two     maximums  ;      in 

April,  May,  and  June,  and  again 

in  October  and  November.     Here 

they  occur  during  the  light,  baffling 

breezes  which  prevail  between  the 

periods   of    the   steadily  blowing 

monsoons. 


THE  ORIGIN  OF  TROPICAL 
CYCLONES 

276.    The    convectional    theory. 

— The  distribution  of  the  meteoro- 
logical elements  about 

.  The  convec- 

a   tropical    Cyclone    has    tional  theory 

been  fullv  considered,  wiiibepre- 

,     . ,      " .  f      ,       sented  from 

and    the   facts   of   ob-  the  deduc- 


tive  stand- 
point. 


servation  concerning 
the  regions  of  occur- 
rence, the  path  followed,  and  the 
frequency  at  different  times  of 
year  have  been  stated.  It  now 
remains  to  discuss  the  origin  and 
development  of  these  storms,  and 
to  account  for  all  these  facts  of 
observation.  The  so-called  con- 
vectional theory  is  the  one  usually 


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278  METEOROLOGY 

given,  and  it  will  be  presented  here  from  the  deductive  standpoint. 
The  fundamental  principles  in  connection  with  convection,  the  effect  of 
the  earth's  rotation  on  moving  air,  and  the  generalizations  in  connec- 
tion with  the  general  wind  system  of  the  world  will  be  used  as  a  basis  ; 
and  from  these,  theoretical  deductions  will  be  drawn  as  to  the  varied 
characteristics  which  a  storm  thus  produced  should  have.  An  exact 
agreement  between  the  theoretical  deductions  and  the  observed  facts 
will  stamp  both  the  deductions  and  the  observational  work  as  correct. 
277.  Convection  is  most  vigorous  in  stagnant,  overheated,  moisture- 
laden  air.  Suppose  that  a  large  mass  of  quiet,  warm,  moist  air  exists 
somewhere.  At  some  point,  due  probably  to  local  causes,  a 


and  devS      convectional  rise  will  take  place.     The  warm,  moist  air  as  it 


opment  of  a  rises  will  expand,  ,  grow  cooler,  reach  its  dew  point,  become 
cyclone.  cloudy,  and  perhaps  yield  precipitation.  The  latent  heat 
liberated  by  the  condensation  of  the  water  vapor  into  cloud 
and  rain  will  lessen  the  rate  of  cooling  of  the  rising  air  and  cause  still 
further  rise.  Due  to  the  excess  of  temperature  and  moisture,  the 
pressure  would  be  slightly  less  than  in  surrounding  regions.  The  cooler 
air,  forcing  its  way  in  to  replace  the  rising  air,  would  be  deflected  to  the 
right,  in  the  northern  hemisphere,  by  the  earth's  rotation,  and  would 
approach  the  center  in  a  counterclockwise-turning  spiral.  This  rota- 
tion of  the  air  about  the  center  would  cause  centrifugal  force,  which 
''  would  hold  the  air  away  from  the  center,  thus  causing  a  still  lower 
barometric  pressure.  This  increased  difference  in  pressure  between  the 
center  and  surrounding  regions  would  drive  in  the  air  with  greater 
vigor  and  cause  a  further  rise  of  air  in  the  center.  This  would  cause 
more  clouds,  more  precipitation,  more  latent  heat,  and  a  still  more 
vigorous  rise.  This  in  turn  would  cause  still  more  air  to  come  towards 
the  center,  which  would  make  the  whirl  still  more  vigorous,  causing 
more  centrifugal  force  and  a  still  lower  barometric  pressure  in  the  center. 
This  again  would  drive  the  whole  circulation  with  still  greater  vigor, 
and  in  this  way  the  violence  becomes  greater  and  greater  until  a  tropical 
cyclone  is  the  result.  The  air  rising  in  the  center  would  continue  to 
rise  until  it  cooled,  due  to  expansion,  to  the  temperature  of  its  surround- 
ings, when  it  would  spread  out  laterally,  still  carrying  a  slight  excess  of 
moisture  with  it.  It  would  then  settle  down  in  surrounding  regions, 
causing  a  slightly  higher  barometric  pressure. 

One  would  thus  expect  a  convection-caused  storm  of  this  kind  to 
consist  of  an  area  of  low  barometric  pressure,  with  violent  winds  blowing 
spirally  inward  and  turning  counterclockwise  in  the  northern  hemi- 


THE  SECONDARY  CIRCULATION  OF  THE   ATMOSPHERE     279 

sphere.  One  would  furthermore  expect  to  find  dense  nimbus  clouds  near 
the  center  with  copious  rain.  Due  to  the  outflow  aloft,  one  would  expect 
to  find  thin  clouds,  cirrus  or  cirro-stratus,  radiating  from  the  center, 
and  the  whole  formation  surrounded  by  a  ring  of  slightly  higher  pressure 
due  to  the  congestion  and  settling  down  of  this  air  discharged  aloft. 

278.  The  rapidity  of  rotation  and  the  centrifugal  force  grow  greater 
as  the  radius  grows  smaller.     Thus,  if  a  tropical  cyclone  were  particu- 
larly violent,  it  might  be  expected  that  many  miles  from  the 

center  the  centrifugal  force  would  be  sufficient  to  balance  central  calm 
the  pressure  gradient,  tending  to  drive  the  air  in  towards  the  would  be 
center;  and  the  result  would  be  that  the  spirally  inflowing 
air  would  not  penetrate  all  the  way  to  the  center,  but  the  cyclone  would 
have  a  calm  central  eye.  The  outflow  of  water  from  a  circular  wash- 
bowl through  a  central  vent  is  exactly  analogous.  The  outflowing  water 
begins  to  rotate  and  rotates  faster  and  faster  until  a  hollow  core  is  often 
formed  at  the  center.  In  the  case  of  the  violent  tropical  cyclone,  this 
calm  central  cylinder  of  air  would  be  surrounded  by  a  ring  of  rapidly 
whirling  and  slowly  rising  air.  This  would  entangle  some  of  the  air 
lying  next  to  it  and  carry  it  up  with  it.  As  a  result,  there  would  form  a 
vacuum  in  the  center  of  a  tropical  cyclone  if  the  air  were  not  replaced 
by  a  gentle  descending  air  current  from  above.  This  air  coming  from 
the  upper  atmosphere  would  be  fairly  moisture  free  and  would  build 
no  clouds  as  it  is  descending,  not  ascending.  It  would  be  heated  both 
by  compression  and  by  absorbing  the  insolation  of  the  sun,  and  thus 
should  have  a  fairly  high  temperature  and  a  correspondingly  low  rela- 
tive humidity. 

One  would  thus  expect,  in  the  center  of  a  violent  tropical  cyclone,  a 
calm  area  several  miles  in  diameter  with  a  gently  descending  air  current, 
without  clouds,  and  with  comparatively  high  temperature  and  low  rela- 
tive humidity. 

279.  Where  can  a  mass  of  calm,  warm,  moist  air  be  found  which 
might  serve  as  the  starting  point  of  a  tropical  cyclone?     Surely  not  in 
the  region  of  the  prevailing  westerlies  or  of  the  trade  winds, 
because  here  the  air  is  far  from  calm  and  the  mixing  is  too 
thorough   to   allow   excessive   temperature   or   amounts   of  would  be 
moisture ;   surely  not  in  the  horse  latitude,  because  here  the  originate.*0 
temperatures  are  too  low  and  we  have  to  do  with  descending 

air  currents  which  are  always  dry.  It  is  only  in  the  perpetual  summer 
sultriness  of  the  doldrums,  with  their  frequent  calms,  high  temperature, 
excessive  moisture,  and  violent  local  convection  that  tropical  cyclones 


280  METEOROLOGY 

could  be  expected  to  originate.     One  would  furthermore  expect  tropical 

cyclones  to  form  over  the  ocean  rather  than  over  the  land.     It  is  the 

ocean  which  can  readily  furnish  the  necessary  moisture,  and  a  land  sur- 

face is  too  irregular  and  offers  too  much  friction  to  permit  the  building 

of  the  regular  violent  wind  circulation  necessary  for  the  building  of  such 

a  storm.     If  the  trade  winds  blow  from  the  land  over  the  ocean,  they 

bring  but  little  moisture  with  them  to  the  doldrums.     If,  however,  they 

have  come  a  long  distance  over  the  ocean,  they  bring  with  them  to  the 

doldrums  immense  quantities  of  warm,  moisture-laden  air.     Since  the 

trade  winds  blow  from  the  northeast  in  the  northern  hemisphere  and 

the  southeast  in  the  southern,  it  is  the  west  side  of  an  ocean  which 

would  have  the  largest  amount  of  moisture  and  thus  be  the  most  likely 

place  of  origin  of  tropical  cyclones.     The  doldrums  never  invade  the 

NORTH  south    Atlantic,    and    one    would    thus    expect    this 

^  ocean  to  be  free  from  tropical  cyclones. 

l\  One  would  thus  expect  the  tropical   cyclones  to 

^_L  originate  in  the  doldrums,  always  over  the  ocean,  on 

f        j  the  west  side  of  an  ocean,  and  never  in  the  south 

\  ___  /  Atlantic. 

j4f  280.   What  would  be  the  course  naturally  followed 

!/  by  a  tropical  cyclone?     According  to  Fen-el's  law 

f  The  track       (see  secti°n  145),  if  air  starts  to  move  on 

which  tropi-    the  earth's  surface,  it  will  be  deflected  to 


right  in  the  northern  hemisphere  and 
Law  to  the  Air  expected  to  the  amount  of  the  deviation  will  depend 
Tterfrt  Cylne.a  *  upon  the  velocity  and  latitude.  Further- 

more,  it  can  be  seen  from  the  table  there  given 
(section  145)  that  for  any  given  velocity  the  deviation  is  nothing  at  the 
equator  and  steadily  increases  with  increasing  latitude.  If  this  is  ap- 
plied to  the  air  coming  into  a  tropical  cyclone  from  the  north  and  from 
the  south,  as  is  indicated  in  Fig.  117,  and  was  first  shown  by  Eerrel, 
it  will  be  seen  that  the  air  coming  from  the  south  moves  more  directly 
towards  the  storm  center  than  the  air  which  comes  from  the  north. 
As  a  result,  the  whole  formation  is  pushed  away  from  the  equator.  The 
other  factor  which  would  cause  a  motion  of  the  tropical  cyclone  is  the 
movement  of  the  wind  system  in  which  it  may  find  itself.  One  would 
thus  expect  a  tropical  cyclone  to  be  pushed  steadily  away  from  the  equa- 
tor and  at  the  same  time  to  drift  with  the  wind  system  in  which  it  finds 
itself.  One  would  thus  expect  a  parabolic  course  convex  towards  the 
west,  with  the  turning  point  or  vertex  in  the  horse  latitudes,  that  is, 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     281 

30°  north  and  about  25°  south  latitude.  One  would  not  expect  a 
tropical  cyclone  to  form  very  near  the  equator,  because  the  deviation  of 
the  air  coming  into  it  would  be  too  slight  to  cause  the  vigorous  whirl 
which  generates  the  centrifugal  force  and  the  low  barometric  pressure 
upon  which  the  life  of  the  storm  depends.  At  the  equator,  the  air 
would  come  straight  towards  the  center,  filling  up  the  depression,  and 
the  cyclone  would  never  develop.  Near  India  in  the  Bay  of  Bengal  and 
in  the  Arabian  Sea,  the  tropical  cyclones  never  escape  from  the  dol- 
drums on  account  of  the  inclosure  by  land.  As  a  result,  one  would  expect 
the  paths  to  be  short  and  somewhat  irregular  with  a  northerly  tendency. 
One  would  thus  expect,  as  regards  path  (except  in  the  case  of  those 
near  India),  a  parabolic  form  starting  out,  not  at  the  equator,  but  some 
8  to  12°  from  it  and  recurving  in  the  horse  latitudes. 

281.  At  what  time  of  year  would  tropical  cyclones  be  most  frequent? 
Since  tropical  cyclones  cannot   originate   at   the  equator,  one  would 
expect  the  largest  number  when  the  doldrums  in  their  migra-  _ 

tion    are    farthest   from   the   equator.     The   doldrums    are  year  when 
farthest  north  of  the  equator  in  the   late  summer,  August,  trop^a1 
September,  and  October,  and  for  the  northern  hemisphere  would  be 
the  maximum  number  of  cyclones  should  occur  then.     The  exPected  to 
doldrums  are  farthest  south  in  February,  March,  and  April, 
the  late  summer  of  the  southern  hemisphere,  and  in  that  hemisphere 
the  maximum  number  of  cyclones  should  occur  then.     Near  India  the 
monsoon  is  the  all-controlling  wind.     Here  one  would  expect  tropical 
cyclones  when  the  air  is  calm,  warm,  and  moist ;  that  is,  during  the 
frequent  calms  which  exist  in  the  intermissions  between  the  monsoon 
periods.     As  there  are  two  such  transition  periods  each  year,  one  would 
expect  two  periods  of  maximum  frequency  of  tropical  cyclones. 

One  would  thus  expect,  as  regards  time  of  occurrence,  that  the  maxi- 
mum number  (except  in  the  case  of  India)  would  occur  during  the  late 
summer,  with  almost  none  during  the  other  half  of  the  year. 

282.  Comparison    with    the    observed    facts.  —  Starting    with    the 
fundamental  principles  in  connection  with  convection,   the  effect  of 
the  earth's  rotation  on  moving  air,  and  the  generalizations 

in  connection  with  the  general  wind  system  of  the  world,  it  pa^so^f 
has  been  determined  in  connection  with  a  convection-caused  deduction 
storm  what  one  would  expect  as  regards  the  distribution  of  Ration. S* 
the  elements  about  it,  the  regions  of  occurrence,  the  track 
followed,  and  the  time  of  occurrence.     If  these  theoretical  deductions 
are  compared  with  the  facts  of  observation,  an  exact  agreement  will  be 


282  METEOROLOGY 

found.     This  stamps  the  whole  process  of  reasoning,  the  deductions, 
and  the  observational  work  as  correct. 

One  caution  should  perhaps  have  been  given  here.  The  convectional 
theory  requires  the  temperature  at  any  given  level  in  a  tropical  cyclone 
to  be  higher  than  at  the  same  level  in  surrounding  regions.  This  has 
never  been  verified  by  observation,  and  some  doubt  it.  If  it  should  be 
found  that  it  is  not  so,  it  would  necessitate  the  remodeling  of  the  present 
theory,  and  perhaps  some  other  theory  would  have  to  be  substituted  for  it. 

THE  COMPARISON  OF  A  TROPICAL  CYCLONE  AND  THE  CIRCUMPOLAR 
WHIRL  IN  THE  GENERAL  WIND  SYSTEM 

283.  A  tropical  cyclone  and  the  circumpolar  whirls  in  the  general 
wind  system  of  the  world  have  many  things  in  common,  although  in 
some  respects  they  stand  in  sharp  contrast. 

In  the  first  place,  both  are  areas  of  low  pressure  with  spirally  inflowing 
winds,  a  calm  center,  and  surrounded  by  a  ring  or  belt  of  high  pres- 
sure. In  the  case  of  the  tropical  cyclone,  it  is  the  whirl 
cyclone  and  caused  by  the  violent,  spirally  inflowing  winds  above 
the  circum-  fae  surface  of  the  earth  which  develops  the  centrifugal 
have  many  force,  which  causes  the  low  central  pressure.  The  central 
things  in  calm  may  be  from  fifteen  to  thirty  miles  in  diameter,  and  the 

common. 

pericyclonic  ring  is  the  belt  of  high  pressure  which  surrounds 
the  whole  formation.  In  the  case  of  the  circumpolar  whirls  it  is 
the  centrifugal  force  due  to  the  air  currents  moving  spirally  towards 
the  poles  on  the  outside  of  the  atmosphere  which  causes  the  low 
polar  pressures.  The  air  is  calm  at  the  poles,  and  the  horse  latitudes 
are  the  surrounding  belts  of  high  pressure. 

Land  also  affects  both  in  the  same  way.  If  a  tropical  cyclone  runs 
ashore,  it  weakens,  grows  less  violent,  and  is  perhaps  entirely  destroyed. 
The  absence  of  land  in  the  southern  hemisphere  is  the  reason  for  the 
lower  barometric  pressure  and  the  more  violent  winds  in  the  southern 
hemisphere. 

In  the  matter  of  temperature,  the  two  formations  stand  in  sharp 

contrast.     The  tropical  cyclone  has  a  warm  center,  while  the  circum- 

.      polar  whirls  have  cold  centers.     The  air  in  a  tropical  cyclone 

sharp  con-      is  accordingly  discharged  upward,  and  spreads  out  laterally 

trast  in          aloft.     In  the  case  of  the  circumpolar  whirls,  the  discharge 

is  downward,  and  the  air  makes  its  way  equatorward  in  the 

middle  layer  of  the  atmosphere. 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     283 
B.   EXTRATROPICAL    CYCLONES   AND    ANTICYCLONES 

DEFINITION  AND  DESCRIPTION  OF  AN  EXTRATROPICAL  CYCLONE 

284.  Definition  and  chief  characteristics.  —  An  extratropical  cyclone, 
as  the  name  implies,  is  a  whirling  storm  which  occurs  outside   the 
tropics;  that  is,  in  the  temperate  latitudes.     These  storms  The  extra_. 
are  depicted  on  the  daily  weather  maps   as  areas  of  low  tropical  cy- 
pressure,  and  appear  in  great  number  and  in  almost  infinite  th^J^of 
variety  as  regards  position  and  form.     The  ceaseless  changes  the  weather 
in  our  weather  are  due  almost  entirely  to  the  approach  and  map* 
passage    of    these    areas    of    low    pressure.      For    this    reason   they 
are  sometimes  spoken  of  as  the  lows  of  the  weather  map,  or  simply 
"  lows."     They  have  many  things  in  common  with  the  whirling  storms 
which  occur  within  the  tropics,  that  is,  the  tropical  cyclones,  but,  as 
will  be  seen  later,  in  many  respects  they  stand  in  sharp  contrast. 

An  extratropical  cyclone  can  be  best  defined  and  described  by  stating 
its  chief  characteristics.     It  is  an  area  of  low  barometric  pressure  with 
spirally   inflowing   winds   turning   counterclockwise   in   the  Definition 
northern  hemisphere  and  clockwise  in  the  southern.     The  and  descrip- 
wind   velocity   is   generally   moderate ;    the   accompanying  *!on  in  out~ 
cloud  area  is    immense ;    precipitation    usually  falls ;     the 
changes  in  temperature  and  moisture  are  large  and  well  marked.     The 
whole  formation  is  from  a  few  hundred  to  several  thousand  miles  in 
diameter  and  moves  with  moderate  velocity  from  some  westerly  to  some 
easterly  quarter. 

285.  Distribution   of   the   meteorological   elements   about   a   typical 
extratropical  cyclone.  —  The  exact  structure    and    nature  of  an  extra- 
tropical  cyclone  can  best  be  learned  by  studying  the  distribution  of  the 
meteorological  elements  about  one  that  is  fully  developed  and  typical, 
and  this  distribution's  shown  graphically  in  the  accompanying  diagram, 
Fig.  118. 

The  isobars  are  usually  oval  in  form,  the  ratio  of  the  two  axes  being 
1.9  to  1,  and  the  direction  of  the  longer  axis  is  northeast-southwest. 
The  central  pressure  averages  about  29.6  inches,  although  this  varies 
all  the  way  from  a  little  less  than  30  to  even  below  28  inches  in  some 
cases.  The  isobars  are  generally  packed  a  little  closer  together  in  the 
southwest  quadrant,  and  are  farthest  apart  in  the  northeast  quadrant. 
This  means  that  the  pressure  gradient  is  greatest  in  the  southwest 
quadrant. 

The  winds  blow  spirally  inward  towards  the  center,  turning  counter- 


284 


METEOROLOGY 


clockwise  in  the  northern  hemisphere  and  clockwise  in  the  southern. 
The  wind  direction  makes  an  angle  with  the  isobaric  lines  which  is 
greatest  in  the  northeast  quadrant,  where  it  averages  from  30  to  40° 


1200    MILES 


Fro.  118.  —  The  Distribution  of  the  Meteorological  Elements  about  an  Extratropical 

Cyclone. 


and  is  least  in  the  southwest  quadrant,  where  its  value  is  from  15  to 
25°.  The  wind  velocity  is  usually  moderate,  and  only  in  rare  cases 
enough  to  be  destructive.  It  is  least  on  the  outside  and  near  the 
center,  and  greatest  in  between. 


THE  SECONDARY.  CIRCULATION  OF  THE  ATMOSPHERE     285 

There  is  a  marked  rise  of  temperature  on  the  south  and  east  side  of 
the  storm,  where  the  winds  blow  from  some  southerly  quarter,  and  a 
decided  drop  in  temperature  on  the  west  side,  where  the  A  detaUed 
winds  blow  from  some  northerly  quarter.     The  moisture  description 
changes  are  not  shown  in  the  diagram,  but  they  follow  the  butionof  the 
temperature  changes  in  a  general  way.     On  the  southern  meteoro- 
and  eastern  sides  of  the  storm,  the  absolute  humidity  in-  elements 
creases  very  rapidly  with  the  rise  of  temperature,  and  it  aboutan 
increases  so  fast  that  even  the  relative  humidity  sometimes  Cai  cyclone 
increases  in  spite  of  the  increased    capacity  of  the  air  for  orlow- 
moisture,   due  to    the    higher    temperature.     In   the   central  part  of 
the  storm,  both  the  absolute  and  relative  humidity  are  high.     On  the 
west  side,  the  absolute  humidity  is  very  small,  where  the  winds  are  from 
the  north  and  the  temperature  is  low,  but  the  relative  humidity  remains 
high  on  account  of  the  small  capacity  of  the  air  for  water  vapor. 

The  cirrus  clouds  are  almost  entirely  lacking  on  the  west  side,  but 
extend  far  out  to  the  east.  The  nimbus  cloud  area  which  also  marks 
the  region  of  precipitation,  is  not  concentric  with  the  isobars,  but  is 
located  chiefly  in  the  southeast  quadrant.  There  are  two  series  of 
transition  clouds  from  the  cirrus  to  the  nimbus.  The  cirrus  may  be- 
come heavier,  and  then  cirro-stratus,  next  alto-stratus,  then  stratus  or 
fracto-stratus,  and  finally  nimbus.  The  cirrus  may  also  become  cirro- 
cumulus,  then  alto-cumulus,  next  strato-cumulus,  and  finally  nimbus. 
Both  forms  of  transition  may  sometimes  be  seen  in  different  parts  of 
the  sky  at  the  same  time.  On  the  west  side,  the  transition  from  nimbus 
to  clear  sky  is  usually  this  :  the  nimbus  becomes  fracto-nimbus,  disclos- 
ing often  an  upper  cloud  area  of  cirrus  or  cirro-cumulus.  The  upper 
cloud  area  extends  but  a  short  distance  from  the  center,  and  then  ceases. 
The  fracto-nimbus  usually  becomes  strato-cumulus,  then  fracto-cumulus, 
and  finally  cumulus  or  a  clear  sky. 

The  diameter  of  the  formation  averages  about  1200  miles  and  varies 
all  the  way  from  a  few  hundred  miles  to  several  thousand. 

The  weather  map  for  8  A.M.,  Dec.  30,  1907,  which  is  given  as  Chart 
XXVIII,  shows  a  very  typical  extratropical  cyclone  or  low  with 
its  center  over  the  Great  Lakes.  The  weather  maps  for  April  3,  1905 ; 
Jan.  3,  1906 ;  Oct.  9,  1909 ;  Jan.  18,  1910,  also  show  very  typical  lows. 

286.  It  must  be  held  in  mind  that  the  distribution  of  the  meteoro- 
logical elements  which  was  pictured  in  the  diagram  and  has  been  so 
fully  described  is  the  typical  one  and  applies  to  the  eastern  part  of  the 
United  States.  This  typical  distribution  is  slightly  different  in  dif- 


286  METEOROLOGY 

ferent  parts  of  the  world,  is  somewhat  different  in  summer  than  in  winter, 
and  the  actual  distribution  in  the  case  of  any  individual  storm  may 
depart  widely  from  the  type  form.1 

The  ratio  of  the  longer  to  the  shorter  axis  of  the  oval  is  about  1.9 

to  1  in  the  United  States,  1.7  to  1  in  the  Atlantic  Ocean,  and  1.8  to  1  in 

.         Europe.     The  central  pressure  is  lower  over  the  ocean  than 

distribution    in  either  Europe  or  America.     The  direction  of  the  longer 

!f-JUghtiy-       ax*s  is  more  towards  the  east  in  Europe  than  America. 

different  in 

different  The  wind  direction  makes  a  smaller  angle  with  the  isobars 
Partid°f  thC  on  ^e  ocean.  The  chief  differences,  however,  in  the  distri- 
bution of  the  elements  are  to  be  found  in  the  location  of  the 
nimbus  cloud  area  and  the  angle  which  the  wind  direction  makes 
with  the  isobars  in  the  various  quadrants.  These  are  very  dif- 
ferent in  different  places,  and  the  explanation  is  to  be  found  in  the  sur- 
face topography,  i.e.  in  the  nearness  to  the  ocean,  the  direction  towards 
the  ocean,  the  presence  and  characteristics  of  mountain  chains,  etc. 
All  of  these  differences  are,  however,  comparatively  small  and  unim- 
portant. A  low  is  essentially  the  same  formation  the  world  round. 
In  the  winter  the  isobars  are  more  nearly  circular,  and  the  central 
pressure  is  less  than  in  summer.  In  winter,  the  wind  velocity  is  usually 
Thet  ical  ^arSer>  and  the  angle  made  with  the  isobars  larger  than  in 
distribution  summer.  The  temperature  difference  between  the  west 
!?-«°mer-hat  and  east  side  of  the  storm  is  usually  much  larger  in  winter 

ainerent  in       . 

summer  than  in  summer.  The  only  important  difference  between 
Sinter1  winter  and  summer  is  in  the  nimbus  cloud  area.  In  winter, 
there  is  large,  continuous  nimbus  cloud  area  mostly  in  the 
southeast  quadrant,  and  precipitation  falls  steadily  over  this 
area.  In  summer,  this  nimbus  cloud  area  is  usually  lacking.  In  its 
place  are  cirrus,  cirro-stratus,  and  cirro-cumulus  clouds,  and,  whenever 
local  convection  takes  place,  a  thundershower  is  the  result.  Thus  in 
winter  over  this  area  a  continuous  nimbus  cloud  with  steadily  falling 
precipitation  is  expected,  while  in  summer  cirriform  clouds,  sultry 
weather,  and  thundershowers  are  to  be  expected.  The  reason  for  this 
is  the  higher  temperatures  of  summer,  as  the  increased  capacity  of  the 
air  for  water  vapor  permits  it  to  be  held  as  invisible  water  vapor,  and  it 
is  not  condensed  into  a  nimbus  cloud  until  convection  takes  place.  The 
lows  of  spring  and  autumn  are  sometimes  of  one  type  and  sometimes  of 

1  See  Meteorologische  Zeitschrift,  Juli,  1903,  pp.307,  for  the  distribution  about  "Lows," 
at  St.  Louis,  U.S.A.,  and  for  references  to  other  articles.  See  also  Annals  of  Harvard  Col- 
lege Observatory,  Vol.  XXX. 


THE  SECONDARY  CIRCULATION  OP  THE  ATMOSPHERE     287 

the  other.     The  winter  type  may  also  occur  in  summer,  but  the  oppo- 
site is  exceedingly  rare. 

Each  individual  extratropical  cyclone  or  low  has  its  own  peculiar 
characteristics  and  distribution  of  the  meteorological  elements,  and  it 
may  differ  much  from  the  normal  or  type  form.     In  order 
to  gain  anything  like  a  full  understanding  of  these  storms,  ^duafiow 
many  individual    lows   as  portrayed   on  the    daily  weather  may  differ 
maps  must  be  critically  studied.     It  would  probably  be  better 
to  defer  beginning  this  study  until  the  first  part  of  Chapter 
VIII   has  been  reached.     At  least   twenty-five  separate  storms  well 
distributed  throughout  the  year  should  be  carefully  considered.     In 
each  case  the  actual  distribution  of  the  elements  should  be  exactly 
noted  and  compared  with  the  type  form.     If  there  is  any  discrepancy, 
it  should  be  explained.     There  are  three  factors  which  cause 
a  departure  from  the  type  form :    (1)  The  surface  topog- 
raphy  of  the  country ;   such  facts  as  the  location  of  bodies  which  cause 
of  water,  nearness  to  the  ocean,  the  height,  position,  and  ancies50"*" 
direction  of  mountain  chains,  etc.     (2)  The  meteorological 
condition  of  the  country ;  such  facts  as  to  whether  the  ground  is  snow 
covered  or  not,  whether  the  temperature  and  moisture  are  large  or  not, 
etc.     (3)  The  neighboring  meteorological  formations ;  that  is,  the  posi- 
tion of  the  various  lows  and  their  antitheses,  the  highs,  with  reference 
to  the  low  in  question.     All  departures  from  a  normal  or  type  form  in 
connection  with  the  distribution  of  the  meteorological  elements  can 
be  explained  on  the  basis  of  these  three  factors. 

287.   There  is  one  modification  of  the  distribution  of  the  meteoro- 
logical elements  aboulXan  extratropical  cyclone  as  just  given,  which  is 
so  marked,  of  so  much  practical  importance,  and  of^such  Manylows 
general  occurrence  that  it  is  worthy  of  special  considera-  have  a  wind 
tion.     In  the  southern  quadrants  of  a  low,  a  so-called  wind  s 
shift  line  often  develops.     This  occurs  in  connection  with  about  one 
low  in  seven  in  this  country,  and  is  much  more  common  in  Europe. 
The  distribution  of  the  meteorological  elements  about  a  low  with  a  well- 
developed  typical  wind  shift  line  is  shown  in  Fig.  119. 

The  oval  isobars  have  a  projection  in  the  southern  quadrants  which 
appears  like  a  pocket  or  a  V-shaped  bulge  pointing  usually  towards  the 
southwest.  On  the  east  side  of  this  line,  the  wind  continues  from  the 
south  with  small  velocity ;  the  precipitation  area  usually  disappears,  and 
the  nimbus  cloud  is  replaced  by  the  cirriform  transition  clouds;  the 
temperature  and  moisture  continue  high.  On  the  west  side  of  this  line, 


288 


METEOROLOGY 


there  is  a  narrow  belt  of  nimbus  cloud  with  strong  northwest  winds. 
The  temperature  lines  are  also  packed  close  together  in  this  precipita- 
tion area.  Apart  from  this  modification  in  the  southern  quadrants,  the 
distribution  of  the  elements  is  the  same  as  before.  The  weather  map 


1000  MILES 


FIG.  119.  —  Distribution  of  the  Meteorological  Elements  about  a  Law  with  a  Typical 

Winci  Shift  Line. 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     289 

for  8  A.M.,  March  3,  1904,  which  is  reproduced  as  Chart  XXIX,  shows 
an  extratropical  cyclone  with  a  pronounced  wind  shift  line.  The 
weather  maps  for  Jan.  23,  1906 ;  Jan.  4,  1907 ;  Jan.  21,  1908 ;  May  6, 
1909;  Nov.  17,  1909;  Nov.  23,  1909;  Aug.  30,  1910,  also  show  lows 
with  typical  well-marked  wind  shift  lines. 


O°F. 


O'F. 


15  F. 


15°  F 


30'  F/        45  F./ 

EARTH'S  SURFACE 


1  MILE 


CIRRUS  CLOUD  MOTION 


4  MILES 
FIG.  120.  —  The  Structure  of  an  Extratropical  Cyclone  at  Various  Levels. 

288.   The  distribution  of  the  meteorological  elements  about  an  extra- 
tropical    cyclone,    its    characteristics    and    structure    at    the    earth's 
surface,    have    been    fully   considered.     It    now    remains,  The  struc_ 
in  order  to  complete  our  knowledge  of  this  formation,  to  ture  of  an 
consider  its  structure  at  various   levels  above  the  earth's  c?cydane 
surface.     This  information  has  been  gained  from  observa-  at  various 
tions  made  on  mountains  and  by  means  of  balloons  and    eves' 
kites  and  by  noting  the  direction  and  velocity  of  motion  of  clouds.    The 
four  small   diagrams  given  as   Fig.  120  will  help   to  make  clear  the 


290  METEOROLOGY 

structure  of  this  storm  at  various  levels.  The  first  diagram  shows 
again  the  well-known  distribution  of  pressure  and  temperature  about 
an  extratropical  cyclone  at  the  earth's  surface.  The  central  pressure 
has  been  assumed  to  be  29.6  inches,  and  the  temperature  difference 
between  the  northwest  and  southeast  parts  has  been  considered  45°  F. 
This  difference  is  rather  large,  but  not  at  all  unusual.  The  second  dia- 
gram has  been  obtained  from  the  first  by  computation  and  shows  the 
isobaric  lines  at  the  height  of  one  mile  above  the  earth's  surface.  It  has 
been  assumed  in  making  the  computation  that  the  vertical  decrease  in 
temperature  was  everywhere  the  same  and  equal  to  the  average  vertical 
temperature  gradient ;  namely,  1°  F.  for  300  feet.  Since  the  air  in  the 
southeast  quadrant  is  warm,  and  thus  light,  the  drop  in  pressure  due  to 
one  mile  of  elevation  will  be  less  there  than  in  the  northeast  quadrant, 
where  the  air  is  cold  and  heavy.  As  a  result,  the  longer  axis  of  the  oval 
isobars  now  has  a  northwest-southeast  direction,  which  is  the  direction 
of  the  greatest  temperature  contrast.  The  oval  has  also  opened  up  so 
that  the  northwest  portion  is  missing,  and  the  center  has  also  been  dis- 
placed towards  the  northwest.  This  displacement  of  the  center  has  been 
a  matter  of  frequent  observation.  On  the  summit  of  Mt.  Washington 
(elevation  6279  feet)  the  storm  center  passes  on  the  average  about  three 
hours  later  than  at  the  base  of  the  mountain.  This  corresponds  to  a 
displacement  of  between  one  and  two  hundred  miles  in  this  small  height. 
The  third  diagram  was  determined  from  the  first  in  the  same  way  as  the 
second,  and  shows  the  isobaric  lines  at  a  height  of  about  four  miles. 
The  oval  form  has  now  practically  disappeared,  and  only  a  certain 
amount  of  distortion  in  the  lines  is  now  apparent.  With  increasing 
height  this  distortion  grows  less  and  less,  and  the  isobaric  lines  become 
more  nearly  straight,  running  from  west  to  east. 

At  the  earth's  surface  the  air  motion  is  a  whirl  about  the  area  of  low 
pressure,  blowing  spirally  inward,  making  an  angle  with  the  oval  iso- 
baric lines  and  turning  counterclockwise  in  the  northern  hemisphere. 
With  increasing  elevation,  the  isobars  become  circular,  even  before  the 
level  of  the  clouds  is  reached,  and  the  air  moves  nearly  tangential  to  the 
isobars,  rising  slowly.  By  the  time  a  height  of  one  mile  has  been  reached, 
as  has  been  already  shown,  the  isobars  have  again  become  oval,  but  with 
the  axis  now  northwest-southeast,  the  center  has  been  displaced,  and  the 
northwest  portion  of  the  storm  is  missing.  An  atmospheric  whirl  can 
now  barely  be  said  to  exist,  as  the  northwest  portion  is  missing.  The 
air  motion  approximates  the  form  of  two  currents  flowing  side  by  side 
in  opposite  directions.  On  the  right  there  is  a  warm  current  coming 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     291 

from  the  southeast,  and  on  the  left  a  cold  current  coming  from  the 
northwest.  With  increasing  height  the  isobars  become  less  and  less 
oval,  and  finally  become  distorted  lines  which  gradually  grow  straighter 
and  straighter,  as  was  shown  in  the  diagrams.  While  this  is  taking  place, 
the  whirling  nature  of  the  air  motion  becomes  less  and  less  apparent,  and 
the  two  currents  flowing  in  opposite  directions  become  more  pronounced. 
There  is  also  a  motion  outward  from  the  center,  as  the  rising  air  must  be 
injected  into  the  general  circulation  aloft.  With  increasing  elevation 
these  air  currents  grow  weaker,  and  by  the  time  the  level  of  the  cirrus 
clouds  has  been  reached  (five  miles  on)  only  a  slight  distortion  of  the 
west  to  east  motion  of  the  outer  layer  of  the  atmosphere  is  apparent. 
The  fourth  diagram  gives  the  motion  of  the  cirrus  clouds  about  an  area 
of  low  pressure  as  observed  at  the  Blue  Hill  Observatory  near  Boston, 
under  the  direction  of  Mr.  A.  L.  Rotch.1  The  distortion  caused  by  the 
currents  is  only  slightly  apparent.  An  extratropical  cyclone  is  thus  a 
formation  of  the  lower  part  of  the  atmosphere.  It  is  often  stated  that 
their  influence  ceases  at  five  miles,  but,  considering  the  fact  that  some 
cross  the  Rocky  Mountains,  a  greater  height  must  be  inferred  in  some 
cases  at  least. 

289.   A   comparison  of   the  tropical  and   extratropical  cyclones.  —  A 
tropical  cyclone  and  an  extratropical  cyclone  when  considered  super- 
ficially may  seem  very  similar,  but  when  the  details  in  their 
structure  are  considered,  they  often  stand  in  sharp  contrast,  ficiaiiy  con-' 
Both  are  areas  of  low  pressure,  with  spirally  inflowing  winds  sidere.d»  thgy 
turning  counterclockwise  in  the  northern  hemisphere.     Both 
are  attended  by  immense  cloud  areas  and  precipitation.     They  differ, 
however,  in  many  ways  :  (1)  In  the  case  of  an  extratropical  cyclone,  the 
isobars  are  oval,  and  they  lie  closer  together  in  the  south-  They  differ 
west  quadrant,  while  in  the  case  of  a  tropical  cyclone,  the  in  many 
isobars  are  more  nearly  circular  and  equidistant  from  each 
other.     The  central  pressure  is  also  lower  in  the  case  of  a  tropical  cyclone. 
(2)  Wind  velocities  are  much  higher,  and  the  angle  made  with  the  isobars 
less,  in  the  case  of  a  tropical  cyclone.     (3)  In  the  case  of  a  tropical 
cyclone,  the  temperature  and  moisture  are  the  same  in  all  quadrants  ; 
that  is,  they  are  symmetrical  with  respect  to  the  center.     In  the  case  of 
an  extratropical  cyclone,  the  values  of  temperature  and  moisture  are 
much  higher  in  the  eastern  part  than  in  the  western.     (4)  In  the  case 
of  a  tropical  cyclone,  the  cirrus  cloud  extends  out  in  all  directions  from 

1  See  Annals  of  the  Harvard  College  Observatory,  Vol.  XXX.,    also  Monthly  Weather 
Review,  March,  1907. 


292  METEOROLOGY 

the  center,  and  the  transition  clouds  are  the  same  in  every  quadrant. 
In  the  case  of  an  extratropical  cyclone,  the  cirrus  cloud  is  found  only  on 
the  east  side,  and  the  transition  clouds  are  entirely  different  on  the  east 
and  west  sides.  (5)  The  rain  area  is  concentric  with  the  low  in  the 
case  of  the  tropical  cyclone,  while  it  lies  mostly  in  the  southeast  quad- 
rant in  the  case  of  an  extratropical  cyclone.  All  these  contrasts  are  well 
brought  out  if  the  two  diagrams  (Figs.  110  and  118)  which  represent 
the  distribution  of  the  meteorological  elements  about  a  tropical  and  an 
extratropical  cyclone  are  carefully  compared. 

The  great  difference  in  the  two  storms  is  noticed  in  connection  with 
the  central  calm  or  eye.  A  tropical  cyclone  usually  has  a  calm  central 
area  where  the  pressure  is  lowest,  the  clouds  break  through, 
difference  is  ^ne  temperature  rises,  and  the  relative  humidity  is  much 
the  presence  less.  An  .extratropical  cyclone  probably  never  has  such  a 
centrafeye.  central  calm  or  eye.  Some  have  thought  that  if  the  extra- 
tropical  cyclone  is  very  violent,  an  approximation  to  a  calm 
central  eye  exists.  This  is  probably  not  the  case,  however,  and  the 
suggestion  of  an  eye  was  given  by  the  fact  that  the  low  in  question  had 
a  pronounced  wind  shift  line.  If  such  a  low  passes  a  little  to  the  north  of 
an  observer,  the  precipitation  may  stop,  the  clouds  change  from  nimbus 
to  a  cirriform  variety,  the  temperature  and  moisture  may  remain  high, 
and  the  wind  may  continue  to  blow  gently  from  the  south.  Soon  after 
it  may  be  again  raining,  with  the  wind  blowing  sharply  from  the  north- 
west and  the  temperature  dropping  rapidly.  This  would  give  the 
impression  of  an  eye,  but  it  is  really  only  the  passage  of  a  wind  shift 
line.  How  this  sequence  of  changes  is  possible  can  be  readily  seen  from 
the  diagram  (Fig.  119),  which  gives  the  distribution  of  the  meteoro- 
logical elements  about  a  low  with  a  typical  wind  shift  line. 

290.  Description  of  the  approach  and  passage  of  an  extratropical 
cyclone.  —  The  first  signs  of  the  approach  of  an  extratropical  cyclone 
or  low  are  usually  these :  The  wind  begins  to  blow  gently 
quence'of  from  the  east,  the  pressure  decreases  slightly,  cirrus  clouds 
weather  make  their  appearance,  and  the  temperature  and  moisture 
tothTap-116  begin  to  increase.  Next  the  barometer  drops  a  little  more, 
proach  and  the  wind  direction  changes  to  the  southeast  and  the  velocity 
alow?6  °  becomes  a  little  greater,  the  cirrus  clouds  thicken  to  cirro- 
stratus  or  cirro-cumulus,  and  the  temperature  and  moisture 
continue  to  rise.  In  the  winter  time,  as  the  popular  phrase  goes, 
the  weather  has  begun  to  moderate.  In  the  summer  time,  it  is  the 
beginning  of  a  period  of  sultriness.  The  pressure  now  drops  still  more, 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     293 

the  wind  veers  a  little  and  blows  harder,  the  cirriform  clouds  go  through 
their  regular  transition  into  nimbus,  and  the  temperature  and  moisture 
are  high  and  increasing.  Next  comes  a  period  of  rain  or  snow,  with  the 
barometer  still  dropping  and  finally  reaching  its  lowest.  The  wind, 
meantime,  has  slackened  somewhat  and  veered  a  little  and  is  perhaps 
now  blowing  from  the  south  or  southwest.  The  temperature  and  mois- 
ture still  continue  high.  The  wind  now  veers  rather  quickly  into  the 
southwest,  then  west,  and  finally  northwest.  The  barometer  begins  to 
rise,  the  precipitation  grows  less,  and  the  temperature  and  moisture 
decrease.  Soon  the  nimbus  clouds  break  up  into  fracto-nimbus,  perhaps 
disclosing  the  upper  cloud  area.  The  fracto-nimbus  then  changes  into 
strato-cumulus,  and  finally  cumulus  or  fracto-cumulus,  with  a  clear  sky 
at  night.  In  the  meantime,  the  wind  blows  from  the  northwest  with 
increasing  velocity,  the  barometer  is  rising,  and  the  temperature  drops 
rapidly.  The  air  also  becomes  much  dryer.  In  the  summer,  the  dry, 
cool,  northwest  wind  has  replaced  the  oppressive  sultriness  of  a  few  days 
before.  In  the  winter,  the  thaw  or  warm  spell  has  been  replaced  by  a 
cold  snap.  How  often  this  sequence  of  weather  changes  has  been 
noted  by  every  one  without  realizing  that  it  was  but  the  approach  and 
passage  of  an  extratropical  cyclone.  This  series  of  weather  changes 
requires  from  two  to  four  days  in  winter  for  its  completion  and  nearly 
double  the  time  in  summer. 

The  series  of  weather  changes  which  has  just  been  described  applies 
to  a  nearly  central  passage  of  a  low.  If  it  passes  to  the  north  or  south 
of  an  observer,  the  sequence  would  be  different  and  depend  veering  and 
upon  the  distance  of  the  center  of  the  low.  Just  what  the  backing 
sequence  would  be,  can  be  determined  at  once  by  consider- 
ing the  diagram  given  as  Fig.  118,  movable  and  moving  it  from  left 
to  right  centrally  over  or  above  or  below  a  given  point.  It  is  worthy  of 
mention  that  if  the  low  passes  north  of  the  observer,  the  wind  veers, 
while  if  it  passes  south  of  the  observer,  the  wind  backs.  Since  most 
lows  pass  north  of  the  United  States,  veering  winds  are  the  rule  and 
backing  the  unusual.  If  the  low  has  a  wind  shift  line,  the  sequence  of 
weather  changes  can  be  determined  in  the  same  way  by  using  the  dia- 
gram given  as  Fig.  119. 

In  the  early  part  of  the  last  century,  when  this  sequence  of  weather 
changes  was  first  noticed,  it  was  explained,  as  was  first  done  The  cause 
by  Dave,  by  assuming  that  there  were  two  currents  of  air,  of  weather 
one  warm  and  moisture  laden  from  the  south  and  the  other 
cold  and  dry  from  the  north.     These  were  supposed  to  flow  above  each 


294  METEOROLOGY 

other  in  the  tropics  and  side  by  side  in  the  temperate  latitudes.  It 
was  to  the  ceaseless  struggle  of  these  opposing  air  currents  and  to  the 
temporary  victory  of  first  one,  then  the  other,  that  the  weather  changes 
were  ascribed.  This  gave  rise  to  the  popular  expression  that  the  wind 
makes  the  weather.  It  is  now  known  that  the  mechanism  back  of  the 
ceaseless  changes  in  our  weather  is  the  approach  and  passage  of  extra- 
tropical  cyclones  or  lows. 

ANTICYCLONES 

291.  Just  as  two  valleys  are  an  impossibility  without  a  hill  or  ridge 
of  land  between  them,  in  the  same  way  two  areas  of  low  pressure  can- 
not exist  without  a  region  of  high  pressure  between.     These 

clones'or  areas  of  high  pressure  stand  in  many  ways  in  sharp  con- 
highs  are  trast  with  the  lows,  and  have  many  characteristics  which 
of  lows.08  are  exactly  the  opposite.  For  this  reason  they  are  called 

anticyclones,  or,  sometimes,  simply  highs. 

An  anticyclone  or  high  can  be  best  defined  and  described  by  stating 
its  chief  characteristics.  It  is  an  area  of  high  barometric  pressure  with 
Outline  spirally  outflowing  winds  turning  clockwise  in  the  northern 
description,  hemisphere  and  the  opposite  in  the  southern;  the  wind 
velocity  is  usually  very  moderate,  and  calms  are  frequent ;  but  few 
clouds  are  to  be  seen,  and  precipitation  is  usually  lacking ;  the  changes 
in  temperature  and  moisture  are  large  and  well  marked.  The  whole 
formation  is  from  several  hundred  to  several  thousand  miles  in  diameter, 
and  moves  with  moderate  velocity,  sometimes  loitering  for  a  day  or 
two,  from  some  westerly  to  some  easterly  quarter. 

292.  The  distribution  of  the  meteorological  elements  about  a  typical 
anticyclone  is  shown  in  Fig.  121. 

The  isobars  are  often  irregular  in  form,  and  in  the  center  there  may  be 
several  highs  instead  of  a  single  peak  of  pressure.  The  more  usual 
form,  however,  is  the  oval,  and  the  longer  axis  generally 
extends  northeast-southwest,  north  and  south,  or  north- 
butionofthe  west-southeast.  The  central  pressure  averages  about  30.6 
^sdliemeSs  mcnes>  although  this  varies  all  the  way  from  a  little  over 
about  an  30  to  more  than  31  inches. 

or  high10*  The  winds  blow  spirally  outward  from  the  center,  turning 

clockwise  in  the  northern  hemisphere.  The  wind  direction 
makes  an  angle  with  the  isobars  which  is  least  in  the  northeast  portion, 
where  it  averages  about  20  or  30°  and  is  greatest  in  the  southwest  por- 
tion, where  the  average  is  from  60  to  70°.  The  wind  velocity  is 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     295 


very  moderate  and  decreases  towards  the  center,  where  calms  are  fre- 
quent, particularly  at  night. 

There  is  a  decided  drop  in  temperature  in  the  northeast  portion, 
where  the  winds  are  from  the  north,  and  a  decided  rise  in  temperature 


\ 


\ 


\ 


FIG.  121.  —  The  Distribution  of  the  Meteorological  Elements  about  a  Typical  "^ 
Anticyclone  or  High. 

in  the  southwest  and  west  portions,  where  the~winds  are  from  the  south. 
The  moisture  changes  are  not  shown  in  the  diagram,  but  they  follow  the 
temperature.  In  the  northeast  portion,  both  the  absolute  and  the 
relative  humidity  are  low,  while  in  the  southwest  portion  both  are  high. 


296  METEOROLOGY 

On  the  east  side  the  strato-cumulus  clouds  of  the  departing  low  may 
be  still  tisible.  In  the  middle  portion,  convection-caused  cumulus  or 
fracto-cumulus  clouds  may  be  found  These  usually  disappear  at  night 
and  are  most  numerous  on  the  east  side  of  the  center.  In  the  winter,  a 
thin  alto-stratus  cloud  due  to  radiation  may  be  formed  during  the  night 
and  be  visible  in  the  early  morning.  On  the  west  side,  the  cirrus  and 
cirro-stratus  of  the  coming  low  may  have  already  made  their  appear- 
ance. Precipitation  very  seldom  falls  in  connection  with  an  area  of 
high  pressure.  Occasionally  a  cumulus  cloud  may  become  sufficiently 
overgrown  to  yield  a  snow  flurry  in  winter  or  a  few  raindrops  in 
summer.  " 

The  diameter  of  the  whole  formation  averages  perhaps  2000  miles 
and  varies  from  several  hundred  to  several  thousand  miles. 

The  weather  map  for  8  A.M.,  April  23,  1906,  which  is  given  as  Chart 
XXX,  shows  a  very  typical  anticyclone  or  high  with  its  center  over 
Illinois.  The  weather  maps  for  May  1,  1905  ;  Oct.  28,  1907  ;  Jan.  3, 
1908  ;  Jan.  30,  1909  ;  March  22,  1909  ;  April  10,  1909  ;  June  18,  1909  ; 
Sept.  26,  1909;  Oct.  29,  1909;  March  14,  1910;  Dec.  21,  1910,  also 
show  typical  highs  at  various  times  of  year. 

293.  It  must  be  remembered  that  the  distribution  of  the  meteorologi- 
cal elements  about  an  anticyclone  or  high  which  has  just  been  so  fully 
stated  is  the  typical  one,  and  applies  to  the  eastern  part  of 
the  United  States.  This  typical  distribution  is  slightly 
is  different  different  in  different  parts  of  the  world  and  at  different  times 
countries11*  °^  vear>  an(^  the  actual  distribution  in  the  case  of  any  indi- 
and  at  dif-  vidual  high  may  depart  widely  from  the  type  form. 

11  ^n  Europe,  the  direction  of  the  longer  axis  of  the  oval  is 


more  northeast-southwest  than  in  the  United  States,  and 
the  quadrant  in  which  the  wind  direction  makes  the  smallest  angle  with 
the  isobars  is  also  different. 

In  winter,  the  highs  are  larger  and  the  central  pressure  is  higher 
than  in  summer. 

Each  individual  high  has  its  own  peculiar  characteristics,  and  distri- 

bution of  the  meteorological  elements,  and  it  may  depart  much  from  the 

type  form.     The  only  way  to  gain  detailed  knowledge  about 

highs  differ     this  formation  is  to  study  carefully  many  individual  cases 

from  the        as  they  occur  on  the  daily  weather  maps.     In  each  case, 

note  carefully  the  actual  distribution  of  the  elements,  com- 

pare this  with  the  type  form,  and  then  explain  all  discrepancies.     The 

three  causes  of  departure  from  type  are  the  same  as  for  lows  ;  namely, 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     297 


X 


(1)  the  surface  topography  of  the  country,  (2)  the  meteorological  condi- 
tion of  the  country,  (3)  the  neighboring  meteorological  formations. 

294.   The  characteristics  of  a  high  at  the  earth's  surface  have  now 
been  fully  stated,  and  it  remains  to  consider  its  structure  at  various 
levels  above  the  earth's  surface.     The  change  in  form  with 
elevation  is  very  similar  to  that  in  the  case  of  a  low,  and  for  ^  Jj^~ 
that  reason  it  will  only  be  sketched  in  outline  here.     At  the  anticyclone 
earth's  surface  the  air  motion  is  a  whirl  about  an  area  of  fevdJ10118 
high  pressure.     The  air  moves  spirally  outward,  making  an 
angle  with  the  oval  isobaric  lines,  and  turning  clockwise  in  the  northern 
hemisphere.     Since  the  northern  portion  is  much  colder  than  the  south- 
ern, the  pressure  decrease  with  elevation  will  be  greater  in  the  northern 
portion  than  in  the  southern.     By  the  time  a  height  of  one  mile  has  been 
reached,  the  center  has  been  displaced  towards 
the  cold  area  (that  is,  towards  the  north),  the 
oval  isobars  have  opened  out,  and  the  northern 
portion  of  the  formation  is  lacking.     An  atmos- 
pheric whirl  can  now  be  hardly  said  to  exist, 
and  the  air  motion  approximates  the  form  of 
two  currents  flowing  side  by  side  in  opposite 
directions.      On    the    right    there    is    a    cold 
current  coming  from  the  north,  and  on  the 
left  a  warm  current*  coming  from  the  south. 
With   increasing   elevation,  the   oval    isobars 

open  out  more  and  more,  and  finally  become  distorted  lines  which  tend 
to  become  straighter. 

While  this  is  taking  place,  the  whirling  nature  of  the  air  motion  be- 
comes less  apparent,  and  the  two  currents,  flowing  in  opposite  directions, 
become  more  pronounced.  There  is  also  a  component  of  motion  in 
towards  the  center,  for  the  air  which  moves  spirally  outward  at  the 
earth's  surface  is  replaced  by  a  gentle  descending  air  current  which  must 
be  supplied  from  above.  With  ever  increasing  elevation,  the  isobars 
become  straighter  and  straighter  and  the  two  air  currents  become  less 
and  less  pronounced,  until,  by  the  time  the  level  of  the  cirrus  clouds 
have  been  reached  (five  miles  on),  only  a  slight  distortion  of  the  west 
to  east  motion  of  the  outer  layer  of  the  atmosphere  is  apparent.  The 
motion  of  the  cirrus  clouds  about  an  area  of  high  pressure  as  observed 
at  Blue  Hill  is  given  in  Fig.  122.  This  distortion  caused  by  the  air 
currents  beneath  is  only  slightly  apparent.  Thus  the  high,  like  the  low, 
is  a  formation  of  the  lower  atmosphere. 


X 


FIG.  122.  — The  Motion  of 
the  Cirrus  Clouds  about  an 
Area  of  High  Pressure. 


298  METEOROLOGY 

295.  The  sequence  of  weather  changes  brought  about  by  the  approach 
and  passage  of  an  anticyclone  or  high  has  been  experienced  many  times 
by  every  one  without  knowing  the  cause  of  the  changes.  A 
quence'of  ^ow  nas  Pr°bably  just  passed;  its  rain  area  has  gone  by; 
weather  the  cloud  form  has  changed  to  strato-cumulus ;  the  wind 
tothTap^116  nas  g°ne  to  the  northwest;  the  temperature  has  dropped 
proachand  markedly;  the  air  has  become  much  dryer;  the  barometer 
hfgh^6  (  *  ig  rismg-  All  this  may  be  expressed  meteorologically  by 
saying  that  a  low  has  passed  and  the  weather  con- 
trol is  being  transferred  to  a  coming  high.  It  may  be  expressed 
popularly  by  saying  that  it  has  cleared  off  and  several  days  of  good 
weather  are  in  store.  The  wind  continues  in  the  northwest  and  de- 
creases in  velocity.  In  fact  the  nights  are  almost  perfectly  calm  and 
the  wind  blows  only  during  the  day.  The  barometric  pressure  con- 
tinues to  rise,  usually  holding  about  steady  during  the  daytime  and 
rising  quite  a  little  during  the  night  and  early  morning.  The  strato- 
cumulus  clouds  become  cumulus  or  fracto-cumulus  and  decrease  in 
number  and  size.  Since  these  are  convection-formed,  they  disappear 
at  night  and  are  visible  only  during  the  daytime.  The  temperature 
drops  fairly  low  at  night  and  rises  rapidly  during  the  day  so  that  the 
daily  range  is  excessive.  Soon  the  center  of  the  high  is  reached.  There 
is  an  almost  perfect  calm  both  day  and  night.  The  pressure  is  at  its 
highest.  The  sky  is  cloudless  or  at  most  shows  a  few  cumulus  clouds. 
The  air  is  dry  and  the  daily  range  in  temperature  continues  large.  Then 
the  barometer  begins  to  fall.  The  wind  goes  to  the  northeast  or  east 
and,  blowing  at  first  very  gently,  soon  increases  in  velocity.  The 
temperature  begins  to  rise,  and  soon  the  cirrus  cloud  puts  in  its  appear- 
ance. The  barometer  continues  to  fall;  the  wind  blows  harder  and 
perhaps  shifts  to  the  southeast ;  the  temperature  rises  rapidly ;  the  air 
becomes  more  moist;  the  cirrus  clouds  thicken  into  cirro-stratus  or 
cirro-cumulus ;  and  the  weather  control  passes  from  the  departing  high 
to  the  coming  low.  This  series  of  weather  changes  requires  from  two 
to  four  days  in  winter  for  its  completion  and  nearly  double  the  time  in 
summer. 

The  series  of  weather  changes  which  has  just  been  described  applies 
to  the  nearly  central  passage  of  a  high.  If  it  passes  to  the  north  or 
south  of  an  observer,  the  sequence  would  be  somewhat  different.  Just 
what  the  sequence  would  be  can  be  determined  by  moving  the  diagram 
given  as  Fig.  121  from  left  to  right  centrally  over  or  above  or  below  a 
given  point. 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     299 

In  summer,  if  a  high  loiters  for  several  days,  there  is  a  series  of  days 
with  rather  high  temperature  during  the  day  but  with  cool  nights.  If  it 
loiters  unduly  long,  a  spell  of  dry  weather  is  the  result.  In  winter  the 
low  temperatures,  sometimes  many  degrees  below  zero,  occur  during  the 
first  two  days  of  a  high.  In  autumn  the  morning  fogs  occur  usually 
during  anticyclonic  weather,  because  it  is  then  that  the  daily  range  of 
temperature  is  so  large.  In  spring  the  destructive  frosts  usually  occur 
just  after  the  weather  control  has  passed  from  a  low  to  a  strong  coming 
high. 

THE  TRACKS  AND  VELOCITY  OF  MOTION  OF  EXTRATROPICAL  CYCLONES 

296.    Tracks  in  the  northern  hemisphere.  —  If  an  observer  far  above 
the  earth  could  look  down  upon  the  whole  northern  hemisphere,  he  would 
see  a  ceaseless  procession  of  lows  between  the  thirtieth  and 
eightieth  parallels  of  latitude,  moving  eastward  and  encir-  eastward 


cling  the  poles.  Their  big  cloud  areas  would  gleam  white  and 
in  the  reflected  sunlight.  In  Fig.  123  (after  Loomis)  the 
tracks  of  a  number  of  lows  are  shown.  Taking  the  northern  hemisphere 
as  a  whole,  the  tracks  followed  by  lows  may  be  characterized  by  stating 
that  they  move  eastward  and  slightly  poleward.  The  reason  for  this 
eastward  motion  is  because  this  is  the  prevailing  direction  of  both  the 
surface  winds  and  the  fast-moving  upper  air  currents  in  these  latitudes. 
The  lows  thus  drift  with  the  general  wind  system. 

Lows  may  originate  anywhere.     A  good  part  of  those  which  visit 
Europe  originate  over  the  Atlantic  Ocean.     In  the  United  Theymay 
States,  a  favorite  place  of  origin  is  the  Mississippi  Valley  originate 
just  east  of  the  Rocky  Mountains.     The  length  of  the  path  anywhere- 
followed  varies  from  a  few  hundred  miles  to,  in  a  few  rare  cases,  more 
than  half  of  the  circumference  of  the  globe. 

The  tropical  cyclones  come  up  from  the  tropics  and  join  the  extra- 
tropical  ones  in  two  places  :    over  the  West  Indies  and  over  the  Philip- 
pines and  Japan.     As  soon  as  a  tropical  cyclone  enters  the         . 
extratropical    region  it  loses  its  violence;    the  central  eye  cyclones 
disappears;    the  rain   area  becomes  excentric;  its  velocity 
of  motion  is  larger,  and  it  soon  assumes  all  the  characteristics 
of  an  extratropical  cyclone.     In  fact,  in  a  few  cases,  it  has  been  known 
to  merge  with  an  extratropical  cyclone. 

297.    Tracks  across  Europe.  —  If  a  smaller  portion  of  the  world  is 
considered,  as  for  example  Europe,  it  is  found  that  not  all  areas  are 


300 


METEOROLOGY 


covered  by  the  same  number  of  lows.  The  lows  seem  to  prefer  to 
The  Van  travel  along  certain  rather  definite  paths;  and  if  the  actual 
Bebber  sys-  paths  followed  by  the  lows  for  a  number  of  years  are  general- 
tra'k&f  f  r  *ze<^  or  summarized,  ^  *s  found  that  a  more  or  less  definite 
Europe.  system  of  storm  tracks  is  the  result.  This  has  been  done 


Fio.  123.  —  The  Tracks  of  a  Number  of  Lows  in  the  Northern  Hemisphere. 
(After  LOOMIS.) 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     301 


Zugslrasse 

der 
Minima,. 


with  particular  skill  by  Van  Bebber,  the  head  of  the  German  meteoro- 
logical service,  and  the  resulting  storm  tracks  for  Europe  are  shown  in 
Fig.  124.  The  width 
of  the  track  is  pro- 
portional to  the  fre- 
quence with  which 
it  is  followed  by  lows. 
Track  I  is  used  more 
than  any  other,  and 
chiefly  in  the  autumn 
and  winter.  It  is 
but  little  frequented 
in  spring.  Tracks 
II,  III,  and  IV  are 
traversed  at  all  times 
of  the  year.  Tracks 
II  and  III  are  used 
a  little  more  during 
the  cold  portion  of 
the  year  and  IV  in 
summer  and  autumn. 
Track  Va  is  almost 
never  traversed  in  summer, 
all  times  of  year. 

298.  Tracks  across  the  United  States.  —  The  tracks  of  lows  across  the 
United  States  have  been  generalized  by  Bigelow,  Russell,  and  Van  Cleef 
and  the  results  of  all  three  investigators  are  presented  here. 

In  Fig.    125  the    Bigelow  system    of    tracks  is  shown.  ™*t^**{ 
The  main  track  follows  the  northern  boundary  of  the  United  tracks 
States  across  the  Great  Lakes  and  out  the  St.  Lawrence  Valley,  £*.<£*  the 
This  main  track  is  joined  by  three  others  coming  up  from  states, 
the  south.     One  comes  up  from  Colorado   and  Utah  and 
joins  it  near  Lake  Superior.     Another  comes  up  from  Texas  and  joins 
it  near  Lake  Huron.     The  third  comes  up  the  Atlantic  coast  and  joins 
it  near  Nova  Scotia.     There  is  a  second  main  track  across  The  Bige_ 
Texas  and  the  Gulf  States  to  the  Atlantic  coast,  where  it  low  system 
either  turns  northward  or  goes  out  over  the  ocean.     The  oftracks- 
broken  lines  show  the  average  daily  movement.     This  system  of  tracks 
is  rather  too  highly  generalized.     It  has  been  made  simple  by  throwing 
out  .too  many  tracks  as  erratic  or  exceptional.     The  percentage  of  lows 
which  will  follow  these  tracks  has  thus  been  reduced. 


FIG.  124.  — Storm  Tracks  for  Europe. 
(From  BERBER'S  Lehrbuch  der  Meteorologie.) 

Tracks  V6,  Vc,  and  Vd  are  frequented  at 


302 


METEOROLOGY 


FIG.  125.  —  The  Bigelow  System  of  Storm  Tracks  across  the  United  States.     (The  Tracks 
of  Highs  are  also  shown.)     (U.  S.  Weather  Bureau.) 


FIG.  126.  —  The  Russell  System  of  Tracks  across  the  United  States. 
(From  RUSSELL'S  Meteorology.) 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     303 


299.  In  Fig.  126  the  Russell  system  of  tracks  is  shown.     Since  there 
are  eleven  tracks,  this  system  is  not  as  highly  generalized  as  The  Russeli 
the  first  one,  and  a  larger  per  cent  of  the  lows  will  follow  system  of 
some  one  of  these  tracks.  tracks. 

300.  In  Fig.  127  the  Van  Cleef  system  of  tracks  is  shown.      This 
is  the  least  generalized  of  the  three  and  thus  accounts  for  the  paths 
followed   by    the   largest   number  of  lows.      In    preparing  The  Van 
this  diagram,  the    tracks  of  the  lows  from  1896  to  1905,  Cleef  system 
1160  in   number,   were    used.      Of    these    1160    only    57  oftracks- 
were  erratic  and  did  not  follow  some  one  of    the  tracks.     The    fre- 
quency with  which  any  track  is  traversed  is  indicated  by  its  width. 


FIG.  127.  —  The  Van  Cleef  System  of  Storm  Tracks  across  the  United  States.     (Twenty- 
seven  Tracks  are  represented.)     (U.  S.  Weather  Bureau.) 

301.   If  a  low  does  not  originate  in  the  United  States,  it  enters  it 
from  the  northwest,  west,  or  south.     Those  which  enter  north  of  the 
middle  of  the  country  have  a  tendency,  in  crossing  the  Missis-  A  smnmary 
sippi  Valley,  to  move  towards  the  south  and  then  recurve  of  the  three 
towards  the  northeast.     In  approaching  the  Atlantic  coast,  syst< 
all  lows  move  towards  the  northeast.     Although  lows  may  originate 
within  the  country  or  enter  it  from  various  directions,  they  nearly  all 


304 


METEOROLOGY 


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leave  it  by  way  of  the 
St.  Lawrence  Valley  and 
Newfoundland.  The 
phrase  "  all  roads  lead 
to  Rome "  might  be 
paraphrased  in  connec- 
tion with  lows  into  "all 
storm  tracks  lead  to 
New  England." 

It  must  not  be  sup- 
posed that  all  lows  fol- 
low one  or  the  other  of 
these  many  tracks  with- 
out exception.  A  low 
may  loiter  in  one  place 
for  a  day  or  two,  or 
turn  sharply  aside  in 
one  direction  or  another, 
or  move  erratically  in 
almost  any  direction. 
These  tracks  simply 
represent  normal  be- 
havior. In  order  to 
gain  familiarity  with 
the  paths  followed  by 
lows,  many  individual 
cases,  as  indicated  on 
the  daily  weather  maps, 
must  be  studied. 

302.    Velocity  of  mo- 
tion. —  Several   impor- 
tant facts  in 

The  number 

and  velocity  Connection 
of  motion  wjth  the 
of  lows. 

velocity  of 
motion  of  lows  are 
brought  out  by  the 
accompanying  table, 
which  gives  the  number 
and  velocity  of  motion 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     305 

of  lows  for  the  ten  years  (1882   to    1891),  classified  according  t.o  the 
Russell  system  of  tracks. 

The  average  velocity  for  all  tracks  for  all  times  of  year  is  31.7  miles 
per  hour.  The  winter  velocity  is,  however,  nearly  twice  as  large  as  the 
summer  (January,  40;  June  and  July,  24.9).  The  velocity  of 
motion  does  not  differ  much  in  the  different  tracks  and  is 
greatest  for  the  tracks  which  have  the  greatest  curvature,  the  velocity 


Some  tracks  are  much  more  frequented  than  others  ;    com-       ™01011  °f 


pare,  for  example,  I  or  V  with  IV  or  XI.  Some  tracks  are 
also  more  frequented  at  certain  times  of  year  ;  I,  for  example,  with  3  in 
April  and  25  in  December,  or  VIII  with  1  in  July  and  13  during  both 
December  and  February.  It  is  also  a  very  important  fact  that  the 
number  of  lows  in  winter  is  more  than  double  the  number  in  summer  ; 
compare,  for  example,  December,  71  with  June,  33. 

Lows  move  faster  across  the  United  States  than  any  other  country. 
One  authority,  which  gives  the  average    velocity  for  the  velocity  in 
United  States  as  26  miles  per  hour,  finds  for  Japan  a  veloc-  various 
ity  of  24;  Russia,  21;  the  north  Atlantic,  18;  and  Europe,  countries- 
16.     That  lows  move  faster  in  winter  than  in  summer  is  found  to  be 
true  for  all  parts  of  the  world. 

It  must  not  be  supposed  that  the  velocity  of  motion  of  individual  lows 
always  conforms  to  the  normal.     A  low  may  loiter  for  a  day  or  two, 
drifting,  perhaps,  but  one  or  two  hundred  miles  in  that  individual 
time.     Another  low  may  rush  across  the  country,  covering  a  lows  are 
distance  of   sixteen  or  seventeen  hundred   miles  in  a  single 
day.     As  has  been  so  often  stated,  the  only  way  to  learn  the  character- 
istics of  lows  and  highs,  their  tracks  and  velocity  of  motion,  is  to  study 
them  as  they  are  portrayed  from  day  to  day  on  the  weather  maps.     In 
making  this  study,  the  normal  behavior  should  be  held  in  mind,  com- 
parisons made,  and  discrepancies  explained.  ' 


THE  TRACKS  AND  VELOCITY  OF  MOTION  OF  ANTICYCLONES 

303.   Anticyclones    or   highs   are  more    erratic    than   lows,  both  as 
regards  the  track  followed  and  the  velocity  of  motion. 

In  Fig.  125,  the  Bigelow  system  of  tracks  for  highs  was 
shown.     Highs  usually  enter  the  country  from  the  extreme  low  system 
northwest  or  over  California.     To  those  entering  from  the  £?  hgCks  for 
extreme  northwest,  two  courses  are  open.     They  may  move 
eastward  and  slightly  southward  along  the  northern  boundary  of  the 


306 


METEOROLOGY 


United  States  until  the  Atlantic  coast  is  reached,  where  they  turn  to- 
wards the  northeast  and  proceed  in  the  direction  of  Iceland.  The  highs 
entering  from  the  northwest  may  also  move  southeast  over  Kansas  and 
the  Gulf  states  to  the  Atlantic  coast  near  Florida.  The  highs  coming  in 
over  California  usually  move  southeast  and  join  this  track  just  south  of 
Kansas.  After  leaving  the  Atlantic  coast,  near  Florida,  the  highs  usually 
continue  to  move  towards  the  southeast  in  the  direction  of  Bermuda. 


Paths  classified 741 

Paths  miscellaneous 98 

Paths  incomplete 
Total  paths 


FIG.  128.  —  The  Van  Cleef  System  of  Tracks  for  Highs  across  the  United  States. 
(U.  S.  Weather  Bureau.) 

The  Van  Cleef  system  of  tracks  for  highs  is  shown  in  figure  128.  This 
The  Van  system  of  tracks  is  based  upon  the  highs  from  1896  to  1905, 
Cleef  sys-  928  in  number.  Only  98  of  the  928  were  eratic  and  did  not 
tracks  for  follow  one  of  the  indicated  tracks.  The  width  of  the  track 
highs.  is  proportional  to  the  frequency  with  which  it  is  traversed. 

304.  The  velocity  of  motion  of  highs  averages  somewhat  less  than 
that  of  lows,  but  this  is  because  a  large  high  often  remains  almost  sta- 
The  veioc-  tionary  for  a  day  or  two  and  perhaps  a  week,  and  this  brings 
ity  of  mo-  down  the  average  velocity  of  motion.  Small  highs,  par- 
g  s*  ticularly  those  wedged  in  between  lows,  usually  move  with 
about  the  same  velocity  as  the  lows  themselves. 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     307 


The  following  table,  compiled  by 
C.  F.  von  Herrmann,  is  taken  from 
the  Bulletin  of  the  Mount  Weather 
Observatory,  Vol.  II,  part  4,  p.  196. 
It  c6ntains  the  average,  highest,  and 
least  velocity  of  motion  of  both  highs 
and  lows  for  the  various  months  and 
for  the  year.  The  data  summarized 
cover  the  twenty-seven  years  from 
1878  to  1904.  For  the  purpose  of 
comparison,  the  values  for  lows  de- 
termined by  Loomis  for  the  period 
1872-1884  are  also  added.  The 
truth  of  the  generalizations  which 
have  been  stated  for  the  velocity  of 
motion  of  both  highs  and  lows  is  at 
once  apparent  from  this  table. 

305.  The  four  weather  maps  for 
8  A.M.,  Jan.  30,  Jan.  31,  Feb.  1,  and 
Feb.  2,  1908,  which  are 

Four  illus- 

given  as  Charts  XXXI-  trative 
XXXIV,  will  show  the 
actual  distribution  of  the 
meteorological  elements  about  highs 
and  lows  in  individual  cases,  and 
will  also  illustrate  the  statements 
which  have  been  made  concerning 
the  tracks  followed  and  the  velocity 
of  motion  on  the  part  of  both  highs 
and  lows.1  The  weather  maps  for 
Dec.  26  to  29,  1904;  Jan.  2  to  5, 
1906 ;  Jan  14  to  17,  1906 ;  Dec.  8 
to  12,  1907;  Dec.  21  to  24,  1907; 
Dec.  28  to  31,  1907;  Jan.  10  to  13, 
1908 ;  Feb.  3  to  7,  1908 ;  Feb.  13  to 

1  For  other  sets  of  weather  maps  to  illustrate 
typical  lows,  see :  Monthly  Weather  Review 
1904,  end  of  volume  ;  Climatic  charts  of  the 
United  States ;  Climatology  of  the  United 
States  (Bulletin  Q  of  the  U.  S.  Weather 
Bureau). 


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308  METEOROLOGY 

16, 1908  ;  Feb.  16  to  20,  1908  ;  Jan.  28  to  31,  1909  ;  Feb.  17  to  20,  1909  ; 
Feb.  22  to  25,  1909 ;  April  5  to  8,  1909 ;  April  28  to  May  1,  1909 ;  Oct. 
31  to  Nov.  3,  1909,  show  more  or  less  typical  highs  and  lows  following 
well-recognized  paths. 

STATISTICS  ON  EXTKATROPICAL  CYCLONES  AND  ANTICYCLONES 

306.  It  is  probably  safe  to  affirm  that  no  subject  in  meteorology  has 
been  more  thoroughly  studied  by  the  statistical  method,  that  is,  by 

generalizing  statistics,  than  have  the  highs  and  lows.  Since 
eraiizations  the  files  of  daily  weather  maps,  particularly  for  the  United 
have  been  States  and  Europe,  cover  a  period  of  from  thirty  to  forty 

years,  the  material  for  such  a  study  is  ample.  All  the  state- 
ments which  have  been  made  concerning  the  distribution  of  the  meteor- 
ological elements  about  the  highs  and  lows,  their  tracks  and  velocity  of 
motion,  have  been  gained  by  the  inductive  method  of  reasoning  and  are 
based  upon  the  general  conclusions  which  have  been  drawn  from  tables 
of  statistics  about  highs  and  lows.  Many  other  tables  of  statistics  have 
been  prepared  and  general  conclusion  drawn.  Some  of  these  have  been 
incorporated  in  various  books  on  meteorology,  but  for  most  of  them  the 
reader  must  be  referred  to  the  periodical  literature  of  the  subject. 

THE  ORIGIN  AND  GROWTH  OF  AN  EXTRATROPICAL  CYCLONE  AND 

ANTICYCLONE 

307.  Theories  as  to  the  origin  of  extratropical  cyclones  and  anti- 
cyclones. —  When  the  tropical  cyclone  was  being  considered,  the  con- 

vectional  theory  of  its  origin  and  growth  was  given,  and  it 
tionai  theory  was  found  that  the  requirements  of  the  theory  and  the  ob- 
of  the  origin  served  facts  were  in  absolute  agreement.  Since  tropical 

of  lows 

cyclones  and  extratropical  cyclones,  when  superficially  con- 
sidered, are  so  similar  and  have  so  many  things  in  common,  it  is  but 
natural  to  ascribe  to  both  the  same  cause,  namely,  convection.  In  fact, 
for  a  long  time,  even  during  the  last  part  of  the  last  century,  all  cyclones 
and  lows  were  referred  to  this  same  cause. 

If  convection  is  the  cause  of  extratropical  cyclones,  they  must  origi- 
nate in  pockets  of  warm,  moist,  quiet  air.  Now,  as  a  matter  of  fact, 
The  three  lows  may  originate  anywhere.  They  form  over  the  frozen 
objections,  snow-covered  areas  of  the  extreme  northwest  when  the 
temperatures  are  far  below  zero  and  the  air  is  very  dry.  They  also 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     309 

originate  in  the  fast-moving  winds  over  the  Atlantic  Ocean.  In  short, 
they  originate  in  large  numbers  when  pockets  of  warm,  moist,  quiet 
air  are  certainly  lacking,  in  fact,  when  just  the  opposite  conditions  are 
present.  Furthermore,  if  they  are  of  convectional  origin,  there  ought 
to  be  more  in  summer  than  in  winter,  for  the  quieter  air,  the  higher 
temperature,  and  the  larger  amounts  of  moisture  during  the  summer 
time  should  be  more  conducive  to  their  formation  than  the  opposite 
conditions  which  exist  during  the  winter.  Statistics,  however,  show 
that  there  are  more  than  twice  as  many  lows  during  the  winter  as  during 
the  summer.  Again,  if  lows  are  of  convectional  origin,  the  temperature 
at  any  given  level  above  the  earth's  surface  in  the  center  of  a  low  ought 
to  be  higher  than  at  the  same  level  in  surrounding  regions.  It  will  be 
remembered  that  it  is  the  liberation  of  latent  heat  when  the  moisture 
condenses  to  form  cloud  and  precipitation  which  heats  the  air,  maintains 
it  at  a  higher  temperature  than  its  surroundings,  causes  still  further  rise, 
and  thus  supplies  the  energy  for  growth  or  continuance.  Now,  observa- 
tions of  the  temperature  in  the  center  of  a  low  and  in  surrounding 
regions  at  the  same  level  have  been  made  at  mountain  observatories 
and  by  means  of  balloons  and  kites,  and  it  is  found  that  the  temperature 
in  the  center  of  a  low  is  often  not  as  high  as  in  surrounding  regions  at 
the  same  level.  Thus,  on  account  of  those  three  objections,  convection 
as  the  origin  of  all  extratropical  cyclones  or  lows  must  be  abandoned. 
Lows  have  been  found  with  centers  warmer  than  their  surroundings. 
It  may  be  that  most  of  the  lows  of  summer  and  some  of  those  of  winter 
are  convection-caused,  but  convection  as  the  universal  cause  of  all  lows 
must  certainly  be  given  up. 

An  attempt  has  been  made  to  obviate  some  of  these  difficulties  by 
ascribing  the  origin  of  lows  to  convection,  not  at  the  earth's  surface,  but 
high  up  in  the  atmosphere.     It  is  known  from  balloon  ascen- 
sions that  there  are  many  layers  in  the  atmosphere  of  very  ti0n^ghtC" 
different  temperature  and  moisture.     If  a  warm,  moist  layer  occur  high 
should  find  itself  underneath  a  colder  and  less  moist  layer,  atmosphere, 
there  would  be  unstable  equilibrium  and  convection  would 
surely  take  place.     The  convectional  motion  high  up  in  the  atmosphere 
would  make  itself  felt  at  the  earth's  surface  as  an  area  of  low  pressure. 
Since  in  winter  the  layers  of  the  atmosphere  have  been  found  to  differ 
most  in  temperature  and  moisture,  it  is  then  that  the  most  convection 
high  up  in  the  atmosphere  would  be  expected. 

308.    The  origin  of  lows  is  sometimes  ascribed  to  eddies  formed  in  the 
general  wind  system.     If  a  stream  of  water  flows  into  a  quiet  pond  or  if 


310  METEOROLOGY 

two  streams  flow  past  or  over  each  other  in  different  directions  or 

with  different  velocities,  eddies  are  often  formed.     Now  the  general 

circulation  of  the  atmosphere  in  extratropical  regions  takes 

The  driven  T        ,1  ,1  -,         >      -,  ^ 

eddy  theory  place  in  three  layers.  In  the  northern  hemisphere,  the 
of  the  origin  Upper  layer  moves  from  the  west,  the  middle  layer  from 
the  northwest,  and  the  surface  layer  from  a  little  south  of 
west.  These  layers  also  have  different  velocities  of  motion,  so  that  it 
is  easy  to  think  of  eddies  as  forming  where  these  layers  meet. 

If  a  stone  is  dropped  into  a  fast-flowing  stream,  an  eddy  often  results. 
Now  the  air  which  moves  from  the  equator  poleward  in  the  outer  layer 
of  the  atmosphere  must  drop  down  in  extratropical  regions  to  commence 
its  journey  back  to  the  equator.  This  dropping  down  of  air  masses 
might  readily  cause  eddies. 

If  a  stream  flows  over  a  rough,  irregular  bed,  eddies  are  usually  formed. 
The  surface  of  the  continents  is  very  rough  and  irregular  and  the  air 
moving  over  it  might  readily  have  eddies  formed  in  it. 

There  are  thus  at  least  three  ways  in  which  the  formation  of  eddies 
in  the  atmosphere  is  easily  thinkable.  Each  eddy  would  develop  a 
small  amount  of  centrifugal  force,  and  this  would  cause  a  slightly  lower 
barometric  pressure  and  a  low  has  thus  originated.  Since  the  circula- 
tion of  the  atmosphere  is  more  vigorous  in  winter  than  in  summer,  eddies 
would  form  in  greater  numbers  in  winter  than  in  summer. 

This  objection  at  once  suggests  itself:  the  direction  of  rotation  of  an 
eddy  would  be  entirely  a  matter  of  chance,  so  that  as  many  would  rotate 
An  objec-  one  wa^  as  tne  otner-  Thus,  when  fully  grown,  the  whirl 
tion  and  its  about  lows  should  be  clockwise  as  well  as  counterclockwise, 
answer.  Observations,  however,  show  that  the  whirl  about  a  low  is  al- 
ways counterclockwise  in  the  northern  hemisphere.  A  partial  answer  can 
be  given  to  this  objection.  Due  to  the  earth's  rotation,  the  deviation 
is  always  to  the  right,  and  the  whirl  must  be  counterclockwise.  If  an 
eddy  turned  in  the  other  direction,  it  would  be  immediately  stopped  and 
put  out  of  existence,  so  that  only  those  eddies  turning  in  the  right  direc- 
tion could  continue  to  exist  and  grow. 

309.  Another  way  of  accounting  for  lows  is  to  consider  them  simply 
the  antitheses  or  results  of  highs.  An  area  of  high  pressure  is  accom- 
LOWS  may  panied  by  descending  air  currents  and  winds  blowing  spirally 
be  caused  outward  at  the  earth's  surface.  Unless  the  air  is  to  congest 
by  highs.  ^  ^e  earth's  surface,  there  must  be  some  upward  escape. 
At  some  point  the  air  rises  to  relieve  this  congestion,  and  thus  a  low  is 
formed.  If  this  explanation  of  the  origin  of  lows  is  accepted,  the 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     311 

question  has  simply  changed  from  how  lows  originate  to  how  highs 
originate. 

310.   During  the  last  few  years  another  very  important  theory  as  to 
the  origin  of  lows  has  been  developed.     This  is  the  work  of 
Bigelow  of  the  U.  S.  Weather  Bureau  and  is  sometimes  called 
the  counter  current  theory.     According  to  this  theory,  the  rent  theory 
origin  of  a  low  is  to  be  found  in  two  great  air  currents  of  oppo- 
site  direction  and  very  different  temperature. 

It  was  shown,  when  the  general  wind  system  of  the  world  was  being 
considered,  that  if  the  surface  of  the  earth  were  level  and  the  same  every- 
where, the  belts  of  high  and  low  pressure  would  be  uniform  The  cause 
and  would  extend  all  around  the  earth  and  the  exchange  of  of  the  coun- 
air  between  the  equator  and  pole  in  the  extratropical  regions  ter  currents< 
would  take  place  in  three  regular  layers,  one  above  the  other.  The 
surface  of  the  earth  is,  however,  not  level,  and  it  is  a  diversified  surface, 
being  part  land  and  part  water.  As  a  result,  these  belts  of  pressure 
break  up  into  peaks  and  depressions,  that  is,  permanent  highs  and  lows, 
which  change  their  intensity  and  position  between  summer  and  winter. 
The  prevailing  westerly  air  currents  in  the  outer  layer  of  the  atmosphere 
seem  to  preserve  their  identity  and  regularity  in  spite  of  this  breaking 
up  of  the  belts  of  pressure,  but  the  two  lower  layers  are  very  much  con- 
fused and  mixed,  so  that  here  the  exchange  of  air  between  the  equatorial 
and  polar  regions  takes  the  form  of  great  jets  or  air  currents  which  move 
now  equatorward,  now  poleward.  Those  from  the  equator  are,  of 
course,  warm  and  moisture-laden,  while  those  from  the  polar  regions  are 
relatively  cold  and  dry. 

The  region  of  strongest  air  currents,  and  thus  most  vigorous  exchange 
of  air  between  equator  and  pole,  seems  to  be  at  an  elevation  of  about 
1.5  miles  above  the  earth's  surface.     Suppose  that  there  are 
two  great  air  currents  at  this  level,  flowing  side  by  side,  but  counter  cur- 
of  opposite  direction  and  of  very  different  temperature,  the  jjents  pro" 
one  on  the  right  being  warm  and  flowing  poleward  and  the 
one  on  the  left  being  cold  and  flowing  equatorward.     Due  to  this  dif- 
ference of  temperature,  at  the  earth's  surface  there  will  exist  an  area  of 
low  pressure  with  spirally  inflowing  winds  turning  counterclockwise 
in  the  northern  hemisphere.     The  truth  of  these  statements  becomes 
evident  when  the  structure  of  a  low  above  the  earth's  surface  as  given 
in  section  288  is  considered.     It  was  there  shown  that  the  atmospheric 
whirl  which  exists  at  the  earth's  surface,  degenerates  at  a  height  of  a 
little  more  than  a  mile  into  two  counter  currents,  flowing  in  different 


312  METEOROLOGY 

directions  with  very  different  temperatures.  The  converse  is  equally 
true.  Two  currents  in  the  upper  atmosphere  necessitate  a  low  with 
inflowing  winds  at  the  earth's  surface.  A  low,  then,  with  its  spirally 
inflowing  winds  may  be  considered  simply  as  the  surface  effect  of  two 
powerful  counter  currents  of  very  different  temperature  in  the  upper 
atmosphere.  Since  the  exchange  of  air  between  equator  and  pole  is 
more  vigorous  in  winter  than  in  summer,  the  number  and  intensity  of 
lows  should  be  greater  in  winter  than  in  summer.  This  theory,  then, 
seems  to  meet  all  the  observed  facts,  to  be  free  from  objections,  and  to  be 
plausible,  but  not  so  easy  to  picture  as  some  of  the  other  theories. 

311.  Highs  and  lows  are  very  closely  related,  so  that  the  origin  of 
anticyclones  or  highs  must  be  accounted    for  as  well  as  the  origin  of 
Highs  may     extratropical  cyclones  or  lows.     One  way  of  explaining  the 
be  caused      origin  of  highs  is  to  consider  them  the  antitheses  or  results  of 

lows.  The  air  rises  in  areas  of  low  pressure.  This  air, 
ejected  from  lows,  must  collect  somewhere  and  increase  the  pressure  or 
form  a  high.  A  high,  then,  may  originate  in  the  congestion  of  the  air 
ejected  from  lows.  If  this  explanation  of  the  high  is  accepted,  the  real 
question  has  become  the  question  as  to  the  origin  of  the  lows. 

312.  Another  way  of  accounting  for  highs  is  to  ascribe  their  origin  to 
the  congestion  of  air  currents  in  the  outer  layer  of  the  atmosphere  due 

to  the  convergence  of  the  meridians.  In  the  outer  layer  of 
be  caused  the  atmosphere  the  air  moves  spirally  from  the  equator 
by  the  con-  towards  the  pole.  All  meridians  (that  is,  north  and  south 
the  poleward  lines)  converge  towards  the  pole  so  that  the  area  steadily 
moving  air  -  decreases  with  increasing;  latitude.  As  a  result  the  air  must 

currents. 

congest  and  finally  drop  or  be  forced  down  to  start  equator- 
ward  in  the  lower  layer.  This  congesting  of  the  air  currents  may  give 
rise  to  areas  of  high  pressure. 

313.  A  high  may  also  be  caused  by  radiation.     If  there  is  an  area 
particularly  free  from  clouds  and  moisture,  the  radiation  of  heat  from 

the  ground  and  the  lower  air  will  go  on  with  greater  rapidity. 

Highs  may  .11  •   i          i  • 

be  caused      particularly  at  night,  than  in  surrounding  regions.     As  a 
by  radia-        result,  the  air  here  will  become  particularly  cold,  dense,  and 
heavy.     It  will  contract  somewhat  and  air  will  come  in  aloft 
to  take  its  place.     As  a  result,  an  area  of  high  pressure  has  formed. 
Bi  elow's  3I4'   Bigelow's  counter  current  theory  may  also  be  used  to 

counter  cur-  account  for  highs.  If  at  the  height  of  a  mile  or  so  there  are 
of'th^ori^  *wo  coun^er  currents,  one  on  the  right,  cold  and  dry,  flowing 
of  highs.  equatorward  and  one  on  the  left,  warm  and  moist,  flowing 


THE  SECONDARY  CIRCULATION  OF  THE   ATMOSPHERE     313 

poleward,  there  will  exist  of  necessity  at  the  earth's  surface  an  area 
of  high  pressure  with  spirally  outflowing  winds  turning  clockwise  in 
the  northern  hemisphere.  When  the  structure  of  a  high  above 
the  earth's  surface  was  being  considered,  it  was  found  that  it  too 
degenerated  from  an  atmospheric  whirl  at  the  earth's  surface  to  two 
counter  currents  at  an  elevation  of  a  little  more  than  a  mile.  A  high, 
then,  can  be  considered  simply  as  the  surface  effect  of  two  powerful 
counter  currents  in  the  upper  air. 

Four  theories  as  to  the  origin  of  a  low  have  been  given  and  four  for  the 
origin  of  a  high.  These  theories  can  be  better  contrasted  and  compared 
after  the  methods  of  growth,  development,  and  maintenance  of  both 
highs  and  lows  have  been  considered. 

315.    Growth  of  an  extratropical  cyclone.  —  The  crucial  question  in 
connection  with  the  growth  or  continued  existence  of  an  extratropical 
cyclone  is  the  source  of   the  energy.     A  full-grown  extra-  How  is  the 
tropical  cyclone  has  an  activity  of  millions  of  horse  power  energy 
and  does  work  at  this  rate  for  days  or  perhaps  weeks.     This  gamed? 
immense  amount  of  energy  must  be  gained  from  some  source,  and  the 
method  by  which  this  energy  is  gained  will  tell  the  story  of  the  growth  and 
development  of  an  extratropical  cyclone.    There  are  three  possible  sources 
of  this  energy :  the  latent  heat  liberated  by  condensation  of  The  ^fee 
moisture  to  form  cloud  and   precipitation ;    the   energy  of  sources  of 
motion  of  the  general  wind  system  of  the  world ;  the  relative  e 
displacement  of  masses  of  cold  and  warm  air. 

If  the  energy  is  gained  from  the  condensation  of  moisture,  then  what- 
ever may  have  been  the  origin  of  a  low,  it  will  build  itself  up  in  accord- 
ance with  the  convectional  theory  and  will  be  essentially  a  How  ^ 
convectional  formation.     The  method  of  growth  has  been  energy  from 
treated  in  full  in  connection  with  the  tropical  cyclone.     A  neat  may* 
small  area  of  low  pressure  has  originated  with  rising  air  build  up  a 
currents.      The  rising    air    cools,    reaches    the    dew    point, 
forms  cloud  and  precipitation,  and  liberates  an  immense  amount  of 
latent  heat.     This  heats  the  rising  air,  maintains  it  at  a  higher  tem- 
perature than  its  surroundings,  causes  further  rise,  and  thus  acts  like 
forced  draft  in   a  chimney.      A  more  violent  indraft  at  the  earth's 
surface  now  takes  place.     It  becomes  a  whirl  due  to  the  earth's  rotation. 
Centrifugal  force  is  developed  and  the  central  pressure  becomes  lower. 
The  formation  has  grown,  due  to  the  latent  heat,  and  acquired  more 
energy.     It  builds  up  and  continues  until  the  supply  of  energy  lessens, 
when  it  gradually  dies  out.     There  have  been  violent  lows  with  very 


314  METEOROLOGY 

little  cloud  and  no  precipitation.  It  would  thus  seem  that  they  must 
have  some  other  source  of  energy.  Furthermore  lows  are  most  violent 
in  winter,  when  the  temperatures  are  lowest  and  the  air  is  dryest  and  thus 
the  supply  of  available  energy  is  least.  It  would  thus  seem  that  the 
latent  heat  due  to  condensation  cannot  be  the  only  source  of  energy  for 
lows. 

When  the  eddy  theory  for  the  origin  of  lows  was  being  worked  out,  it 
was  thought  that  these  eddies  might  gain  their  energy  from  the  general 
winds  of  the  world,  just  as  water  eddies  gain  their  energy 
nughTcome    ^rom  the  currents  in  which  they  are  formed.     If  this  were 
from  the        the  case,  the  energy  acquired  by  a  low  would  be  taken  from 
system. W       ^ne  general  wind  system  and  the  circulation  would  thereby 
be  hindered  and  slowed  down.     It  seems  incredible,  however, 
that  a  low  five  miles  thick  and  3000  miles  wide,  thus  resembling  a  piece 
of  paper,  and  of  a  size  comparable  with  the  great  air  currents  them- 
selves, could  receive  all  its  energy  from  these  currents,  just  as  water 
eddies  receive  their  energy  from  the  currents  which  cause  them. 
If  the  Bigelow  counter  current  theory  of  lows  is  accepted,  then  the 
.        energy  comes  from  the  relative  displacement  of  masses  of 
might  come    cold  and  warm  air.     These  two  counter  currents,  due  to 
their  temperature  difference,  produce  the  area  of  low  pressure 
ment  of         and  the  atmospheric  whirl  at  the  earth's  surface.     The  more 
warm  and      vigorous  these  currents,  the  greater  the  temperature  differ- 
ences and  thus  the  more  vigorous  the  effect,  namely,  the 
low,  which  they  produce  at  the  earth's  surface. 

There  is  nothing  to  prevent  all  three  of  these  sources  of  energy  from 

contributing  to  the  growth  of  a  low  at  the  same  time  and 

sources  of      such  is  probably  the  case.     Careful  observations  and  many 

energy  of   them,  particularly  in  connection  with  the  beginning  of 

tribute  at       lows  and  at  various  levels  in  the  atmosphere,  would  permit 

the  same        ^he  relative  importance  of  these  three  sources  of  energy  to  be 

determined.     The  last  one  is  sufficient  in  itself  to  account 

for  lows,  the  first  two  are  not. 

316.  Growth  of  an  anticyclone.  — In  connection  with  the  growth  of 
There  are  an  anticyclone  or  high,  the  source  of  the  energy  is  again  the 
three  chief  thing  to  be  considered.  If  a  high  is  due  to  the  accumu- 

energySfor      lation  of  air  from  lows  or  to  radiation,  it  is  gravity  which 
the  growth      supplies  the  energy ;  and  either  of  these  processes  which  origi- 
nated the  high  may  continue  indefinitely  and  become  more 
vigorous  and  thus  account  for  the  growth  or  continuance  of  the  high. 

v-  A  \  . 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     315 

If  highs  are  due  to  the  congestion  of  poleward  moving  air  currents, 
then  the  energy  comes  from  the  general  wind  system,  and  this  process 
can  continue  an  indefinite  time. 

If  highs  are  due  to  counter  currents  in  the  upper  air,  then  the  source 
of  energy  is  the  relative  displacement  of  masses  of  warm  and  cold  air. 
The  high  at  the  earth's  surface  is  simply  the  effect  of  these  currents, 
and  the  more  vigorous  the  currents  the  more  vigorous  the  high. 

There  is  nothing  to  prevent  all  four  of  these  processes  from  working 
simultaneously  to  build  up  and  develop  a  high.  Sometimes  it  would 
seem  that  one  or  the  other  of  the  processes  stands  out  conspicuously 
as  the  origin  of  a  particular  high,  but  it  is  impossible  to  say  that  the 
others  are  not  present.  Any  one  of  the  four  is  sufficient  by  itself  to 
produce  a  high. 

317.    The  characteristics  which  lows  and  highs  should  have  and  the 
comparison  with  the  observed  facts.  —  The  various  theories  as  to  the 
origin  of  lows  and  highs  and  the  sources  of  the  energy  which 
causes  their  growth  and  development  have  been  fully  con-  inconnec- 
sidered.     The  general  features  of  both  the  lows  and  highs  ti°nwitjj 
have  been  accounted  for.     There  now  remain  many  details  highs  must 
in  connection  with  the  distribution  of  the  meteorological 
elements  about  typical  lows  and  highs,   their  paths,   and 
velocity  of  motion  in  connection  with  which  the  demands  of  theory  on 
the  one  hand  and  observed  facts  on  the  other  must  be  noted  and  a 
comparison  made. 

On  the  east  side  of  an  area  of  low  pressure  the  wind  is  from  some 
southerly  quarter,  and  this  transports  warm,  moisture-laden  air,  raising 
the  temperature  and  humidity.  On  the  west  side,  the  wind  Tfa 
direction  is  from  some  northerly  quarter  and  here  the  air  is  terfstics  of 
cold  and  dry.  The  great  difference  in  temperature  and 
moisture  between  the  two  sides  of  a  low  is  thus  due  to  the 
wind  direction.  The  oval  form  of  the  isobars  and  the  direction  of  the 
oval  is  due  to  the  temperature  difference  between  the  two  sides.  With 
increasing  altitude  this  northeast-southwest  oval  first  becomes  circular 
in  form,  then  again  oval,  but  with  the  longer  axis  northwest-southeast. 
All  this  change  in  form  is  due  simply  to  the  temperature  differences  on 
the  two  sides.  The  cloud  and  rain  area  is  located  chiefly  in  the  southeast 
quadrant  because  here  the  temperature  and  moisture  have  their  largest 
values.  The  cirrus  cloud  is  formed  only  on  the  east  side  because  the 
air  which  rises  in  a  low  is  injected  into  the  rapidly  moving  westerly  air 
currents  of  the  outer  layer  of  the  atmosphere.  This  air,  still  containing 


316  METEOROLOGY 

considerable  moisture,  is  carried  rapidly  eastward,  but  makes  very  little 
headway  towards  the  west  against  the  air  current. 

In  connection  with  an  area  of  high  pressure,  the  cloud  forms  on  the 
east  side  are  the  last  reminders  of  a  departing  low,  while  the  cloud 

forms  on  the  west  side  are  the  first  heralds  of  a  coming  low. 
teristics  of  Only  convection-caused  cumulus  clouds  or  ^radiation  clouds 
a  high  are  m  ^he  early  morning  are  found  in  the  central  part  of  a  typical 

high.     The  northeast  portion  of  a  high  is  the  coldest  and 

the  southwest  the  warmest,  due  to  the  transportation  of  air  by  the  wind. 

Lows  and  highs  move  in  general  eastward.    .  This  is   because  both 

the  surface  winds  and  the  upper  air  currents  move  from  a  little  south  of 

west  towards  the  east.     In  other  words  lows  and  highs  drift 

The  direc-       .  .  _.  .  -11 

tionandve-  m  the  general  wind  system.  If  a  small  area  is  considered, 
locity  of  as  f or  example  the  United  States  or  Europe,  the  path  followed 
lows  and  by  any  individual  low  or  high  is  determined  by  four  factors : 
highs  are  (i)  ^he  topography  of  the  country,  (2)  the  meteorological 
condition  of  the  country,  (3)  the  surrounding  highs  and  lows, 
(4)  the  distribution  of  the  meteorological  elements  about  the  individual 
low  or  high  itself.  These  four  factors  and  the  motion  of  individual 
lows  and  highs  will  be  considered  more  fully  in  the  chapter  on  weather 
prediction.  Since  these  four  factors  are  for  the  most  part  constant 
or  subject  to  periodic  variations,  it  is  to  be  expected  that  lows  and  highs 
will  follow  more  or  less  exactly  a  rather  definite  system  of  tracks. 

Lows  and  highs  move  faster  in  winter  than  in  summer.  This  is  be- 
cause the  general  winds  of  the  world,  which  carry  the  lows  and  highs, 
move  faster  in  winter  than  in  summer. 


THE  EFFECT  OF  PROGRESSION  ON  THE  DIRECTION  AND  VELOCITY  OF 

CYCLONIC  WINDS 

318.   The  conception  which  is  often  held  of  a  low  (or  a  high)  is  that 

.         the  air  which  constitutes  the  formation  moves  bodily  forward 

companied  a  over  the  earth's  surface.     A  low  moves  usually  eastward  and 

low,  the         slightly  poleward.     In  the  southern  quadrants  of  a  low  the 

ity  on  the       air  motion  with  respect  to  the  center  is  from  the  southwest  to 

south  side      ^e  northeast.     Here,  then,  the  motion  of  progression  and 

would  be        the  motion  of  revolution  about  the  center  agree  and  the 

very  dif-        velocities  would  be  added.     In  the  northern  quadrants  of  a 

low  the  air  motion  with  respect  to  the  center  is  from  the 

northeast  to  the  southwest.     Here  the  motion  of  progression  and  the 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     317 


motion  of  revolution  are  opposed  and  the  velocities  would  be  sub- 
tracted. To  make  the  picture  more  definite,  suppose  that  a  low  is 
moving  towards  the  northeast  with  a  velocity  of  thirty  miles  an 
hour.  Suppose  also  that  the  velocity  of  revolution  is  thirty  miles 
an  hour.  If  the  air  moved  with  the  formation,  there  would  be  then 
a  wind  velocity  of  sixty  miles  an  hour  in  the  southern  quadrants 
and  a  dead  calm  in  the  northern  quadrants.  Now,  no  such  difference 
in  wind  velocity  as  this  exists  between  the  northern  and  southern 
quadrants. 

The  truth  of  the  matter  is  that  the  low,  the  formation,  goes  forward, 
but  the  air  which  constitutes  it  at  any  moment  does  not.     A  low  takes 
in  air  at  the  front,  moves  it  a  short  distance,  and  abandons 
it.     The  low  as  a  formation  moves  forward,  but  the  air  which  tion  moves  ; 
constitutes  it  at  different  stages  in  its  journey  is  entirely  *he  *"" 

does  not. 

different. 

This  is  exactly  analogous  to  the  progress  of  an  ocean  wave  in  deep 
water.     Each  mass  of  water  describes  a  little  oval  path  and  A  wave 
comes  back  nearly  to  where  it  was  before,  but  the  wave  anal°gy- 
has  gone  on.     The  wave  as  a  formation  has  gone  on,  but  the  water 
which  constitutes  it  is  entirely  different. 

Over  the  ocean,  in  connection  with  tropical  cyclones,  and  at  consider- 
able altitudes,  the  velocity  of  the  wind  is  found  to  be  greater  in  the 
southern  quadrants  of  lows  than  in  the 
northern.  This  means  that  in  these 
cases  there  is  a  slight  carrying  forward  of 
the  air  which  constitutes  the  low.  Over 
the  land,  however,  and  near  the  earth's 
surface  the  friction  is  too  great  to  permit 
the  actual  transportation  of  air  by  the  low. 

319.  If  a  low  progresses  as  a  forma- 
tion, it  now  remains  to  consider  what 
actually  determines  the  direc- 

.  .  »          .  .  (.   .-,         .  j    The  factors 

tion  of  motion  of  the  air  and  which 


FIG.  129.  —  Diagram  Illustrating  the 
Determination  of  the  Wind  Direc- 
tion and  Velocity  at  any  point 
near  a  Passing  Low. 


its  velocity  at  any  point  when  mine  the  di- 

a  low  passes.      Let  US  Consider,    ^6^? 

for  the  sake  of   definiteness,  and  its  ve- 

,  ,  .    ,       Tr         j     -IT-    •       locity  at  any 

the  two  points  X  and   Y  in  point 

Fig.  129,  at  the  earth's  surface 

on  opposite  sides  of  a  passing  low  in  the  United  States.     These  points 

are  located  in  the  region  of  the  prevailing  westerly  winds  and  thus  at 


318  METEOROLOGY 

each  point  there  would  be  the  component  A,  which,  in  direction  and 
length,  is  to  represent  these  prevailing  westerly  winds  in  which  the 
low  drifts.  Of  course,  this  component  marked  A  is  really  the  com- 
bination of  two  things :  the  exchange  of  air  between  equator  and  pole 
and  the  deviation  due  to  the  earth's  rotation.  If  the  earth  did  not 
rotate,  the  surface  wind  direction  in  extratropical  regions  in  the  northern 
hemisphere  would  be  south.  This  is  deviated  to  the  right  by  the  earth's 
rotation  and  becomes  the  prevailing  westerly  winds  with  a  direction  from 
a  little  south  of  west.  Since  a  low  is  near,  there  is  a  pressure  gradient 
towards  the  center,  and  the  air  tends  to  move  along  the  gradient  at 
right  angles  to  the  isobaric  lines,  but  it  is  deviated  to  the  right  by  the 
earth's  rotation.  This  cyclonic  component  is  indicated  in  the  figure  by 
B  in  each  case,  and  is  the  combination  of  two  things  :  the  pressure 
gradient  towards  the  center,  and  the  deviation  due  to  the  earth's  rotation. 
On  account  of  friction,  the  air  has  its  greatest  velocity  of  motion  about 
a  low,  not  at  the  earth's  surface,  but  some  distance  above,  where  the 
isobars  are  nearly  circular  and  the  direction  of  the  air  motion  is  nearly 
tangential  to  the  isobars.  Due  to  fluid  friction  this  more  rapid  motion 
of  the  upper  air  would  drag  around  the  surface  air  to  a  slight  extent 
with  it.  If,  for  example,  an  iron  ring  were  suspended  in  a  tank  of  water 
and  made  to'  revolve  rapidly,  the  water  would  have  a  certain  amount  of 
motion  imparted  to  it  through  friction.  In  the  same  way  the  rapidly 
revolving  ring  of  air  above  the  earth's  surface  will  impart  a  certain 
amount  of  motion  to  the  more  slowly  moving  air  above  and  below.  This 
component  is  indicated  by  C  in  each  case.  If  these  three  components 
are  summed  up,  the  resultant  R  in  each  case  is  obtained.  It  will  be 
seen  that  the  velocity  is  larger  at  X  than  at  Y  and  that  the  direction 
is  more  nearly  tangential  to  the  isobars  at  X  than  at  Y.  If  the  dis- 
tribution of  the  meteorological  elements  about  a  low  is  considered, 
it  will  be  seen  that  all  this  is  in  exact  accord  with  the  results  of 
observation. 

If  other  points  at  different  levels  in  connection  with  lows  and  highs 
are  considered,  two  more  components  must  at  times  be  added.  In  the 
upper  part  of  a  low  the  rising  air  is  forced  out  and  injected  into  the 
surrounding  air.  There  is  here  a  component  directed  directly  from  the 
center.  In  the  case  of  the  upper  part  of  a  high,  air  is  drawn  in  to  take 
the  place  of  the  descending  air  current.  Here  there  is  a  component 
directed  directly  in  towards  the  center.  The  other  component  to  be 
considered  is  due  to  the  forward  motion  of  the  low  or  high.  If  no  air 
were  carried  along  with  the  formation,  this  component  would  not  exist, 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     319 

but  as  a  small  amount  particularly  at  higher  levels  and  over  the  ocean 
is  carried  along  with  the  formation,  there  is  this  last  component  which 
has  the  direction  of  the  low  or  high. 

If  it  is  desired  to  determine  the  wind  direction  and  velocity  at  any 
level  and  at  any  point  in  connection  with  either  a  low  or  a  high,  it  is 
only  necessary  to  determine  these  five  components  in  direction  and 
relative  magnitude  and  find  their  resultant. 

THE  CORRELATION  OF  THE  METEOROLOGICAL  ELEMENTS 

320.   Suppose,  for  example,  the  wind  direction  and  the  fact  as  to 
whether  precipitation  was  falling  or  not  have  been  recorded  for  the 
same  hour  each  day  for  many  years.     A  table  of  statistics 
can  be  prepared  which  would  show  for  each  wind  direction  J^icai^ie-" 
the  number  of  times  precipitation  had  occurred  with  that  mentsare 
direction.     It  would  be  found  for  the  northeastern  part  of  fated"6" 
the  United  States  that  precipitation  occurred  with  the  great- 
est frequency  when  the  wind  was  southeast.     East,  south,  and  south- 
west would  also  stand  high.     It  would  also  be  found  that  precipitation 
occurred  the  least  number  of  times  when  the  wind  direction  was  north- 
west.   West,  north,  and  northeast  would  also  stand  low.     There  is  thus 
a  decided  connection  or  correlation  between  the  wind  direction  and  the 
frequency  of  precipitation.     The  nature  of  the  correlation  is  determined 
by  summarizing  statistics* 

The  correlation  of  any  observation  of  any  element  with  any  other  can 
be  worked  out.  Many  of  these  correlations  are  of  no  practical  value  or 
importance.  A  few  of  those  which  are  of  some  interest  are  the  follow- 
ing :  maximum  temperature  and  wind  direction ;  minimum  temperature 
and  wind  direction;  pressure  and  temperature;  temperature  and 
precipitation. 

The  reason  why  a  correlation  of  the  meteorological  elements  should 
exist  must  now  be  considered.     To  do  this,  the  question  as  to  what 
causes   our   weather   must   be   raised   and   answered.     Our  jhe  reason 
weather  consists  of  two  things :    the  typical  weather  which  for  the  cor- 
would  exist  if  there  were  no  disturbances,  and  the  influence  of  * 
the  passing  lows  and  highs.     Now,  the  typical  weather  which  would 
exist  if  there  were  no  disturbances  is  indicated  by  the  normal  values  of 
all  the  meteorological  elements  together  with  their  diurnal  and  annual 
variations.     Lows  and  highs  are  definite,  well-known  formations  mov- 
ing along  well-recognized  tracks.     A  correlation  of  the  meteorological 


320  METEOROLOGY 

elements  therefore  exists  because  the  weather  is  not  haphazard  or  a 
matter  of  chance,  but  is  the  resultant  of  two  factors,  both  of  which  are 
definite,  well-known,  and  nearly  always  the  same. 

C.    THUNDERSHOWERS 

DEFINITION  AND  DESCRIPTION 

321.  Definition    and    chief    characteristics.  —  A   shower   is   usually 
defined  as  a  copious  rainfall  of  short  duration.     The  intermittent  down- 
pours which  occur  during  certain  types  of  spring  weather 

thunder-        are,  for  example,  spoken  of  as  April  showers.     If  a  shower 
shower  de-     js  accompanied  by  thunder  and  lightning,  it  is  considered 
a  thundershower  or  thunderstorm.     Some  would  reserve  the 
word  thunderstorm  for  a  more  than  usually  violent  thundershower, 
but  the  distinction  is  seldom  made.     The  presence  of  thunder  and  light- 
ning while  it  is  raining  does  not  necessarily  mean  that  a 
and  Hght-       thundershower  is  in  progress,  for  all  violent  rains  which  fall 
ningaccom-    from  thick  clouds  are  accompanied  by  thunder  and  light- 
occurrences    nmS-     Thunder  and  lightning  accompany  tropical  cyclonesV 
tornadoes,  desert  whirlwinds,  and  volcanic  eruptions  as  wel, 
as  thundershowers.     It  was  formerly  thought  that  thunder  and  light- 
ning were  the  essential  things  in  connection  with  a  thundershower; 
in  short,  they  were  considered  the  cause  of  the  storm.     It  is 
and  light-       now  known,  however,  that  they  are  extremely  secondary 
ning  a  result,  an(j     pjay  a  verv  unimportant  part.     In  fact,  they  are  a 
result  and  not  a  cause.     They  are  the  inevitable  consequence 
and  accompaniment  of  copious  condensation  in  a  thick  cloud,  and  play 
no  part  whatever  in  the  mechanism  of  a  thundershower. 

A  thundershower  is  a  too  well-known  phenomenon  to  need  a  careful 
definition  and  a  full  description.  It  can  be  best  defined  and  described 
^  ,.  j  by  stating  its  chief  characteristics.  A  thundershower  is 

Outline  de-  .  . 

scription  of     an  immense  cumulo-nimbus  cloud  accompanied  by  copious 

a  thunder-  precipitation,  a  marked  drop  in  temperature,  and  a  peculiar, 
often  violent,  outrushing,  squall  wind  which  just  precedes 
the  rainfall.  Thunder  and  lightning  are  always  present  and  hail  some- 
times falls.  It  is  a  violent,  local  storm,  covering  a  comparatively  small 
area  and  lasting  but  a  short  time.  Damage  is  often  caused  by  wind, 
hail,  and  lightning.  • 

322.  Description  of  the  approach  and  passage  of  a  thundershower.  — 
Thundershowers  occur  everywhere  from  the  equator  to  the  pole  and 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     321 

have  thus  been  more  generally  observed  than  any  other  storm.  It  is 
the  typical  thundershower  on  a  hot  summer  afternoon  whose  approach 
and  passage  is  here  described  in  outline. 

It  has  been  a  hot,  sultry,  oppressive  day  in  summer.  The  air  has 
been  very  quiet,  perhaps  alarmingly  quiet,  interrupted  now  and  then 
by  a  gentle  breeze  from  the  south.  The  pressure  has  been 

i      11  -i  r™_       i       •     i  •  Description 

gradually  growing  less.  The  sky  is  hazy  ;  cirrus  clouds  are  Of  the  ap- 
visible ;  here  and  there  they  thicken  to  cirro-stratus  or  Proach  and 
cirro-cumulus.  The  temperature  has  risen  very  high  and  atypical 
the  absolute  humidity  is  very  large,  but  owing  to  the  high  th"nder 
temperature,  the  relative  humidity  has  decreased  some- 
what. The  combination  of  high  moisture  and  temperature  and  but 
little  wind  has  made  the  day  intensely  sultry  and  oppressive.  In  the 
early  hours  of  the  afternoon,  amid  the  horizon  haze  and  cirro-stratus 
clouds  in  the  west,  the  big  cumulus  clouds,  the  thunderheads,  appear. 
Soon  distant  thunder  is  heard,  the  lightning  flashes  are  visible,  and  the 
dark  rain  cloud  beneath  comes  into  view.  As  the  thundershower 
approaches,  the  wind  dies  down  or  becomes  a  gentle  breeze  blowing 
directly  towards  the  storm.  The  temperature  perhaps  drops  a  little 
as  the  sun  is  obscured  by  the  clouds,  but  the  sultriness  and  oppressive- 
ness remain  as  before.  The  thundershower  comes  nearer,  and  the  big 
cumulus  clouds  with  sharp  outlines  rise  like  domes  and  turrets  one  above 
the  other.  Perhaps  the  loftiest  summits  are  capped  with  a  fleecy,  cirrus- 
like  veil  which  extends  out  beyond  them.  If  seen  from  the  side,  the 
familiar  anvil  form  of  the  cloud  mass  is  noticed.  Just  beneath  the 
thunderheads  is  the  narrow,  turbulent,  blue-drab  squall  cloud.  The 
patches  of  cloud  are  now  falling,  now  rising,  now  moving  hither  and 
thither  as  if  in  the  greatest  commotion.  Beyond  the  squall  cloud  is  the 
dark  rain  cloud,  half  hidden  from  view  by  the  curtain  of  rain.  The 
thunderheads  and  squall  clouds  are  now  just  passing  overhead.  The 
lightning  flashes,  the  thunder  rolls,  big,  pattering  raindrops  begin  to 
fall  or  perhaps,  instead  of  these,  damage-causing  hailstones.  The  gentle 
breeze  has  changed  to  the  violent  outrushing  squall  wind,  blowing  directly 
from  the  storm,  and  the  temperature  is  dropping  as  if  by  magic.  Soon 
the  rain  descends  in  torrents,  shutting  out  everything  from  view.  After 
a  time,  the  wind  dies  down  but  continues  from  the  west  or  northwest ; 
the  rain  decreases  in  intensity;  the  lightning  flashes  follow  each  other 
at  longer  intervals.  An  hour  or  two  has  passed ;  it  is  growing  lighter 
in  the  west;  the  wind  has  died  down;  the  rain  has  almost  stopped. 
Soon  the  rain  ceases  entirely;  the  clouds  break  through  and  become 


322  METEOROLOGY 

fractostratus  or  cirriform ;  the  temperature  rises  somewhat,  but  it  is  still 
cool  and  pleasant;  the  wind  has  become  very  light  and  has  shifted 
back  to  the  southwest  or  south.  Now  the  domes  and  turrets  of  the 
retreating  shower  are  visible  in  the  east ;  perhaps  a  rainbow  spans  the 
sky;  the  roll  of  the  thunder  becomes  more  distant;  the  storm  has 
passed,  and  all  nature  is  refreshed. 

323.    Distribution  of  the  meteorological  elements  about  a  thunder- 
shower.  —  The  distribution  of  the  meteorological  elements   about  a 
thundershower  is  best  shown  by  means  of  the  accompanying 
The  changes  diagram,  Fig.  130,  which  depicts  the  changes   in    the  ele- 

in  the  mctc~  . 

oroiogical  ments  during  a  hot  summer  day  with  a  typical  thunder- 
elements  shower  between  three  and  five  in  the  afternoon. 

which  occur 

onahotsum-  The  temperature  rises  unduly  high  during  the  day,  and 
mer  day  with  reaches  a  maximum  just  before  the  coming  of  the  storm, 
thunder-  It  drops  slightly  when  the  sun  is  obscured  by  the  coming 
shower  in  clouds,  but  the  large  drop  in  temperature,  which  may  amount 
noon.  to  from  6°  to  20°  F.,  occurs  during  the  first  twenty  minutes 

of  the  thundershower.  The  temperature  then  continues  to 
drop  slowly  until  the  end  of  the  thundershower  is  reached.  After 
the  storm  passes,  the  temperature  usually  rises  again,  but  does  not 
begin  to  attain  the  height  reached  just  before  its  coming. 

The  pressure  usually  sags  somewhat  during  the  day.  When  the  squall 
wind  begins  to  blow,  there  is  a  sudden  increase  in  pressure  of  six  or  seven 
hundredths  of  an  inch  and  the  pressure  oscillates  up  and  down  slightly 
all  through  the  storm.  At  its  end,  the  pressure  is  generally  a  little  higher 
than  just  before  the  beginning.  The  pressure  sometimes  begins  to  fall 
after  the  shower  has  passed,  but  it  generally  rises,  particularly  if  the 
shower  has  been  a  heavy  one  and  no  more  are  following  in  quick  suc- 
cession. These  oscillations  of  pressure  are  easily  traceable  in  the 
indications  of  a  barograph  and  from  an  interesting  record  of  a  thunder- 
shower.1 

The  wind  is  light  and  from  the  south  during  the  day.  As  the  thunder- 
shower  approaches,  it  shifts  to  the  east  and  blows  gently  directly  towards 
the  approaching  storm.  This  is  suddenly  replaced  by  the  violent, 
sometimes  damage-causing,  squall  wind  which  blows  from  the  west  or 
northwest  directly  from  the  thundershower.  During  the  thundershower 
the  wind  holds  its  direction  but  steadily  lessens  in  velocity.  After  the 
storm  passes,  it  is  light  and  often  shifts  back  to  the  southwest  or  south. 

1  For  thundershower  barograph  curves,  see  Monthly  Weather  Review,  1898,  Vol.  XXVI, 
p.  592. 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     323 

The  relative  humidity  is  high,  but  drops  during  the  day,  due  to  the 
great  rise  in  the  temperature.  When  the  thundershower  commences 
and  the  temperature  drops,  it  rises  rapidly  and  attains  a  value  of  85  or 
90  per  cent.  After  the  storm  passes  it  drops  again  slowly,  due  to  the 


DURATION  OF  RAINFALL 


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PIG.  130.  —  The  Change  in  the  Meteorological  Elements  during  a  Hot  Summer  Day 
with  a  Typical  Thundershower  in  the  Afternoon. 


324 


METEOROLOGY 


rise  in  temperature.  Haze  and  cirriform  clouds  are  prevalent  during  the 
day.  As  the  storm  passes,  the  thunderheads,  squall  cloud,  and  nimbus 
clouds  follow  in  succession,  the  whole  being  called  a  cumulo-nimbus 
cloud.  After  the  thundershower  passes,  stratiform  clouds  exist  for  a 
while,  but  the  cirriform  clouds  usually  again  make  their  appearance. 

The  rain  starts  with  a  few  large  pattering  drops,  then  falls  in  torrents, 
and  then  gradually  lessens  in  intensity  until  the  end  of  the  thunder- 
shower  is  reached. 

The  average  duration  of  a  thundershower  is  a  little  less  than  two 
hours.  The  first  thunder  is  ordinarily  heard  about  an  hour,  or  a  little 
more,  before  the  coming  of  the  rain,  and  the  last  thunder  is  heard  about 
the  same  time  after  the  cessation  of  the  rain. 

70  MILES 


FIG.  131.  — The  Cross  Section  of  a  Typical  Thundershower. 

It  must  always  be  held  in  mind  that  the  changes  in  the  meteorological 
elements  which  have  been  so  fully  described  and  illustrated  in  the  dia- 
gram apply  to  the  typical  thundershower.  Thundershowers  are  slightly 
different  in  different  parts  of  the  world,  and  the  thundershowers  which 
occur  in  winter  and  at  night  are  somewhat  different  from  those  which 
occur  on  a  hot  summer  day.  Each  individual  thundershower  may  also 
differ  widely  from  the  type  form.  There  is  no  more  interesting  or 
profitable  study  than  to  watch  the  progress  of  a  thundershower  with  the 
type  form  always  in  mind. 

324.  Cross  section  of  a  thundershower.  —  In  the  cross  section  of  a 
typical  thundershower,  Fig.  131,  the  air  circulation  and  the 
section  of  a  distribution  of  the  cloud  masses  are  shown.  The  rising  air 
typical  thun-  wnich  builds  the  storm,  the  descending  and  forward  mov- 
ing air  currents  underneath  the  thundershower,  and  the 
whirl  in  the  squall  cloud  with  the  peculiar  outrushing  squall  wind  are 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     325 

all  shown.  The  thunderheads  with  their  cirruslike  caps,  the  immense 
nimbus  cloud,  the  stratiform  clouds  at  the  back,  and  the  turbulent 
squall  cloud  are  also  depicted.  The  vertical  and  horizontal  scales  are 
not  the  same  because  a  thundershower  has  an  average  length  of  perhaps 
seventy  miles  and  an  average  height  of  only  about  four  miles.  The 
anvil-like  form  of  the  immense  cumulo-nimbus  cloud  is  here  clearly 
seen. 

Two  cumulo-nimbus  clouds  are  represented  among  the  illustrations 
of  the  cloud  forms  in  connection  with  section  217.  In  one  the  thunder- 
head  is  especially  prominent  while  in  the  other,  which  is  a  side  view  of 
a  thundershower,  the  anvil-like  form  of  the  cloud  is  particularly  notice- 
able. 

325.   The  observations  of  thundershowers.  —  Many  special  stations 
for  a  certain  period  of  years  have  been  maintained  by  several  states  in 
the  United  States  and  by  several  countries  in  Europe  for  the 
detailed  study  of  thundershowers.     Most  of  the  information  tions  have 
which  we  have  has  been  gained  from  the  observations  made  be.en  main- 
at  such  stations.     Since  a  thundershower  is  such  a  small 
local  formation,   the  regular  and  cooperative  stations  of  the  U.   S. 
Weather  Bureau  are  too  far  apart  to  make  a  detailed  study  of  them 
possible. 

If  instruments  are  not  used,  the  observations  ordinarily  made  are : 
the  time  of  beginning  and  ending ;  the  direction  of  motion ;  the  violence 
of  the  thunder  and  lightning ;   the  presence  of  hail ;   the  in- 
tensity of  the  precipitation;    the  amount  of  the  drop  in  vations 
temperature ;   the  violence  of  the  wind.     If  instruments  are  wh*?h  are 
at  hand,  the  observations  include  a  record  of  the  changes  in 
all  the  meteorological  elements,  together  with  the  observation  of  the 
time  of  beginning  and  ending,  direction  of  motion,  thunder  and  light- 
ning, and  hail.     In  determining  the  time  of  beginning  and  ending  in  the 
case  of  a  thundershower,  it  is  the  time  of  occurrence  of  the  first  and  last 
thunder  that  is  usually  taken.     This  is  better  than  to  use  the  time  of 
occurrence  of  the  first  and  last  lightning,  as  the  lightning  can  be  seen  so 
much  farther  at  night  than  during  the  daytime.     In  fact,  the  so-called 
"  heat  lightning  "  usually  indicates  the  presence  of  a  thundershower,  for 
it  is  probably  the  reflection  from  clouds  or  the  hazy  sky  of  lightning 
which  is  accompanying  a  shower  which  is  below  the  horizon  of  the 
observer. 


326  METEOROLOGY 

THE  REGIONS  AND  TIME  OF  OCCURRENCE 

326.  Geographical  distribution.  —  Thundershowers  occur  in  nearly 
every  part  of  the  world,  but  the  number  decreases  rapidly  from  the 

d  equator  towards  the  pole.  Within  the  tropics  there  are 

showers  many  places  where  there  are  nearly  200  days  in  the  course  of 
occur  every-  a  vear  with  thundershowers.  The  number  of  days  with 
thundershowers  decreases  rapidly  with  latitude,  until  in  the 
polar  regions  but  one  or  two  thundershowers  in  the  course  of  several 
years  may  be  recorded.  Fewer  thundershowers  occur  over  the  ocean 
than  over  the  land,  and  mountainous  regions  have  far  more  than  level 
country. 

In  the  United  States  the  largest  number  occurs  in  the  Gulf  States, 
where  there  are  on  the  average  about  sixty  days  in  the  course  of  a  year 
with  thundershowers.  The  number  decreases  both  north  and  west. 
In  New  England  the  average  is  not  much  over  fifteen.  The  accompany- 
ing table  gives  the  normal  number  of  days  with  thundershowers  for 
several  stations  in  the  United  States.1  These  normals  are  based  on  the 
ten  years,  1901  to  1910,  inclusive. 

Statistics  as  to  the  number  of  days  with  thundershowers  or  the  number 
of  thundershowers  are  very  unreliable  and  cannot  be  compared.  The 
Statistics  reason  is  because  there  is  no  uniformity  among  observers  as 
are  unre-  to  what  constitutes  the  presence  of  a  thundershower.  Some 
liable.  count  only  the  presence  of  rain,  others  the  audibility  of 

thunder  or  the  visibility  of  lightning  or  the  occurrence  even  of  the  so- 
called  heat  lightning. 

327.  Relation  to  extratropical  cyclones  and  V-shaped  depressions.  — 
Nearly  all  thundershowers  which  occur  in  extratropical  regions  are  to 
Thunder-       De  f°und  in  the  southern  quadrants  of  a  low.     The  thunder- 
showers         showers  which  are   due  to  purely  local  causes,  such  as  a 
southern*116    mountain  or  a  peculiarity  of  the  general  wind  system,  are, 
quadrants       of  course,  to  be  expected.     It  will  be  remembered  that  the 

great  difference  between  a  winter  and  summer  low  was 
the  fact  that  the  nimbus  cloud  area  is  unusually  absent  in  summer 
and  its  place  is  taken  by  cirriform  clouds,  with  sultry  weather  and 
thundershowers.  Chart  XXXV,  which  gives  the  daily  weather  map 

1  For  the  number  of  thundershower  days  in  the  United  States  during  1903  see  Monthly 
Weather  Review,  1903,  end  of  volume.  For  a  chart  of  the  normal  number  of  thunder- 
shower  days  in  the  United  States,  see :  Climatology  of  the  United  States,  by  A.  J.  HENRY 
(Bulletin  Q  of  the  U.  S.  Weather  Bureau).  For  the  number  of  thundershowers  at  Albany, 
N.Y.,  each  year  from  1884  to  1910,  see  section  28. 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     327 


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328  METEOROLOGY 

for  8  A.M.,  Sept.  2,  1904,  shows  a  low  with  several  thundershowers  in  its 
southern  quadrants.  The  presence  of  a  thundershower  is  indicated  by  /~2. 
The  weather  maps  for  June  30,  1904;  July  14,  1904;  March  1,  1907; 
July  17,  1908 ;  April  30,  1909 ;  May  6,  1909 ;  May  15,  1909,  are  also 
particularly  good  illustrations  of  this. 

Thundershowers  also  occur  in  great  numbers  in  connection  with 
V-shaped  depressions.  Whenever  an  isobaric  line,  instead  of  being 
V-shaped  straight  or  uniformly  curved,  is  bent  so  as  to  have  the  form 
depressions.  of  a  pOCket  Or  trough,  it  is  spoken  of  as  a  V-shaped  depres- 
sion. Whenever  a  low  has  a  pronounced  well-developed  wind  shift 
line,  the  isobars  in  the  southern  quadrants  nearly  always  have  this 
Thunder-  peculiar  V-shaped  bulge.  This  has  been  fully  described  and 
showers  are  illustrated  in  section  287.  If  a  low  with  a  wind  shift  line 
wmTaiTv-  and  this  peculiar  V-shaped  bulge  crosses  the  country  during 
shaped  de-  the  summer,  it  is  nearly  always  attended  by  thundershowers 
ins'  particularly  along  the  wind  shift  line.  A  V-shaped  depres- 
sion sometimes  indicates  that  a  secondary  low  is  forming.  These 
secondary  lows  are  very  common  in  Europe,  but  not  so  frequent 
in  this  country.  They  sometimes  develop  all  the  characteristics  of  a 
regular  low  and  are  even  more  violent.  They  usually  form  in  the 
southern  quadrants  of  a  low,  and  their  motion  is  eastward  and  north- 
ward usually  with  a  greater  velocity  than  the  parent  low.  As  a  result, 
they  seem  to  circle  about  it  in  a  counterclockwise  direction.  Whenever 
these  secondaries  occur  in  summer,  they  are  nearly  always  attended  by 
thundershowers.  Sometimes  thundershowers  will  occur  in  connection 
with  a  V-shaped  depression  when  it  is  neither  a  wind  shift  line  or  the 
beginning  of  a  secondary  low.  Thus  whenever  a  weather  map  is  being 
studied,  all  V-shaped  depressions  should  be  carefully  noted  and  the 

characteristics    of    the    meteorological    elements    about 

them  critically  examined. 

328.    Path  across    a   country.  —  When  an  overgrown 

cumulus  cloud  first  develops  into  a  cumulo-nimbus  cloud 
FIG.  132.  —  The  Form  of  a       an(^    becomes    a    thundershower,    it    usually 
Typical  Form  thunder-        covers   but   a  small    area.     It  is  perhaps   a 

of  a  Thunder-    shower.  f  -i         i  j  -i  •  j 

shower.  few  miles    long    and    a    mile   or   two    wide. 

As  it  moves  across  the  country,  it  becomes  constantly 
larger,  so  that  at  the  end  of  six  or  seven  hours,  which  is  about  the 
average  life  of  a  thundershower,  it  has  a  front  some  150  or  200 
miles  long  and  is  perhaps  40  miles  wide.  Its  form  is  that  shown  in 
Fig.  132. 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     329 


FIG.    133.  —  The    Typical 
Path  of  a  Thundershower. 


The  typical  path  of  a  shower  across  the  country  is  thus  pear-shaped 
and  is  shown  in  Fig.  133.  Much  larger  individual  thundershowers 
have  been  noted.  In  some  cases  Form  of 
they  have  lasted  more  than  twelve  path- 
hours  and  have  covered  a  path  more  than  500 
miles  long  and  nearly  as  wide  in  its  widest  part. 
A  large  thundershower,  which  traversed  Ger- 
many August  9,  1881,  has  been  carefully 
charted  by  Koppen  and  is  shown  in  Fig.  134. 

329.  Direction    and    velocity    of    motion.  — 
Thundershowers    may  move   in    any    direction,   but   in   extratropical 
regions  the  great  majority  of  them  move  from  west  to  east.  Direction  of 
This  agrees  with  the  direction  of  motion  of  the  upper  air  motion- 
currents  and  also  with  the  direction  of  motion  of  the  surface  winds  in 

the  southern  quadrants  of  a  low. 
Those  due  to  purely  local  causes, 
as  a  mountain,  sometimes  remain 
stationary. 

The   average  velocity  for   the 
United  States  is  between  30  and 
40  miles  per  hour;   in  Velocity  of 
Europe,  it  is  between  motion- 
20  and  30  miles.     The  velocity  of 
motion  is  greater  over  the  ocean 
than  on   land  and  is  greater  in 
winter  and  at  night  than  in  sum- 
mer and  during  the  daytime. 

Individual  showers  often  depart 
widely  from  the  type  as  regards 
direction  and  velocity  of  motion. 

330.  Time   of    day   and    season   of    occurrence.  —  Thundershowers 
may  occur  any  hour  of  the  day  or  night,  and  any  month  in  the  year. 
The  great  majority,   however,   occur   during  the  warmest 

months  in  the  year,   June,   July,  August,  and  during  the  and  season 
hottest  part  of  the  day,  3  to  5  P.M.  of  occur~ 

Most    stations    in    the     United    States    have     a    pro- 
nounced   maximum    between    2    and    6    in    the    afternoon,     and    a 
small  secondary  maximum   between  3  and  5  in  the  early  morning. 
The    forenoon    and    the    hours    near    midnight    show    the    smallest 
number. 


FIG.  134.  —  The  Path  of  a  Large  Thunder- 
shower  across  Germany. 


330  METEOROLOGY 

Over  the  ocean  more  occur  during  the  night  than  during  the  day,  and 
the  same  is  true  of  Iceland  and  some  coast  stations. 

331.  Periodicity  of  thundershowers.  —  In  addition  to  the  prominent 
well-marked  diurnal  and  annual  periodicity  in  the  occurrence  of  thunder- 
showers,  it  has  been  thought  that  other  faintly  marked  periods 
are  also  Present-     Some  think  that  the  moon  influences  the 

ficant  occurrence  of  thundershowers ;    the  number  being  slightly 

thToccur!-  larger  during  new  moon  and  first  quarter  than  during  full 
rence  of  moon  and  last  quarter.  Some  think  that  there  is  a  tidal 
showers".  influence,  the  number  being  greater  during  the  high  tide  than 
during  low  tide.  Others  have  thought  that  there  was  a 
26-day  period  corresponding  to  the  time  of  revolution  of  the  sun  and 
an  11-year  period  corresponding  to  the  sun  spot  cycle.  If  there 
were  uniform  observations  at  many  stations  covering,  say  100  years, 
it  would  be  possible  to  answer  the  questions  as  to  the  existence  of 
these  periods  at  once.  As  it  is,  the  observations  are  far  from  uniform 
and  the  records  are  often  short  and  fragmentary.  As  a  result,  it  seems 
impossible  at  present  to  be  sure  whether  these  periods  exist  or  not. 

They  are  certainly  not  large  or  well  marked. 

* 

THE  ORIGIN  AND  GROWTH  OF  A  THUNDERSHOWER 

332.  Three  classes  of  thundershowers.  —  All  thundershowers  can- 
not be  ascribed  to  the  same  cause,  and  for  this  reason,  in  studying  the 
The  three       origin  °f  thundershowers,  it  is  more  convenient  to  divide 
classes  of       them  into  three  classes ;    namely,  heat  or  convection  thun- 
thunder-        dershowers,  cyclonic    thundershowers,   thundershowers   due 

to  local  conditions.  Each  of  these  three  classes  will  be 
considered  separately.  41!  thundershowers  have  this  in  common: 
All  due  to  tnev  are  caused  by  moist  rising  airl  This  moisl;  rising  air 
the  rising  of  cools  due  to  expansion,  reaches  its  dew  point,  and  builds  the 

immense  cumulo-nimbus  cloud  with  its  copious  precipitation. 
As  a  result  of  this  copious  condensation  in  a  thick  cloud,  lightning  and 
thunder  occur. 

333.  Heat-  or  convection-caused  thundershowers.  —  Heat-  or  convec- 
The  origin      tion-caused  thundershowers  have  their  origin  in  masses  of 
opmentVof"a    warm>  m°ist  air.     This  air  rises,  due  to  convection,  cools  by 
convection-    reason  of  expansion,  reaches  its  dew  point,  and  builds  first 
thunder-        a  cumuuis  cloud.     If  the  mass  of  rising  air  is  not  too  large, 
shower.          or  if  it  rises  in  different  places  in  smaller  amounts,  only 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     331 

cumulus  clouds  are  the  result.  If  the  mass  of  rising  air  is  large,  the 
cumulus  cloud  becomes  overgrown  and  the  condensation  of  moisture 
is  sufficiently  vigorous  to  cause  precipitation.  The  cumulus  cloud 
has  now  become  a  cumulo-nimbus  cloud  and  underneath  it  a  de- 
scending air  current  forms.  There  are  three  causes  of  this  descending 
air  current.  In  the  first  place,  the  falling  raindrops  carry  down  air  with 
them.  In  the  second  place,  the  air  underneath  the  cloud  becomes  cooler 
and  heavier  because  of  the  cold  raindrops  which  pass  through  it  and  also 
because  it  is  shielded  from  insolation  by  the  cloud.  In  the  third  place, 
there  is  a  certain  amount  of  reaction  against  the  rising  air,  for  a  large 
amount  of  air  is  injected  into  the  upper  atmosphere  and  this  must 
cause  an  outward  movement  in  all  directions.  Thus,  underneath  the 
cumulo-nimbus  cloud,  there  is  a  descending  air  current  which  is  brushed 
forward  when  it  reaches  the  earth's  surface,  both  as  a  result  of  the  for- 
ward movement  of  the  shower  and  to  take  the  place  of  the  air  which  is 
rising  due  to  convection.  In  between  this  rising  moist  warm  air  in 
front  of  the  thundershower  and  this  descending  air  current  underneath 
the  cloud  mass,  a  vigorous  eddy  or  whirl  about  a  horizontal  The  expla_ 
axis  forms.  This  is  seen  in  the  turbulent  squall  cloud  and  nation  of  the 
the  peculiar  violent  outrushing  squall  wind  is  a  part  of  it.  squaU* 
It  might  seem  that  the  squall  is  very  vigorous  compared  with  the  gentle 
air  currents  which  build  it,  but  it  must  be  remembered  that  the  squall 
cloud  and  wind  are  very  small  compared  with  the  immense  amount  of 
rising  and  descending  air.  Furthermore,  the  squall  cloud  is  near  the 
axis  of  the  whirl.  All  these  air  motions  are  shown  well  in  Fig.  134. 
The  air  motion  in  a  thundershower  is  very  similar  to  what  would  be  pro- 
duced by  placing  a  card  edgewise  on  a  table,  inclining  it  backward  so  as 
to  make  an  angle  of  about  60°  with  the  table,  and  then  moving  it  slowly 
forward.  The  air  would  rise  over  the  card,  descend  back  of  it  to  fill 
the  vacant  space  as  it  was  moved  forward,  and  an  eddy  about  the  upper 
horizontal  edge  of  the  card  would  also  form. 

The  origin  and  growth  of  a  heat-  or  convection-caused  thundershower 
have  now  been  fully  stated  and  the  reason  for  the  structure  of   the 
storm  and  the  changes  in  the  meteorological  elements  during  The  expia_ 
its  approach  and  passage  should  be  apparent.     The  baro-  nation  of  the 
metric  pressure  rises  as  soon  as  the  squall  wind  arrives.     The  the'meteor- 
reason  for  this  is  the  downward  movement  of  the  air  masses  ological  eie- 
underneath   a   thundershower,  and  also   the  fact  that   the  ments* 
air  beneath  the  cloud  mass  is  cooler   and  thus   denser  and  heavier. 
There  are  three  reasons  why  the  temperature  drops  rapidly  when  the 


332  METEOROLOGY 

squall  wind  comes.  Underneath  the  cloud  mass  there  are  cold  descend- 
ing air  currents.  Furthermore  the  air  is  cooled  by  the  colder  raindrops 
which  fall  through  it,  and  it  is  shielded  from  insolation  by  the  cloud 
mass.  The  reasons  for  the  change  in  the  wind,  moisture,  cloud,  and 
precipitation  are  too  evident  to  need  explanation. 

This  class  of  thundershowers  ought  to  occur  in  the  greatest  number 
and  with  the  most  vigor  when  large  masses  of  warm  moist  air  are  most 
Time  and  numerous.  This  would  be  during  the  hottest  time  of  year 
place  of  and  during  the  hottest  time  of  day.  Furthermore,  it  would 
Lce'  be  when  a  place  was  located  in  the  southern  quadrants  of  a 
low,  for  it  is  then  that  the  southerly  winds  are  transporting  the  largest 
amount  of  warm,  moist  air.  It  will  be  seen  that  all  these  requirements 
of  theory  are  in  good  accord  with  the  observed  facts. 

There  are  two  or  three  observed  facts  in  connection  with  thunder- 
showers  which  have  been  observed  so  often  by  professional  meteorolo- 
Rivers  bin-  gists  and  others  as  well  that  an  explanation  of  them  should 
der  thunder-  be  given.  One  is  the  fact  that  a  thundershower  often  seems 
to  be  unable  to  cross  a  large  river.  It  advances  to  one 
bank,  remains  stationary,  and  perhaps  weakens  and  disappears  or  builds 
sidewise  along  the  river.  If  it  crosses  the  river,  it  is  practically  a-  new 
shower  which  builds  on  the  other  bank.  This  has  been  observed  too 
often  to  question  the  fact.  It  must  be  that  the  river  is  colder  than  the 
surrounding  country  and  is  thus  the  seat  of  a  gently  descending  air 
current,  which,  however,  has  sufficient  vigor  to  prevent  the  convec- 
tional  rise  of  air  which  is  the  condition  of  life  and  advance  on  the  part 
of  the  thundershower.  In  winter,  when  the  river  is  warmer  than  the 
land,  this  hindrance  to  the  advance  of  thundershowers  should  not  exist. 
It  also  should  be  much  less  at  night. 

Another  fact,  which  has  been  often  observed,  is  that  it  rains  harder 
after  each  lightning  flash.  If  this  is  true,  it  may  be  that  the  small  rain- 
drops in  the  cloud  are  all  charged  with  the  same  kind  of 
harder  after  electricity  and  kept  from  uniting  by  electrical  repulsion, 
a  lightning  AS  soon  as  they  are  discharged  by  the  lightning  flash,  they 
coalesce  much  more  readily  and  build  the  larger  drops  which 
soon  fall  to  the  earth's  surface.  The  interval  of  time  between  the  lightning 
flash  and  the  increase  in  the  rainfall  ought  to  give  the  time  required  for 
the  raindrop  to  fall.  It  may  also  be  that,  for  some  reason,  the  small  drop- 
lets suddenly  unite  to  form  large  ones,  and  thus,  as  a  result,  the  lightning 
flash  occurs.  In  one  case,  the  lightning  flash  is  the  cause;  in  the  other, 
the  result.  Interesting  observations  in  this  connection  could  be  made. 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     333 

334.  Convection  can  be  caused  by  cooling  the  top  of  a  layer  of  air 
as  well  as  by  heating  the  bottom.     Thus  thundershowers  might  be 
caused  by  cooling  the  upper  layers  of  the  atmosphere  as  well 

as   by   heating  unduly  the  surface   layer.     This   probably  method  of 
often  occurs.     Excessive  radiation  during  the  night  from  beginning  of 
the  upper  layers  of  the  atmosphere,  particularly  if  there  is  a  caused 
thin  cloud  layer,  may  cool  the  upper  air  sufficiently  to  cause  thunder- 
unstable  equilibrium  and  thus  convection  and  a  thunder- 
shower.      These  thundershowers    would    occur    chiefly    in    the   early 
morning  when  the   air   was    coldest.      The    secondary   maximum   in 
the  frequency  of  thundershowers  in  the  early  morning  which  has  been 
observed  at  most  stations  in  the  United   States  is  probably  due  to 
thundershowers  formed  in  this  way. 

335.  Hail.  —  Hail  falls  wherever  thundershowers  occur.     It  is  esti- 
mated that  from  one  half  to  one  tenth  of  all  thundershowers  are  accom- 
panied by  hail.     Hail  practically  never  occurs  except  during  The  charac_ 
the  hottest  part  of  the  year  and  the  hottest  time  of  day.  teristics  of 
In  this  respect,  it  shows  a  much  more  marked  annual  and  a 

daily  periodicity  than  do  thundershowers.  Hail  never  falls  except  dur- 
ing the  beginning  of  a  thundershower.  The  area  covered  by  a  fall  of 
hail  is  very  much  smaller  than  the  area  covered  by  the  thundershower 
which  accompanies  it.  It  is  usually  not  more  than  6  or  7  miles  wide  and 
perhaps  40  or  50  miles  long.  Thundershowers  which  are  accompanied 
by  hail  usually  have  a  very  well-developed  squall  cloud  and  are  more 
than  usually  violent.  The  structure  and  characteristics  of  this  particu- 
lar kind  of  hail  have  already  been  fully  stated  in  section  243. 

The  hailstones  are  formed  in  the  whirling  squall  cloud  of  a  thunder- 
shower.     The  nucleus  is  carried  up  and  coated  with  snow ;  it  then  falls 
or  is  carried  down,  and  is  coated  with  water;    it  is  then  Theforma- 
carried  up  again;    the  process  continues,  adding  coat  after  tionofthe 
coat,  until  the    hailstone    becomes  too  heavy  to  be  longer 
sustained  and  it  falls  to  the  ground.     It  will  be  seen  at  once  that  all  of 
the  facts  concerning  the  structure  of  a  hailstone  and  concerning  the  time 
and  place  of  occurrence  of  hail  are  in  accord  with  this  explanation. 

The  possibility  of  the  existence  of  temperatures  sufficiently  low  to 
cause  hail  to  be  formed  might  be  questioned.     Suppose  a  thundershower 
is  4  miles  thick,  and  that  the    cloud  commences  one  mile  The  cause 
above  the  earth's  surface,  and  suppose,  furthermore,  that  the   of  the  low 
temperature  is  that  of  the  rising  air.    This  air  cools  1.6°  F.  for  temPerature- 
every  300  feet  until  the  cloud  level  is  reached.     Then  the  rate  of  cooling 


334  METEOROLOGY 

will  be  roughly  one  half  of  that  until  the  freezing  point  is  reached  and 
then  still  less.  A  rough  calculation  will  show  that  the  top  of  the  cloud 
ought  to  be  from  75°  to  100°  F.  colder  than  the  temperatures  at  the 
surface.  Thus,  even  if  surface  temperatures  are  well  up  towards  90°  F., 
the  top  of  a  thundershower  must  be  well  below  the  freezing  point  and 
even  in  the  neighborhood  of  zero  and  thus  composed  of  snowflakes  or 
ice  crystals.  There  is  no  difficulty  then  in  finding  sufficiently  low  tem- 
peratures for  the  formation  of  hail. 

336.    Cyclonic  thundershowers.  —  Cyclonic  thundershowers  are  due 
to  the  passing  of  a  low  and  the  coming  of  a  high ;  they  are  thus  condi- 
tioned on  the  change  in  the  weather  control  from  an  area  of 

The  transi-  „  _  .    _ 

tion  from  a  low  pressure  to  an  area  of  high  pressure.  If  a  low  goes  by 
low  to  a  north  of  a  station  so  that  its  southern  quadrants  pass  over 

Du£u* 

the  place  in  question,  warm  moist  air  is  brought  to  the 
place  by  the  southerly  winds  which  accompany  the  low.  A  coming  high 
is  heralded  by  rather  brisk,  cold,  dry  jets  of  air  from  the  northwest. 

The  transition  from  one   to   the  other  is  usually  slow  and  gradual. 
There  are  times,  however,  when  it  is  abrupt,  and  this  is  particularly  the 

case  when  the  low  has  developed  a  prominent  wind  shift 
clonic  thun-  line  in  its  southern  quadrants.  The  jets  of  cold  dry  air 
dershowers  from  the  northwest  may  overrun  or  underrun  the  warm 

are  caused. 

moist  air  of  the  low.  If  the  cold  dry  air  overruns  the  warm 
moist  air,  then  there  is  unstable  equilibrium  and  all  the  conditions  for 
convection  are  fulfilled,  for  there  is  warm  air  below  and  cold,  dense  air 
above.  Convection  will  take  place  and,  in  summer,  thundershowers  will 
often  form.  These  are  really,  in  their  growth  and  development,  con- 
vection thundershowers,  but  their  origin  is  cyclonic  because  it  is  condi- 
tioned upon  the  interaction  of  a  high  and  a  low.  If  the  cold  dry  air  of 
the  coming  high  underruns  the  warm  moist  air  of  the  departing  low, 
this  warm  moist  air  will  be  raised  bodily  and  forced  to  rise.  This  forced 
rise  of  warm  moist  air  in  summer,  and  even  in  winter,  often  causes 
thundershowers.  Thundershowers  formed  in  this  way  by  the  inter- 
action of  a  high  and  low  often  have  a  very  long  front.  It  may  extend 
even  the  whole  length  of  the  wind  shift  line,  and  in  England  these  showers 
are  called  "  line  showers."  Cyclonic  thundershowers  ma3r  occur  any 
hour  of  the  day  or  night  or  at  any  time  of  year. 

How  local  33^'    Thundershowers  due  to  local    conditions. — Thun- 

stationary  dershowers  due  to  local  conditions  are  nearly  always 
showers'are  causec^  kv  the  presence  of  a  mountain  and  a  sea  breeze,  or 
caused.  a  mountain  breeze,  or  a  wind  caused  by  a  passing  high  or 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     335 

low.  If  a  strong  sea  breeze  blows  against  a  mountain  side  near  the 
shore  and  the  air  is  forced  to  rise,  a  permanent  cumulus  cloud  is 
formed  over  the  mountains.  If  the  sea  breeze  is  particularly  strong  or 
moisture-laden,  the  cumulus  cloud  may  become  overgrown  and  develop 
into  a  thundershower.  Such  a  thundershower  would  remain  stationary 
near  the  mountain  and  disappear  in  the  late  afternoon  with  the  dying 
down  of  the  sea  breeze.  A  mountain  breeze  blowing  up  a  mountain 
side  during  the  daytime  could  produce  a  stationary  thundershower  in 
the  same  way.  Many  of  these  stationary  thundershowers  over  moun- 
tain peaks  are  reported  in  the  Alps  and  in  the  regions  near  the  Rocky 
Mountains.  Air  set  in  motion  by  a  passing  high  or  low  and  forced  to 
rise  by  a  mountain  could  also  produce  a  thundershower.  It  will  be 
seen  in  every  case  that  the  cause  of  the  thundershower  was  air  forced  to 
rise  by  a  barrier.  A  cumulus  cloud  is  first  formed,  and  this  may  become 
overgrown  and  develop  into  a  thundershower. 

THUNDER  AND  LIGHTNING 

338.  Since  lightning  and  thunder  are  only  attendant  phenomena 
caused  by  the  copious  condensation  in  a  thick  cloud  and  have  These  wm 
no  part  in  the  mechanism  of  a  thundershower,  they  will  be  be  consid- 
considered  later  in  Chapter  XI  in  connection  with  atmos-  eredlater' 
pheric  electricity. 

D.  TORNADOES 

DEFINITION  AND  DESCRIPTION 

339.  Definition  and  chief  characteristics.  —  The  tornado  is  the  most 
diminutive  and  yet  the  most  violent  and  destructive  Of  all  storms.     It  is 
peculiar  to  the  United  States,  although  in  a  slightly  modified 

form,  it  at  times  occurs  in  other  parts  of  the  world.     The  charactens- 
name  is  derived  from  the  Spanish  and  refers  to  the  twisting  tics  °* a 

5   tornado. 

or  rotating  nature  of  the  storm.  It  is  always  associated 
with  a  violent  thundershower,  which  is  usually  accompanied  by 
hail,  a  pronounced  squall  wind,  and  violent  thunder  and  lightning.  It 
occurs  almost  exclusively  during  the  warmer  months  of  the  year  and 
during  the  hottest  part  of  the  day.  The  term  cyclone  is  still  often  used 
popularly,  and  even  in  the  newspapers,  in  referring  to  these  storms,  but 
they  should  be  called  tornadoes,  and  the  term  cyclone  should  be  reserved 
for  the  tropical  cyclone  or  the  extratropical  cyclone. 


336  METEOROLOGY 

The  most  distinctive  thing  about  a  tornado  is  the  peculiar  black  funnel- 
shaped  cloud  which  extends  downward  from  the  heavy  cloud  masses 
above,  usually  reaches  the  earth's  surface,  and  causes  complete  devasta- 
tion wherever  it  touches.  In  the  United  States,  nearly  a  hundred  lives 
and  several  million  dollars  of  property  are  lost  annually  by  tornadoes, 
while  a  single  violent  one  may  cause  four  or  five  times  this  amount  of 
loss. 

340.  Description  of  the  approach  and  passage  of  a  tornado.  —  Since 
a  tornado  is  always  associated  with  a  heavy  thundershower,  the  charac- 
The  funnel  teristics  of  the  day  and  the  weather  changes  which  precede 
cloud.  ^he  coming  of  a  tornado  are  the  same  as  those  which  herald 

the  coming  of  a  violent  thundershower  on  a  hot,  sultry  summer  after- 
noon. Nearly  all  observers  agree  that  just  before"  the  formation  of  the 
funnel  cloud,  the  clouds  have  an  ominous  greenish  black  appearance, 
and  seem  to  rush  together  and  start  whirling  with  great  violence.  Then 
the  black  funnel  cloud  appears,  which  drops  lower  and  lower  until  the 
surface  of  the  ground  is  reached.  Here  it  enlarges  slightly,  so  that  some 
have  described  the  tornado  cloud  as  having  the  form  of  an  hourglass. 
It  usually  sways  slightly  from  side  to  side  and  often  writhes  and  twists. 
Sometimes  the  funnel  cloud  jumps  a  certain  strip,  only  to  touch  the 
ground  again  farther  on.  The  whole  formation  is  usually  less  than 
1000  feet  in  diameter  and  passes  a  given  point  in  less  than  half  a  minute, 
but  this  is  sufficient  for  complete  destruction. 

The  destruction  wrought  by  a  tornado  seems  to  be  caused  both  by 
excessive  wind  velocities  and  also  by  an  explosive  action.  The  explo- 
Two  causes  s^ve  ac^ion  is  probably  due  to  the  sudden  decrease  in  the 
ofthede-  barometric  pressure.  The  barometric  pressure  is  normally 
between  fourteen  and  fifteen  pounds  per  square  inch.  If  the 
pressure  were  suddenly  reduced  one  half,  it  would  cause  a  pressure 
of  over  seven  pounds  per  square  inch  on  the  inside  surface  of  all  objects 
containing  air  which  could  not  quickly  escape.  The  destruction  wrought 
by  a  tornado  is  often  weird  and  unusual  as  well  as  terrible. 

Due  to  tornadic  action,  large  trees  are  stripped  of  their  branches, 
broken  off  near  the  ground,  or  torn  up  by  the  roots ;  heavy  brick  and 
The  charac  s*one  buildings  are  crushed  and  destroyed  as  if  they  were 
teristics  of  card  houses ;  tin  roofs  are  torn  from  buildings  and  carried 
the^destruc-  manv  miles  through  the  air ;  loaded  cars  and  even  locomo- 
tives have  been  blown  from  the  track;  heavy  iron 
girders  have  been  carried  over  the  tops  of  buildings ;  iron  bridges 
have  been  moved  from  their  foundations.  Straws  have  been  driven 


FIG.  13& —  Two  Views  of  the  same  Tornado  at  Goddard,  Kansas,  May  26,  1903,  about 
4  P.M.     (No  barograph  disturbance  was  noticed  at  Wichita  about  18  miles  distant.) 


FIG.  136.  —  A  Tornado  at  Oklahoma 
City,  May  12,  1896. 


FIG.   137.  —  Damage  caused  by  a  Tornado  at 
Rochester,  Minn.,  August  21,  1883. 


FIG.  138.  —  Wreckage  of  Anchor  Hall,  Jefferson  and  Park  Avenues, 
St.  Louis,  May  27,  1896. 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     337 

through  boards,  laths  through  trees,  and  small  sticks  of  timber 
through  iron  plate.  When  the  funnel  cloud  strikes  a  building,  it  often 
seems  to  explode.  The  roof  is  carried  up  and  the  side  walls  fly  apart. 
Chests  explode ;  corks  are  drawn  from  empty  bottles ;  chickens  are 
stripped  of  their  feathers.  Soot  is  often  seen  to  rise  in  quantity  from 
the  chimneys  of  near-by  houses  and  window  panes  often  fly  outward. 

The   noise   which   accompanies   a   tornado   is   tremendous.     It   has 
been  likened  to  the  noise  of  a  thousand  express  trains  rush-  The  accom_ 
ing  through  tunnels  or  to  the  sound  produced  by  thousands  panying 
of  wagons  loaded  with  iron  and  moving  rapidly  over  an  E 
uneven  pavement.     The  uproar  is  so  great  that  the  crash  of  individual 
buildings  is  seldom  heard. 

The  funnel  cloud  of  a  tornado  is  usually  associated  with  the  front  of 
the   violent   thundershower.     But   little   rain   usually   falls  Reiati0nto 
before  its  coming,  and  hail  usually  follows  it.     The  lightning  the  thunder- 
at  times  is  almost  incessant  so  that  the  funnel  cloud  has 
a  reddish,  lurid  look.     After  a  tornado  passes,  all  the  characteristics 
of  a  violent  thundershower  usually  appear. 

In  Figs.  135  and  136  photographs  of  distant  tornadoes  are  repro- 
duced, and  in  Figs.  137  and  138  some  of  the  damage  caused  by  tor- 
nadoes is  pictured.1 

341.    Distribution  of  the  meteorological  elements   about   a   tornado. 
—  The  changes  in  the  meteorological  elements  brought  about  by  a 
thundershower   have    already   been    considered.     Only   the 
changes  brought  about  by  the  funnel  cloud,   the  tornado  sure  and 
proper,   will  be  here  stated.     The  barometric  pressure  in  wind  in  a 
the  center  of  the  funnel  cloud  of  a  tornado  has  never  been 
determined,  as  the  instruments  have  always  been  smashed  or  exploded. 
Almost  instantaneous  drops  in  pressure  of  nearly  an  inch  have  been 
observed  within  a  few  hundred  feet  of  a  tornado,  but  the  drop  in  the 
center  of  the  'funnel  cloud  is,  without  doubt,  much  larger  than  this. 
From  its  explosive  effect,  it  has  been  estimated  that  the  pressure  per- 
haps drops  to  one  half  its  value  in  the  center.     The  wind  velocity  has 
never  been  measured.     It  certainly  goes  well  over  a  hundred  miles  per 
hour,  and  may  even  reach  500  miles  per  hour,  in  violent  tornadoes. 
The  direction  of  the  whirl  about  the  center  is  always  counterclockwise, 
and  would  thus  seem  to  be  determined  by  the  rotation  of  the  earth,  or 

1  For  additional  pictures  of  tornadoes  and  the  damage  caused  by  them  see  Monthly 
Weather  Review,  July,  1899 ;  September,  1905,  p.  400 ;  June,  1906,  p.  276 ;  June,  1907, 
p.  258. 


338  METEOROLOGY 

perhaps  the  rotation  of  air  about  the  low  in  the  southern  quadrants  of 
which  the  thundershower  and  tornado  usually  form.  The  only  distinc- 
tive cloud  form  is  the  blue-black  funnel  cloud  which  extends  down  from 
the  cloud  masses  above  to  the  earth's  surface.  No  noticeable  changes 
in  temperature,  moisture,  or  precipitation  occur. 

342.  Observation   of  tornadoes.  —  Deliberate,   careful   observations 
are  not  often  made  during  the  passage  of  a  tornado,  not  even  if  the  path 
The  obser-     °^  destruction  is  several  hundred  feet  from  the  observer, 
vations  to  be  Such  observations  are,  however,  much  needed.     They  should 

cover  three  things:  (1)  the  appearance  of  the  clouds,  their 
color  and  motions,  the  direction  and  velocity  of  motion  of  the  tornado, 
the  characteristics  of  the  thundershower  which  it  accompanies;  (2) 
the  changes  in  the  meteorological  elements  during  the  whole  day,  but 
more  particularly  during  the  passage  of  the  thundershower  and  tornado  ; 
(3)  the  kind  and  peculiarities  of  the  destruction  wrought. 

In  making  full  notes  of  any  meteorological  occurrence,  it  is  always 
better  to  record  them  in  full  on  the  spot  and  not  trust  at  all  to  one's 
memory  later. 

THE  REGIONS  AND  TIME  OF  OCCURRENCE 

343.  Geographical    distribution.  —  Tornadoes    occur    almost    exclu- 
sively in  the  United  States,  although  in  a  slightly  modified  form  they 

sometimes  occur  in  the  other  countries  where  violent  thunder- 

The  place 

of  occur-  showers  are  common.  They  do  not  occur  with  the  same 
tornadoes  frequency  in  all  parts  of  the  United  States,  but  visit  chiefly 
the  Mississippi  Valley  and  certain  of  the  southern  states. 
They  are  practically  unknown  in  the  Rocky  Mountain  region  and  near 
the  Appalachian  system.  They  are  most  frequent  over  level  country 
which  is  not  heavily  wooded  and  are  practically  unknown  in  mountainous 
or  forest  covered  regions.  In  Fig.  139,  the  distribution  of  all  recorded 
tornadoes  from  1794  to  1881,  as  determined  by  Lieutenant  J.  F. 
Finley,  is  given.1 

The  number  of  observed  tornadoes  is  increasing  each  year,  but  this 
does  not  necessarily  mean  that  the  number  of  tornadoes  is  increasing, 
The  number  as  the  country  is  becoming  more  thickly  settled  and  the 
of  tornadoes,  occurrence  and  details  of  tornadoes  are  more  fully  published. 
During  the  year  1877  to  1887f  on  the  average,  146  tornadoes  occurred 
annually  in  the  United  States. 

1  See  C.  ABBE,  "  Tornado  Frequency  per  Unit  Area,"  Monthly  Weather  Review,  June, 
1897,  p.  250. 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     339 


FIG.  139.  —  The  Distribution  of  all  Recorded  Tornadoes  from  1794  to  1881. 
(From  GREELEY'S  American  Weather.) 

344.   Relation  to  extratropical  cyclones.  —  Tornadoes  nearly  always 
occur  in  the  southern  or  southeastern  portion  of  an  extratropical  cyclone 
and  from  200  to  800  miles  from  its  center.     It  will  be  remem- 
bered  that  it  is  here  that  violent  thundershowers  form  in  iow  which 
the  largest  numbers.     It  is  sometimes  said  that  the  low  pvesnseto 

tornadoes. 

which  is  attended  by  tornadoes  has  such  definite  and  peculiar 
characteristics  that  the  probability  of  a  tornado  can  almost  be  predicted 
from  the  characteristics  of  the  low.  These  peculiar  characteristics 
are  the  following :  The  isobars  are  distinctly  oval,  and  extend  exactly 
north  and  south.  The  wind  shift  from  south  to  northwest  or  west  is 
very  sharp,  and  often  a  V-shaped  bulge  in  the  isobars  in  the  southern 
quadrants  has  developed.  The  temperature  lines  are  packed  very 
closely  together  along  this  wind  shift  line,  and  the  bend  in  them  is  some- 
times so  sharp  that  they  bend  backward  on  themselves.  The  tempera- 
ture difference  between  the  east  and  west  portion  of  the  low  is  large  and 
well-marked. 

Charts  XXXVI  and  XXXVII  which  give  the  daily  weather  map  for 
3  P.M.  March  11,  1884  and    8  A.M.  April  25,  1906,   repre-  Tornado 
sent  typical  tornado   lows.     In  the   first  case,  there  were  prediction. 


340  METEOROLOGY 

several  distinct  tornadoes  in  the  southeast  portion,  and  in  the  second 
case  a  tornado  occurred  in  Texas.  Unfortunately  these  charac- 
teristics are  not  always  present  when  a  low  is  attended  by  tornadoes. 
In  fact,  a  tornado  can  occur  when  none  of  them  are  present.  A  careful 
study  of,  say,  a  hundred  lows  which  have  been  attended  by  tornadoes 
will  always  leave  the  impression  that  it  is  impossible  to  do  any  predic- 
tion of  the  occurrence  of  a  tornado  from  the  characteristics  of  the  low.1 

345.  Path  across  a  country.  —  The  path  of  a  tornado  is  from  a  few 
feet  to  perhaps  2000  feet  wide,  and  from  a  mile  to  sometimes  200  or  300 

miles  long.     The  direction  of  motion  is  easterly  or  south- 
easterly  and   the  velocity  of  motion  from  20  to  50  miles 

per  hour.      As  a  result  a  tornado  passes  a  given  point  in  less  than 

half  a  minute. 

346.  Time  of  day  and  season  of  occurrence.  —  Tornadoes  are  most 
frequent  from  3  to  5  in  the  afternoon,  and  least  frequent  from  7  to  9 

in  the  morning.     The  diurnal  variation  is  very  large  and 
and  season     well-marked.     Of  those  occurring  in  the  evening,  most  have 

of  occur-  originated  during  the  afternoon  and  have  continued  their 
rence.  . &  . 

existence  into  the  evening. 

Tornadoes  occur  chiefly  during  May,  June,  July,  and  August,  but, 
particularly  in  the  Southern  states,  they  are  common  at  all  times  of 
the  year. 

THE  ORIGIN  AND  GROWTH  OF  A  TORNADO 

347.  The  origin  of  tornadoes.  —  The  method  of  formation  of  the 
various  kinds  of  thundershowers  has  already  been  fully  treated.     In 
considering  the  origin  of  a  tornado  it  is  only  necessary  to  explain  why 
the  tornado  funnel  cloud  forms. underneath  certain  showers.     The  forma- 
tion of  a  tornado  is  usually  ascribed  to  one  of  two  causes :  violent  local 
convection  at  a  given  point  or  the  existence  of  an  energetic  eddy. 

If  a  thundershower  is  viewed  from  the  side,  it  is  often  noticed  that 
one  or  two  thunderheads  rise  much  higher  than  the  rest.  This  means 
Convectional  a  more  violent  convectional  updraft  at  these  points  than 
origin  of  elsewhere.  If,  at  some  point,  the  convectional  updraft  were 
a  tornado.  especially  energetic,  there  would  develop  a  well-marked 
indraft  at  the  earth's  surface  to  replace  the  rising  air.  This  moving 
air  would  be  deviated  to  the  right,  due  to  the  earth's  rotation,  and  a 
vigorous  atmospheric  whirl  would  build  itself  up.  The  growth  and 

1  For  a  long  series  of  weather  maps  when  tornadoes  occurred  see,  FINLET,  "Tornado 
Studies  for  1884,"  Prof.  Papers  of  the  Signal  Service,  No.  XVI. 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     341 

development  would  be  almost  exactly  the  same  as  in  the  case  of  the 
tropical  cyclone  and  would  soon  lead  to  a  fully  developed  tornado. 
The  direction  of  revolution  would  be  here  determined  by  the  rotation 
of  the  earth. 

At  the  level  of  the  lower  clouds  there  are  numerous  air  currents  with 
slightly  different  directions  and  very  different  velocities.     These  could 
easily  form  an  energetic  eddy  which  would  make  itself  felt  Tornadoes 
at  the  earth's  surface.     Since  a  tornado  is  so  small  and  the  may  be  due 
path  covered  so  short  as  compared  with  the  thundershower  * 
which  it  accompanies,  it  is  not  unreasonable  to  think  of  it  simply  as  an 
eddy.     It  is  a  well-known  principle  that  when  one  eddy  forms  inside 
another,  the  small  eddy  takes  the  direction  of  rotation  of  the  larger. 
Now  tornadoes  form  in  the  air  which  is  moving  counterclockwise  about 
an  area  of  low  pressure  and  thus  would  rotate  in  the  same  direction. 

Many  more  exact  observations  of  tornadoes  are  necessary  before 
every  step  in  the  origin  and  development  of  a  tornado  can  be  fully 
stated. 

348.  Explanation  of  the  facts  of  observation.  —  The  theories  as  to  the 
origin  of  a  tornado  account  for  the  existence  of  the  whirl,  its  direction 
of  rotation,  and  the  high  velocity.  There  are  several  other  _ 

£xpl  fl.X13.tlO  U 

facts  of  observation  which   need   explanation.     The  rapid  Of  various 
whirl  causes  centrifugal  force,  and  this  is  the  cause  of  the  facts  °f  ob~ 

.  .      servation, 

very  low  barometric  pressure  at  the  center,  which  in  turn  is  particularly 

the  cause  of  the  tornado  funnel  cloud.     The  air  has  been  the  funnel 

•  - .  cloud, 

relieved  of  perhaps  one  half  of  the  pressure  upon  it ;   it  has 

expanded  quickly,  cooled  below  its  dew  point,  and  produced  a  cloud 
extending  the  whole  length  of  the  whirl.  The  funnel  cloud  has  often 
been  observed  to  touch  the  earth's  surface  for  a  certain  distance,  then 
rise  above  the  earth,  leaving  a  strip  without  destruction,  and  then 
descend  again  to  the  earth's  surface  and  continue  its  work  of  destruction. 
This  simply  means  that  the  whirl  weakened  for  a  certain  distance.  The 
weakening  of  the  whirl  accounts  at  once  for  the  absence  of  destruction. 
It  also  would  mean  less  centrifugal  force,  a  higher  barometric  pressure 
at  the  center,  and  thus  less  cooling  due  to  expansion,  and  perhaps  ab- 
sence of  the  cloud  as  the  dew  point  would  not  be  reached. 


PROTECTION  FROM  TORNADOES 

349.   The  subject  of  protection  from  tornadoes  can  be  treated  from 
two  entirely  different  points  of  view ;  first,  the  precautions  to  be  taken 


342  METEOROLOGY 

in  order  to  secure  safety  if  one  is  overtaken  by  a  tornado  and,  secondly, 
tornado  insurance  again  loss  caused  by  tornadoes. 

If  it  is  a  frame  building,  the  southwest  corner  of  the  cellar  is  probably 
the  safest  place.  This  is  on  the  assumption  that  the  building  will  be 
The  places  carried  away  by  the  first  blast  of  wind.  If  the  building  is  of 
of  greatest  stone  or  brick,  or  if  one  is  caught  out  of  doors,  it  is  better  to 
lie  down  in  the  open.  Do  not  seek  safety  under  a  tree.  In 
places  often  visited  by  tornadoes,  so-called  "  cyclone  cellars  "  are  some- 
times constructed,  and  these  are  even  provided  by  the  more  cautious 
with  tools  and  certain  supplies,  in  case  everything  is  demolished,  or  the 
opening  is  covered  with  debris. 

Insurance  against  loss  by  tornadoes  is  now  provided  by  several 
tornado  insurance  companies.  Until  recently  they  were  not  on  a  very 
Tornado  satisfactory  basis,  but  at  present  the  distribution  of  tornadoes 
insurance.  an(j  the  amount  of  damage  caused  by  them  is  sufficiently 
well  known  to  permit  the  probable  risk  to  be  computed.1 


E.     WATERSPOUTS   AND   WHIRLWINDS 

WATERSPOUTS 

350.  Waterspouts  are  simply  tornadoes  which  occur  over  bodies  of 
water.  This  is  abundantly  proved  by  the  fact  that  whenever  a  tornado 
Waterspouts  crosses  a  river,  pond,  or  body  of  water,  it  immediately  becomes 
are  simply  a  waterspout ;  and  waterspouts  which  run  inland  develop  at 
once  all  the  characteristics  of  tornadoes.  Waterspouts  are 
most  common  in  the  warmer  and  calmer  seas,  although  they  may  occur 
wherever  violent  thundershowers  are  found.  When  the  funnel  cloud 
touches  the  surface  of  the  water,  it  is  greatly  agitated,  and  the  water  has 
been  observed  to  rise  even  eight  or  ten  feet.  This  gives  an  idea  of  the 
diminution  in  the  barometric  pressure  in  the  center  of  a  waterspout. 
If  it  were  a  vacuum,  the  water  would  rise  a  little  more  than  thirty  feet. 
The  spray  is,  of  course,  often  carried  up  much  higher,  but  the  stories  of 
large  quantities  of  water  being  carried  up  from  the  sea  into  the  clouds  is 
pure  myth.  When  a  waterspout  crosses  a  vessel,  it  has  been  found 
that  the  water  in  it  is  fresh.  It  is  thus  a  condensation  product,  and  has 
not  been  carried  up  from  the  sea. 

Waterspouts  have  usually  been  observed  from  considerable  distances 

1  See :  Tornado  Insurance,  by  HOWARD   E.  SIMPSON,  Colby  College,  Waterville,  Me., 
Insurance  Monitor,  February  to  September,  1883. 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     343 

and  with  no  great  care.     The  waterspout  off  Cottage  City,    Mass., 
August  19,  1896,  occurred  under  circumstances  remarkably  advantage- 
ous for  making  observations  and  photographs,  and  has  been  A  notable 
for  this  reason  more  carefully  studied  than  any  other.     It  water- 
has  been  critically  studied  and  treated  by  Frank  H.  Bigelow  spout< 
in  four  articles  in  Vol.  XXXIV  (1906)  of  the  Monthly  Weather  Review. 
One  of  the  photographs  there  given  is  reproduced  as  Fig.  140.      This 
waterspout  occurred  in  connection  with  a  thundershower  which  was  due 
to  the  overrunning  of  the  surface  air  by  a  layer  of  colder  air,  as  the 
weather  control  passed  from  an  area  of  low  to  an  area  of  high  pressure. 

WHIRLWINDS 

351.  The  violent  sandstorms  and  the  dusty  whirlwinds  of  desert 
regions  are  probably  only  tornadoes  of  a  slightly  modified  form  and 
small  intensity  which  are  occurring  under  rather  unusual  Desert 
conditions.  Over  a  dry  desert  region,  the  amount  of  mois-  whirlwinds, 
ture  in  the  air  is  so  small  that  violent  convection  causes  no  precipitation 
or  even  cloud.  The  surface  layer  of  air  is  heated  to  very  high  tempera- 
tures, as  the  sky  is  always  cloudless,  and  energetic  convection  must  often 
take  place.  If  the  sandstorm  is  a  straight  blow  with  rather  long  front, 
then  it  has  all  the  characteristics  of  a  thundershower,  except  that  the 
cloud  and  precipitation  are  lacking.  Whenever  a  dusty  whirlwind  with 
a  small  slender  column  appears,  it  is  probably  a  tornado  of  small  intensity, 
but  without  a  funnel  cloud,  as  there  is  not  moisture  enough  to  form  it. 

In  fact,  even  the  little  whirlwinds  which  spring  up  along  a  dusty 
roadway  on  a  hot  summer  afternoon  and  move  a  short  distance  and 
rise  to  a  height  of  twenty  or  thirty  feet,  remind  one  of  a  diminutive 
tornado.  Even  when  carefully  considered,  as  to  origin  and  character- 
istics, they  still  have  some  things  in  common  with  tornadoes. 


F.  CYCLONIC  AND  LOCAL  WINDS 

INTRODUCTION 

352.   In  the  last  part  of  Chapter  IV  the  general  winds  of  the  world 
were   classified   and   discussed.     It   remains   in   connection   with   this 
chapter  to  treat  those  winds  which  are  due  to  the  passing  of  Cyclonic 
cyclonic  and  anticyclonic  disturbances.     There  are  three  of  winds- 
these  which  are  well  known  in  the  United  States.     They  are  :  first,  the 


344  METEOROLOGY 

warm  moist  south  wind  often  called  by  its  Italian  name,  the  sirocco  ; 
second,  the  cold  dry  northwest  wind  which  accompanies  the  coming 
There  are  of  a  cold  wave,  and  may  under  certain  circumstances  be 
United1  thC  considered  a  blizzard ;  third,  the  chinook,  which  is  perhaps 
states.  better  known  by  its  Swiss  name,  the  foehn. 


THE  CYCLONIC  AND  LOCAL  WINDS  OF  THE  UNITED  STATES 

353-  Warm  wave  (sirocco).  —  If  a  low  is  passing  north  of  a  sta- 
tion, warm  moisture-laden  air  from  the  south  is  transported  to  the 
Cause  of  a  place  in  question.  If  the  low  moves  slowly  or  if  the  in- 
warm  wave.  draft  is  particularly  energetic,  a  decided  rise  in  tempera- 
ture may  take  place,  and  this  is  spoken  of  as  a  warm  wave.  The  pass- 
ing of  a  cyclonic  disturbance  is  particularly  apt  to  cause  a  warm  wave 
in  the  states  east  of  the  Mississippi  Hiver,  as  the  air  transported  from 
the  Gulf  States  is  especially  warm  and  moist. 

In  summer,  the  warm  wave  takes  the  form  of  several  hot,  sultry,  oppres- 
sive days.  The  temperature  and  moisture  are  high,  the  wind  blows 

from  the  south,  and  the  sky  is  hazy  and  partly  cloud-covered, 

usually  with  cirriform  clouds.  Relief  comes  when  the  center 
warm  wave  of  the  low  approaches  sufficiently  near  to  cause  rain,  or  when 
an^winter.  ^ne  ^ow  Passes  and  the  wind  shifts  to  the  northwest.  In 

winter  it  takes  the  form  of  a  thaw,  and  the  sleighing  may  be 
spoiled  or  the  ground  freed  from  snow.  Ice  storms  often  result  from 
the  coming  of  this  south  wind,  since  the  wind  velocity  is  much  greater 
at  the  height  of  a  mile  or  so  above  the  earth's  surface.  This  transports 
warm  moist  air  much  more  rapidly  at  this  level,  so  that  the  upper  air  may 
be  much  warmer  than  the  air  at  the  earth's  surface. 

This  warm  moist  wind  from  the  south  which  causes  a  warm  wave  is 
often  called  by  its  Italian  name,  the  sirocco,  but  this  name  is  ordinarily 
The  sirocco  empl°ved  to  designate  a  particularly  warm  moist  wind,  and 

not  to  designate  the  south  wind  in  front  of  any  coming  low. 
354.  Cold  wave  (blizzard).  —  The  meaning  of  the  term  cold  wave 
has  been  made  definite  by  the  U.  S.  Weather  Bureau.  It  is  defined  as 
Definition  of  a  drop  of  a  certain  number  of  degrees  in  twenty-four  hours 
a  cold  wave.  wjth  a  minimum  below  a  certain  temperature.  The  amount 
of  the  drop  and  the  minimum  are  different  for  different  times  of  year, 
Cause  of  a  and  for  different  groups  of  states.  A  cold  wave  is  brought 
cold  wave,  about  by  the  cold,  dry,  northwest  wind  which  marks  the 
passing  of  a  low  and  the  coming  of  a  high.  In  the  popular  mind,  the 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     345 

drop  in  temperature  and  the  wind  are  associated  together,  although 
technically  the  cold  wave  has  to  do  with  the  drop  in  temperature  only. 
If  it  still  continues  to  snow  after  the  wind  has  gone  to  the  northwest, 
and  the  temperature  has  dropped,  or  if  the  high  northwest  The 
wind  drifts  the  light  snow,  a  blizzard  is  said  to  be  in  progress.  blizzard- 
There  is  no  exact  definition  of  a  blizzard,  but  its  characteristics  are 
supposed  to  be  high  northwest  wind,  driving  snow,  and  low  and  falling 
temperature. 

The  origin  of  the  cold  air  which  constitutes  a  cold  wave  has  been 
ascribed  to  several  causes.     Some  have  claimed  that  the  cold  air  was 
transported  from  the  far  north,  and  that  cold  waves  thus  The  origin 
have  their  place  of  beginning  in  the  extreme  northwest,  of  cold 
Others  have  claimed  that  the  cold  air  descends  from  the  v 
upper  atmosphere,  while  still  others  look  for  the  cause  in  the  con- 
tinued radiation  of  heat  to  the  clear  sky  night  after  night.     That  a  cold 
wave  is  made  on  the  spot,  so  to  speak,  seems  more  plausible  than  that 
the  air  is  transported  long  distances. 

355.  Chinook  (foehn).  —  The  chinook  occurs  chiefly  on  the  eastern 
side  of  the  Rocky  Mountains,  and  is  particularly  common  in  the  states  of 
Wyoming  and  Montana.  It  is  a  hot  dry  wind  coming  from  the  _ 

J  .  •-.  .  Description. 

west  across  the  mountains.  It  usually  makes  its  appearance 
suddenly,  and  the  temperature  may  rise  even  40°  in  fifteen  minutes. 
The  snow  disappears  as  if  by  magic,  for  it  evaporates,  due  to  the  dryness 
of  the  air,  as  well  as  melts.  It  often  removes  as  much  snow  in  a  day  as 
ordinary  spring  thawing  would  remove  in  two  weeks.  If  it  is  common, 
it  raises  the  normal  annual  temperature  of  a  place  by  several  degrees. 

This  wind  was  first  noted  and  studied  in  northern  Switzerland,  where 
it  was  called  the  foehn.  It  occurs  chiefly  on  the  north  side  of  the  Alps, 
although  it  does  appear  on  the  south  side  as  well.  In  some  The  foehn 
valleys,  it  blows  from  30  to  50  days  during  the  months  from  and  its  char- 
November  to  March  and  raises  the  normal  annual  tempera-  a 
ture  considerably.  If  the  foehn  continues  for  several  days,  or  if  it  is 
particularly  vigorous,  not  only  does  all  the  snow  melt,  but  everything 
becomes  so  dry  that  special  precautions  are  taken  to  prevent  a  general 
conflagration  in  case  of  fire.  Sometimes  all  fires  are  extinguished  while 
the  foehn  blows.  This  wind  is  also  found  in  Greenland  and  New  Zea- 
land. In  fact,  it  is  found  wherever  a  mountain  chain  and  passing  lows 
are  associated  together.  This  wind  is  usually  called  the  chinook  in  the 
Western  states,  although  it  is  better  known  as  the  foehn  in  other  parts 
of  the  world,  and  this  last  name  is  fast  becoming  universal. 


346 


METEOROLOGY 


FIG.  141. 


Diagram  Illustrating  the  Formation  of 
the  Foehn  Wind. 


The  cause  of  the  foehn  can  best  be  stated  in  connection  with  a  dia- 
gram (Fig.  141).  The  air  is  passing  over  a  mountain  towards  a 
The  cause  cyclonic  center  and  is  forced  to  rise  by  the  mountain.  It 
of  the  foehn.  expands  and  cools  at  the  rate  of  1.6°  F.  per  300  feet,  soon 
reaches  its  dew  point,  becomes  cloudy,  and  yields  copious  precipitation. 
The  latent  heat  liberated  by  the  condensation  of  the  water  vapor  warms 
the  rising  air  and  prevents  its  cooling  at  so  rapid  a  rate.  In  fact  the 
rate  is  cut  down  from  1.6°  F.  per  300  feet  to  a  little  less  than  half  this 

amount.  After  ^e  top  of 
the  mountain  is  reached,  the 
air  begins  the  descent  on  the 
other  side.  It  is  now  being 
compressed  and  grows  warm 
at  the  rate  of  1.6°  F.  per 
300  feet.  The  precipitation 
ceases,  the  clouds  disappear, 
and  the  air  continues  to  grow 

warm  at  the  same  rate  during  the  whole  of  the  descent.  As  a 
result,  it  may  reach  the  same  level  on  the  other  side  of  the  moun- 
tain 20°  or  even  40°  warmer  than  before  it  started  the  ascent.  It 
has  also  become  very  dry,  as  so  much  moisture  was  removed  by 
precipitation.  This  explanation  of  the  foehn  was  first  given  by 
Hann  of  Vienna.  It  was  formerly  thought  that  the  Swiss  foehn 
was  due  to  hot  dry  air  which  came  in  some  way  from  the  desert  of 
Sahara. 

A  chinook  should  occur  on  the  eastern  side  of  the  Rocky  Mountains 
whenever  a  well-developed  cyclonic  storm  passes  across  the  northern 
The  relation  ^^^  °^  ^ne  United  States.  It  ought  to  occur  on  the  western 
of  the  side  of  the  mountains  when  the  storm  center  passes  across  the 

southern  part  of  the  country.  As  most  cyclonic  storms  pass 
along  the  northern  boundary  of  the  United  States,  the  chi- 
nook has  been  observed  on  the  east  side  much  more  frequently  than  on 
the  west.  A  cyclonic  storm,  central  over  Germany,  ought  to  cause  a 
well-marked  foehn  on  the  northern  side  of  the  Alps,  and  a  foehn  on  the 
south  side  ought  to  be  caused  by  a  low,  central  over  Italy  or  the  Medi- 
terranean Sea.  As  a  matter  of  fact,  the  foehn  is  much  better  developed 
and  of  much  more  frequent  occurrence  on  the  north  side  of  the  Alps 
than  on  the  south  side.  The  reason  is  because  the  air  coming  north 
from  Italy  is  already  warm  and  dry,  and  thus  lends  itself  much  more 
readily  to  the  formation  of  a  foehn  wind. 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     347 


THE  CYCLONIC  AND  LOCAL  WINDS  OF  OTHER  COUNTRIES 

356.   The  various  cyclonic  and  local  winds  which  occur  in  different 
parts  of  the  world  may  be  divided  in  four  groups  according 
to  cause,  and  each  of  these  groups  will  be  treated  in  order,  four  groups 

(I)  The  first  group  includes  those  which  correspond  to 
the  warm,  moist,  south  wind  which  is  the  cause  of  the  warm 

waves  in  the  United  States,  and  is  the  well-known  sirocco  of  Italy. 

The  solano  is  a  very  hot,  dusty,  southeast  wind  which  The  sirocco 
occurs  in  the  Mediterranean,  especially  on  the  eastern  and  similar 
coast  of  Spain. 

The  leveche  is  a  hot,  dry,  southwest  wind,  which  also  occurs  in 
Spain. 

The  leste  is  a  very  hot  parching  south  wind  which  occurs  in  Madeira 
and  northern  Africa. 

All  these  winds  have  the  same  origin.  They  are  due  to  the  indraft 
of  warm  air  from  the  south  towards  the  center  of  an  advancing  low. 
They  will  be  dry  or  moist,  according  as  the  region  over  which  they  ad- 
vance is  dry  or  moist.  In  the  southern  hemisphere,  the  corresponding 
winds  would  come  from  the  north. 

The  brickfielder  of  southern  Australia  is  a  hot,  dry,  north  wind. 

The  zonda  of  the  Argentine  Republic  is  a  hot  northerly  sirocco. 

(II)  The  second  group  includes  those  winds  which  correspond  to  the 
dry,  cold,  northwest  wind  which  ushers  in  the  cold  waves  in  the  United 
States,  and  is  often  a  blizzard. 

The  buran,  or  purga,  is  a  very  cold  northeast  wind,  which  zard  and 


occurs  in  Russia  and  central  Asia.     The  snow  is  blown  by 
the  wind,  and  it  is  often  blizzard-like. 

The  pampero  is  a  dry,  cold;  southwest  wind  which  is  common  in 
southern  Brazil,  Argentina,  and  Uruguay. 

The  tormentos  of  Argentina  is  the  same  kind  of  a  wind. 

All  of  these  winds  are  due  to  the  cold  dry  northwest  wind  (northern 
hemisphere)  which  marks  the  passing  of  a  low  and  the  coming  of  a  high. 

(Ill)    The  winds  of  the  third  group  are  due  to  the  bringing  down  of 
cold  dry  air  from  a  plateau  into  a  valley  by  a  passing  anticyclonic  dis- 
turbance.    Masses  of  air  might  be  brought  down  from  a  The  mistrai 
plateau  into  a  valley  by  a  passing  low  as  well  as  by  a  high,  and  similar 
but  the  air  would  not  be  cold  and  dry,  as  the  presence  of  a 
high  is  necessary  to  give  the  air  over  the  plateau  these  characteristics. 
The  air  is  warmed  somewhat  by  compression  during  the  descent,  but  if 


348  METEOROLOGY 

the  descent  is  not  too  rapid,  it  still  reaches  the  valley  colder  and  dryer 
than  the  valley  air. 

The  mistral  (Italian :  magistrate  =  masterly)  is  a  dry,  cold,  north- 
west wind  found  in  the  Rhone  Valley,  and  due  to  masses  of  air  coming 
into  the  valley  from  the  plateau  in  the  southeastern  part  of  France. 

The  bora  (Greek :  /Jo'peas  =  north  wind)  is  a  furious  northerly  wind, 
coming  into  the  Adriatic  Sea  from  the  plateau  of  Carinthia. 

The  tramontana  is  a  searching  northerly  blast  found  on  the  Italian 
side  of  the  Adriatic. 

The  gregale  of  Malta  is  a  cold,  dry,  unhealthy  wind,  and  is  due  to 
the  same  cause. 

The  williwaus  of  Terra  del  Fuego,  in  Patagonia,  is  perhaps  of  the 
same  origin.  This  is  a  hurricane-like  wind  coming  down  from  the  moun- 
tains and  blowing  with  great  violence  only  8  or  10  seconds. 

(IV)  The  fourth  group  includes  those  winds  which  get  their  peculiar 
characteristics  from  the  topography  and  condition  of  the  regions  from 
The  kham-  which  the  winds  come.  The  wind  is,  of  course,  caused  to 
sin  and  sim-  blow  from  these  particular  directions,  due  to  the  passing  of 
aar  winds.  a  h;gh  or  low 

The  harmattan  is  a  hot,  dusty,  east  wind,  which  is  found  in  the  west 
side  of  the  desert  of  Sahara.  It  is  most  common  during  December, 
January,  and  February,  and  brings  much  sand  with  it. 

The  khamsin  (Arabic :  khamsun  =  fifty)  is  found  in  Egypt  and  is  a 
hot  wind  from  the  desert.  Its  direction  is  usually  from  the  south  or 
southeast,  and  it  blows  from  20  to  50  days  in  the  course  of  a  year. 

It  will  be  seen  that  all  of  these  are  cyclonic  or  local  winds,  in  the 
sense  that  they  are  due  to  the  passing  of  highs  or  lows,  and  in  some 
cases  to  the  characteristics  of  the  surrounding  region. 

QUESTIONS 

(1)  State  the  chief  characteristics  of  the  tropical  cyclone.  (2)  What  names 
are  applied  to  them  in  different  parts  of  the  world?  (3)  Trace  the  historical 
rise  of  our  present  information  about  tropical  cyclones.-"  (4)  Describe  the 
approach  and  passage  of  a  tropical  cyclone.  (5)  Describe  in  detail  the  distri- 
bution of  the  meteorological  elements  about  a  tropical  cyclone.  (6)  Illustrate 
the  distribution  by  means  of  two  diagrams.  (7)  Give  the  life  history  of  some 
tropical  cyclone.  (8)  What  are  the  rules  for  mariners  in  connection  with 
tropical  cyclones?  (9)  How  is  the  storm  center  located?  (10)  Which  is  the 
dangerous  half  of  the  tropical  cyclone  and  why?  (11)  Where  are  the  regions  of 
occurrence  of  tropical  cyclones?  (12)  Describe  the  form  of  the  track  followed. 
(13)  At  what  times  of  year  do  they  occur?  (14)  State  in  full  the  convectional 
theory  of  the  origin  of  tropical  cyclones.  (15)  Explain  the  formation  of  the 
calm  central  eye.  (16)  Explain  the  path  followed  and  the  time  of  occurrence  of 


THE   SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     349 

the  tropical  cyclone.  (17)  Why  do  tropical  cyclones  occur  where  they  do? 
(18)  Compare  the  tropical  cyclone  and  the  circumpolar  whirl  in  the  general  wind 
system.  (19)  State  the  chief  characteristics  of  the  extratropical  cyclone.  (20) 
Describe  in  detail  the  distribution  of  the  meteorological  elements  about  a  low. 
(21)  How  does  the  distribution  differ  at  different  times  of  a  year  and  in  different 
parts  of  the  world?  (22)  What  are  the  factors  which  cause  a  departure  from 
the  type  form?  (23)  Describe  the  distribution  of  the  meteorological  elements 
about  a  low  with  a  wind  shift  line.  (24)  Illustrate  the  distribution  by  means 
of  a  diagram.  (25)  Describe  in  detail  the  structure  of  an  extratropical  cyclone 
at  various  levels  above  the  earth's  surface.  (26)  Contrast  the  tropical  and 
extratropical  cyclone.  (27)  Describe  the  approach  and  passage  of  a  low.  (28) 
Define  a  high  and  state  the  distribution  of  the  meteorological  elements  about  it. 

(29)  Describe  the  structure  of  a  high  at  various  levels  above  the  earth's  surface. 

(30)  Describe  the  sequence  of  weather  changes  as  a  high  approaches  and  passes. 

(31)  Describe  the  tracks  followed  by  lows  in  the  northern  hemisphere.     (32) 
Where  do  they  originate?     (33)  Describe  the  system  of  tracks  across  Europe. 
(34)  Describe  the  three  systems  of  tracks  across  the  United  States.     (35)  Treat 
the  velocity  of  motion  of  lows  across  the  United  States.     (36)  Describe  the 
tracks  followed  and  the  velocity  of  motion  of  highs.     (37)  Treat  in  full  the 
theories  as  to  the  origin  of  extratropical  cyclones  and  anticyclones.     (38)  De- 
scribe the  methods  of  growth  of  an  extratropical  cyclone.     (39)  Describe  the 
growth  of  an  anticyclone.     (40)  In  connection  with  both  high  and  lows  explain 
the  peculiarities  in  the  distribution  of  the  elements  and  the  direction  and  velocity 
of  motion.     (41)  Does  the  ah*  accompany  a  high  or  low?     (42)  What  factors 
determine  the  direction  of  the  wind  and  its  velocity  at  any  point  near  a  low  or 
high?     (43)  What  is  meant  by  the  correlation  of  the  meteorological  elements 
and  why  does  one  exist  ?     (44)  Define  a  thundershower  and  state  its  chief  char- 
acteristics.    (45)  Describe  the  approach    and    passage    of    a    thundershower. 
(46)  Describe  and  illustrate  by  means  of  a  diagram  the  changes  in  the  meteoro- 
logical elements  as  a  thundershower  passes.     (47)  Draw  a  cross  section  of  a 
thundershower.     (48)  What  observations  are  made  of  thundershowers  ?     (49) 
Where  do  thundershowers  occur?     (50)  What  is  the  relation  of  thundershowers 
to  extratropical  cyclones?     (51)  How  are  they  related  to  V-shaped  depressions? 
(52)  Describe  the  form  of  a  thundershower  and  the  form  of  its  path.     (53)  What 
is  their  direction  and  velocity  of  motion  ?     (54)  At  what  time  of  day  and  season 
of  the  year  do  they  occur?     (55)  Are  thundershowers  periodic  in  their  occur- 
rence?    (56)    Slate   the   three    classes  of   thundershowers.     (57)  Describe  in 
detail  the  development  of  a  heat-  or  convection-caused  thundershower.    (58)  De- 
scribe the  structure  and  formation  of  hail.     (59)  How  are  cyclonic  thunder- 
showers  caused  ?     (60)  How  are  thundershowers  due  to  local  conditions  formed  ? 
(61)  Define  a  tornado  and  state  its  chief  characteristics.     (62)  Describe  the 
approach  and  passage  of  a  tornado.     (63)  State  the  changes  in  the  meteoro- 
logical elements  as  the  tornado  passes.     (64)  When  and  where  do  tornadoes 
occur?     (65)  How  are  they  related  to  extratropical  cyclones?     (66)  Explain 
the  origin  and  growth  of  a  tornado.     (67)  Describe  a  waterspout  and  state  its 
cause.     (68)  State  the  cause  of  the  violent  sand  storms  and  the  dusty  whirl- 
winds of  desert  regions.     (69)  Name  the  three  cyclonic  winds  peculiar  to  the 
United  States.     (70)  Describe  the  wind  which  causes  the  warm  wave  and  is 
often  called  the  sirocco.     (71)  Describe  the  wind  which  causes  the  cold  wave  and 
may  sometimes  be  considered  a  blizzard.     (72)  What  is  the  chinook  wind? 
(73)  Where  does  the  foehn  occur  and  what  are  its  characteristics?     (74)  State 
the  cause  of  the  foehn  wind.     (75)  Name  the  four  classes  of  cyclonic  or  local 
winds  and  describe  some  examples  in  each  class. 


350  METEOROLOGY 

TOPICS   FOR   INVESTIGATION 

(1)  The  complete  life  history  and  characteristics  of  some  tropical  cyclone. 

(2)  The  calm  central  eye  of  a  tropical  cyclone  and  the  evidence  of  its  exist- 
ence. 

(3)  The  extent  to  which  a  tropical  cyclone  retains  peculiar  characteristics 
after  entering  extratropical  regions. 

(4)  The  difference  in  the  distribution  of  the  meteorological  elements  about 
a  typical  low  in  Europe  and  in  America. 

(5)  General  laws  derived  from  statistics  on  highs  and  lows. 

(6)  The  theories  as  to  the  origin  of  highs  and  lows. 

(7)  The  complete  life  history  of  some  thundershower. 

(8)  The  periodicity  of  thundershowers. 

(9)  The  theories  as  to  the  formation  of  hail. 

(10)  The  size  and  structure  of  hailstones. 

(11)  The  complete  life  history  and  characteristics  of  some  tornado. 

(12)  The  characteristics  of  the  low  which  is  accompanied  by  tornadoes. 

(13)  Tornado  insurance. 

(14)  The  complete  life  history  and  characteristics  of  some  waterspout. 

(15)  The  characteristics  of  the  blizzard. 

PRACTICAL   EXERCISES 

(1)  Note  the  distribution  of  the  meteorological  elements  about  several  highs 
and  lows  and  in  each  case  explain  in  full  any  departure  from  the  type  form. 

(2)  Note  the  path  followed  by  several  highs  and  lows  and  in  each  case  explain 
any  departure  from  the  normal  path. 

(3)  From  a  10-year  file  of  the  daily  weather  maps  work  up  statistics  on  some 
point  in  connection  with  highs  and  lows. 

(4)  Work  out  the  correlation  of  the  meteorological  elements  for  one  or  more 
stations. 

(5)  Determine  from  a  file  of  weather  maps  the  location  of  thundershowers 
with  reference  to  the  lows  which  they  accompany. 

(6)  Determine  for  one  or  more  stations  the  normal  number  of  days  with 
thundershowers  for  the  various  months  and  for  the  year. 

(7)  Work  up  statistics  as  to  the  time  of  day  and  season  of  occurrence  of 
thundershowers . 

(8)  If  a  thundershower  with  hail  occurs,  study  critically  the  hail  stones. 

REFERENCES 

TROPICAL  CYCLONES 

ALEXANDER,  WILLIAM  H.,  Hurricanes.    Bulletin  32,  U.  S.  Weather  Bureau,  1902. 
ALGUE,  JOSE,  The  Cyclones  of  the  Far  East,  2d  ed.,  4°,  283  pp.,  Manila,  1904. 
BERGHOLZ,  PAUL,  Orkane  des  fernen  Ostens,  xii  +  260  pp.,  Bremen,  1900. 
DAVIS,    WILLIAM    M.,     Whirlwinds,    Cyclones,    and    Tornadoes,    24°,    90  pp., 

Boston,  1884. 
DOBERCK,  W.,   The  Law  of  Storms  in  the   Eastern  Seas,  4th  ed.,  8°,  44  pp., 

Hongkong,  1904. 
ELIOT,  SIR  JOHN,  Handbook  of  Cyclonic  Storms  in  the  Bay  of  Bengal  for  the  Use 

of  Sailors,  2d  ed.,  8°,  2v.,  Calcutta,  1900-1901. 


THE  SECONDARY  CIRCULATION  OF  THE  ATMOSPHERE     351 

FISHER,    ALFRED,    Die    Hurricanes  oder    Drehsturme    Westindiens,   4°,   70  pp., 

Gotha,  1908. 
GARRIOTT,  E.  B.,  West  Indian  Hurricanes,  4°,  69  pp.,  1900.     Bulletin  H,  U.  S. 

Weather  Bureau. 
PIDDINGTON,  HENRY,  The  sailors  Horn-book  for  the  Law  of  Storms,  6th  ed.,  8°, 

408  pp.,  London,  1876. 

For  the  life  history  of  various  tropical  cyclones  and  other  storms  see  the 
periodical-  literature. 

EXTRATROPICAL  CYCLONES  AND  ANTICYCLONES 

For  the  theories  as  to  the  origin  and  nature  of  extratropical  cyclones  and  anti- 
cyclones, see : 

ABBE,  CLEVELAND,  Mechanics  of  the  Earth's  Atmosphere  (2  vols.  of  collected 
papers). 

BIGELOW,  FRANK  H.,  Monthly  Weather  Review  (various  articles  from  1902  on). 

HILDEBRANDSSON,  H.  H.,  ET  TEissERENC  DE  BORT,  Les  bases  de  la  meteorologie 
dynamique,  Paris,  1900-1907. 

Report  of  the  chief  of  the  Weather  Bureau,  1898-1899.  (Report  on  the  inter- 
national cloud  observations.) 

STREIT,  A.,  Das  Wesen  der  Cyclonen,  Wien,  1906. 

For  the  distribution  of  the  meteorological  elements  about  highs  and  lows,  see : 

Annals  of  Harvard  College  Observatory,  Vol.  XXX. 

HANN,  JULIUS,  Lehrbuch  der  Meteorologie. 

HANZLIK,  STANISLAV,   Die  rdumliche   Verteilung  der  meteorologischen    Elemente 

in  den  Antizyklonen,  94  pp.,  Wien,  1898. 
LOCKYER,  WILLIAM  J.  S.,  Southern    Hemisphere  Surface    Air    Circulation,  4°, 

109  pp.,  1910. 

Meteorologische  Zeitschrift,  p.  307,  Juli,  1903. 
Monthly  Weather  Review,  March,  1907. 

For  the  tracks  followed  by  highs  and  lows,  see : 

BIGELOW,  FRANK  H.,  Storms,   Storm  Tracks,  and  Weather    Forecasting,  8°, 

87  pp.,  Washington,  1897.     Bulletin  20,  U.  S.  Weather  Bureau. 
DUN  WOODY,   H.   C.,   Summary,  of  International  meteorological  Observations, 

1878-1887.     Bulletin  A,  U.  S.  Weather  Bureau. 
VAN  CLEEF,  Monthly  Weather  Review,  Vol.  36,  1908. 

THUNDERSHOWERS 

CONGER,  N.  B.,  Report  on  the    Forecasting    of    Thunderstorms    during    the 

Summer  of  1892.     Bulletin  9,  U.  S.  Weather  Bureau,  1893. 
GOCKEL,  ALBERT,  Das  Gewitter,  246  pp.,  Koln,  1905. 
PLUMADON,  J.  R.,  Les  orages  et  la  grele,  8°,  192  pp.,  Paris,  1901. 
TOMLINSON,  CHARLES,  The  Thunderstorm,  London,  1877. 

TORNADOES 

American  Meteorological  Journal,  August,  1890.    (Four  prize  essays  on  tornadoes.) 
FERREL,  WILLIAM,  Cyclones,  Tornadoes,  and  Waterspouts.     Professional  Papers, 
U.  S.  Signal  Service,  No.  XII,  Washington,  1882. 


352  METEOROLOGY 

FINLEY,  J.  P.,  Report  of  the  Tornadoes  of  May  29  and  30,  1879,  in  Kansas, 
Nebraska,  Missouri,  and  Iowa.  Professional  Papers.  U.  S.  Signal  Service 
No.  IV,  1881. 

FINLEY,  J.  P.,  Characteristics  of  six  hundred  tornadoes.  Professional  papers, 
U.  S.  Signal  Service,  No.  VII,  1884. 

FINLEY,  J.  P.,  Tornadoes,  New  York,  12°,  196  pp.,  1887. 

FINLEY,  J.  P.,  Tornado  studies  for  1884.  Professional  papers,  U.  S.  Signal 
Service,  No.  XVI,  1885. 

HAZEN,  H.  A.,  The  Tornado,  New  York,  12°,  143  pp.,  1890. 

Report  of  the  Chief  of  the  Weather  Bureau,  1895-1896.  (A  Study  of  the  Tor- 
nadoes, 1889-1896.) 

WATERSPOUTS 

BIGELOW,  F.  H.,  four  articles  in  the  Monthly  Weather  Review,  1896,  Vol.  34. 


CHAPTER  VII 
WEATHER  BUREAUS  AND  THEIR  WORK 

A  BRIEF  HISTORY  OF  THE  U.  S.  WEATHER  BUREAU,  357 

THE  PRESENT  ORGANIZATION,  358,  359 

THE  STATION  EQUIPMENT  AND  THE  OBSERVATIONS  TAKEN,  360,  361 

THE  DEVELOPMENT  OF  THE  DAILY  WEATHER  MAP,  362 

THE  DAILY  WEATHER  SERVICE  OF  THE  U.  S.  WEATHER  BUREAU 

The  taking  and  sending  of  the  observations,  363. 

The  charting  of  the  observations,  364,  365. 

The  construction  of  the  weather  map,  366. 

The  distribution  of  the  map,  367. 

Other  methods  of  distributing  the  forecasts,  data,  and  warnings,  368. 

The  weather  service  in  the  evening  and  on  Sundays  and  holidays,  369. 

OTHER  WORK  AND  PUBLICATIONS  OF  THE  U.  S.  WEATHER  BUREAU,  370, 

371 

THE  WEATHER  BUREAUS  OF  OTHER  COUNTRIES,  372 
SOME  SPECIAL  METEOROLOGICAL  OBSERVATORIES;    THEIR  EQUIPMENT 

AND  WORK,  373 

A  BRIEF  HISTORY  OF  THE  U.  S.  WEATHER  BUREAU 

357.   In  1747  Benjamin  Franklin  made  a  very  important  discovery 
in  connection  with  storms.     He  had  arranged  with  his  brother  in  Boston 
to  make  some  observations  of  an  eclipse  of  the  moon,  while  Benjamin 
he  took  simultaneous  observations  in  Philadelphia.     Shortly  Franklin  ob- 
before  the  occurrence  of  the  eclipse,  a  strong  northeast  wind  ^stonn^sa 
set  in  in  Philadelphia,  bringing  with  it  clouds  and  rain,  so  moving  for- 
that  the  observations  could  not  be  secured.     Since  the  wind 
came  from  the  northeast,  he  supposed,  of  course,  that  the  observations 
had  not  been  made  in  Boston.     Great  was  his  surprise  when,  several 
days  later,  he  received  word  that  the  observations  had  been  secured  in 
Boston,  and  that  a  heavy  storm  had  commenced  the  following  morning. 
From  further  observations  collected  in  connection  with  this  storm,  and 
from  other  observations  as  well,  he  came  to  the  conclusion  that  a  storm 
was  a  moving  formation  and  that,  although  the  wind  usually  commenced 
to  blow  from  the  east  or  northeast,  its  motion  was  from  some  westerly 
2  A  353 


.1 


354  METEOROLOGY 

to  some  easterly  quarter.  As  soon  as  the  fact  was  fully  recognized  that 
a  storm  was  a  moving  formation,  the  desire  at  once  arose  to  keep  track  of 
it  and  herald  its  coming,  but  the  means  of  communication  were  too  slow 
and  uncertain  to  make  this  practicable.  After  the  invention  of  the 
electric  telegraph,  in  1837,  the  receiving  of  simultaneous  observations 
from  different  parts  of  the  country,  and  the  heralding  of  storm,  was  put 
into  partial  operation,  but  this  was  brought  to  an  end  by  the  breaking 
out  of  the  Civil  War. 

The  storms  on  the  Great  Lakes  had  attracted  attention  on  account 
of  their  severity  and  the  losses  caused  by  them.  In  1869  Professor 

Cleveland  Abbe,  who  was  then  Director  of  the  Observatory 
inge0f°Jhe  "  at  Cincinnati,  was  asked  by  the  Board  of  Trade  of  that  city, 
weather  ser-  to  undertake  the  forecasting  of  these  storms,  and  the  Western 
Abbe.7  Union  Telegraph  Co.  transmitted  the  messages  free  of  charge. 

The  work  was  so  successful  and  the  results  so  satisfactory 
that  the  attention  of  the  whole  country  was  attracted  to  it.  In  1870 
a  bill  was  introduced  into  Congress  by  Hon.  H.  E.  Paine,  from  Wiscon- 
sin, perhaps  at  the  suggestion  of  Professor  I.  A.  Lapham  of  Milwaukee, 
to  appropriate  $20,000  to  make  the  weather  service  national.  The 
bill  was  passed,  and  the  weather  service  of  the  United  States  was  thus 
inaugurated.  It  was  made  part  of  the  Signal  Service  and  placed  under 
the  War  Department,  and  thus  General  Albert  J.  Myer  became  its 
head,  The  first  warning  was  issued  November  1,  1870,  and  the  first 
The  first  daily  weather  map  was  issued  January  1,  1871.  This 
weather  made  the  United  States  the  fourth  country  to  issue  daily 

weather  maps,  as  the  Netherlands,  England,  and  France 
had  already  done  so. 

General  Myer  continued  to  be  the  head  of  the  weather  service  until 
his  death,  August  24,  1880.  He  was  a  strong  man  of  great  executive 
The  differ-  ^lity  and  under  him  the  weather  service  developed  rapidly, 
ent  heads  of  He  also  enjoyed  popular  approval  and  confidence.  He  was 

succee(^e^  ^y  General  William  B.  Hazen  who  continued  in 

office  until  his  death,  January  16,  1887.  His  administra- 
tion was  characterized  by  a  fostering  of  the  scientific  side  rather  than  by 
a  development  of  the  executive  side.  He  was  succeeded  by  General, 
then  Captain,  A.  W.  Greely,  who  continued  in  office  until  the  weather 
service  and  signal  service  were  separated.  This  separation  took  place 
July,  1891,  and  the  weather  service  was  then  made  a  separate  bureau 
and  placed  under  the  Department  of  Agriculture.  Mark  W.  Harring- 
ton, who  was  then  Professor  of  Astronomy  and  Director  of  the  Observa- 


WEATHER  BUREAUS  AND   THEIR  WORK  355 

tory  at  the  University  of  Michigan,  became  the  first  chief  of  the  U.  S. 
Weather  Bureau.  He  was  succeeded  July  4,  1895,  by  Willis  L.  Moore, 
who  is  at  present  the  able  head  of  the  service. 

The  Weather  Bureau  has  always  been  characterized  by  steady  ad- 
vance, development,  and  improvement,  along  both  scientific 
and  executive  lines.      In  1870  there  were  only  24  stations.  Weather 
In  1893  there  were  nearly  136  stations,  and  the  annual  cost 
was  about  $900,000.     At  present  there  are  nearly  200  sta-  IzeTby* 
tions,  and  the  annual  cost  is  about  $1,600,000.     It  is  often  steady  ad~ 
state.d,    even    by   foreign    scientists    and   weather    service 
officials,  that  the  U.  S.  Weather  Bureau  is,  in  many  respects,  a  model 
weather  service. 

THE  PRESENT  ORGANIZATION 

358.   The  central  station  of  the  U.  S.  Weather  Bureau  is,  of  course, 
located  at  Washington,  and  the  rest  of  the  country  is  divided  into  dis- 
tricts in  two  entirely  different  ways,  for  two  different  purposes.  The  ^i^ 
In  the  first  place,  there  are  twelve  climatological  districts,  ciimatoiogi- 
conforming  to  the  twelve  principal  drainage  areas  of  the  c 
United  States.     This  scheme  affords  the  best  system  of  territorial  units 
for  the  compilation  and  discussion  of  climatological  data.     These  twelve 
districts  are :    (1)  North  Atlantic  States,  (2)  South  Atlantic  and  East 
Gulf  States,  (3)  Ohio  Valley,  (4)  Lake  Region,  (5)  Upper  Mississippi 
Valley,  (6)   Missouri  Valley,  (7)   Lower  Mississippi  Valley,  (8)  Texas 
and  Rio  Grande  Valley,  (9)  Colorado  Valley,   (10)  Great  Basin,  (11) 
California,  (12)  Columbia  Valley. 

The  country  is  also  divided  into    six  (formerly  eight)  forecast  dis- 
tricts for  the  purpose  of  forecasting  the  weather  and  pre-  The  six 
dieting  storms.          •  forecast 

Prior  to  1909  there-were  twenty-one  climatic  subdivisions 
for  the  purpose  of  preparing  climatological  data  and  statistics.     The 
observations  were  summarized  for  each  of  these  districts  as  a  whole. 
These  were 

New  England  Lower  Lake  Southern  Slope 

Middle  Atlantic  States  Upper  Lake  Southern  Plateau 

South  Atlantic  States  North  Dakota  Middle  Plateau 

Florida  Peninsula  Upper  Mississippi  Valley          Northern  Plateau 

East  Gulf  States  Missouri  Valley  North  Pacific  Coast  Region 

West  Gulf  States  Northern  Slope  Middle  Pacific  Coast  Region 

Ohio  Valley  and  Tennessee       Middle  Slope  South  Pacific  Coast  Region 


356  METEOROLOGY 

The  country  was  also  divided  into  forty-five  sections  for  the  collection 
and  dissemination  of  weather  observations.  Each  state  was  usually 
The  former  a  section,  but  Maine,  New  Hampshire,  Vermont,  Massa- 
method  of  chusetts,  Connecticut,  and  Rhode  Island  were  grouped 
subdivision.  together  as  New  England,  and  Maryland  and  Delaware, 
and  Oklahoma  and  Indian  Territory,  were  considered  a  single  section  in 
each  case.  Alaska,  Hawaii,  and  Porto  Rico  were  included  in  the  forty- 
five.  Up  to  1909,  each  section,  with  the  exception  of  Alaska,  published 
a  monthly  climatological  report,  and  a  weather  bulletin  weekly  during 
the  summer.  At  present  the  section  practically  does  not  exist,  al- 
though in  the  supervision  of  substations  and  in  the  collection  of  obser- 
vations the  section  directors  still  perform  their  old  duties  within  their 
respective  stations. 

Scattered  throughout  the  country  are  Weather  Bureau  stations  of 

three  different   kinds.     There  are,   first,   the  regular  stations  of  the 

Weather  Bureau,  of  which  there  are  nearly  200.     There  are, 

kinds  of66       m  addition,   nearly  3000_jcooperative  stations,   and  about 

weather         500  special  stations^    Observations  are  also  received  from 

dons**  S1        many  stations  in  Canada,  Mexico,  and  the  West  Indies,  and 

from  some  stations  in  Europe,  Asia,  and  various  islands. 

The  accompanying  map  (Fig.  142)  shows  the  12  climatological  districts 
and  the  6  forecast  sections  into  which  the  country  is  divided.  The 
centers  of  the  12  districts  are  indicated  by  a  •  while  the  centers  of  the  6 
forecast  sections  are  underlined.  Only  about  half  of  the  regular  stations 
are  shown.  For  a  full  list  of  the  regular  stations,  see  Appendix  VII. 

359.  The  chief  of  the  U.  S.  Weather  Bureau  has  his  headquarters  at 
Washington,  and  closely  associated  with  him  in  executive  and  scientific 
The  organi-  WOI>k  are  the  assistant  chief,  the  chief  clerk,  the  professors, 
zation  at  the  heads  of  the  divisions  into  which  the  work  at  the  central 
station  is  divided,  and  the  inspectors.  The  assistant  chief 
acts  in  place  of  the  chief  when  he  so  desires  or  is  absent.  The  chief 
clerk  has  charge  of  the  correspondence  and  all  questions  of  personnel. 
The  professors,  of  whom  there  are  eight  at  present,  are  particularly 
well-trained  men  who  are  connected  with  the  scientific,  rather  than  the 
executive,  work.  The  ten  divisions  into  which  the  work  at  the  cen- 
tral station  at  Washington  is  divided  are :  forecast,  river  and  flood, 
instrument,  publications,  supplies,  telegraph,  accounts,  marine,  library, 
and  climatological,  and  each  is  presided  over  by  an  efficient  head.1  It 

1  The  number  of  divisions,  however,  is  a  matter  which  is  subject  to  frequent  change. 
Very  recently  (1911)  the  forecast,  marine,  and  river  and  flood  divisions  have  been  prac- 
tically consolidated  into  one,  the  division  of  observations  and  reports. 


WEATHER  BUREAUS  AND  THEIR  WORK 


357 


358  METEOROLOGY 

is  the  duty  of  the  inspectors  to  visit  the  regular  stations  from  time  to 
time,  both  to  make  investigations  in  case  anything  has  gone  wrong, 
and  also  to  make  suggestions  as  to  how  the  efficiency  and  usefulness  of 
the  station  may  be  increased. 

At  the  regular  Weather  Bureau  stations  outside  of  Washington  from 
one  to  fourteen  men  are  employed.  The  head  of  each  station  receives 
the  title  of  official  in  charge,  and  the  one  next  in  authority  to  him  is 
usually  called  the  first  assistant. 

At  the  central  station  at  Washington  about  200  are  employed,  and 
at  all  the  regular  stations  outside  of  Washington  about  500,  so  that  there 
Number  of  are,  all  told,  about  700  commissioned  employees  in  the  service 
employees.  of  the  Weather  Bureau.  In  addition,  nearly  100  display 
men  and  observers  receive  compensation  for  their  services.  The  salary 
of  the  chief  is  $6000,  and  that  of  the  assistant  chief  and  chief  clerk,  $3000 
each.  At  the  regular  weather  bureau  stations  outside  of  Washington 
the  salaries  run  from  $3500  down.  The  observers  at  cooperative  sta- 
tions receive  no  compensation  for  their  services,  except  the  publications 
of  the  bureau. 

THE  STATION  EQUIPMENT  AND  THE  OBSERVATIONS  TAKEN 

360.  The  central  station  of  the  U.  S.  Weather  Bureau  is  pictured  in 
Fig.  143,  which  is  used  as  the  frontispiece  of  this  book,  and  consists  of 
The  central  a  mam  building  and  several  adjoining  buildings  located  at 
station  at  24th  and  M  streets  in  Washington.  On  the  ground  floor  of 
the  main  building  are  located  the  rooms  for  the  weather 
forecasting  and  the  library.  The  library  now  contains  nearly  30,000 
volumes.  It  is  the  best  meteorological  library  in  this  country,  and 
one  of  the  best  in  the  world.  On  this  same  floor  are  located  the 
accounts,  telegraph,  and  river  and  flood  divisions.  On  the  second  floor 
are  located  the  rooms  for  the  marine  work,  and  the  offices  of  the  chief, 
assistant  chief,  and  chief  clerk.  The  instrument  division,  the  printing 
outfit,  the  climatological  division,  and  the  division  of  supplies  are  located 
in  the  adjoining  buildings.  The  observations  taken  and  the  instruments 
used  are  essentially  the  same  as  those  at  all  the  regular  stations  of  the 
Weather  Bureau.  There  are,  however,  several  special  instruments  in 
use  at  Washington. 

The  regular  stations  of  the  U.  S.  Weather  Bureau  are  usually  located 
in  the  larger  cities,  as  the  weather  maps  and  weather  forecasts  can  be 
more  quickly  distributed  and  reach  more  people.  From  three  to 


WEATHER  BUREAUS  AND  THEIR  WORK  359 

eight  or  ten  rooms  are  usually  occupied  by  a  Weather  Bureau  station. 
In  some  cases  the  Weather  Bureau  owns  the  building,  but  more  often 
the  rooms  are  simply  rented.  They  are  generally  located 
on  the  top  floor  of  some  high  office  building,  and  a  flat  mien*!!?!?" 
roof  must  also  be  available  for  exposing  the  instruments.  reguiarWea- 
In  New  York  the  rooms  were  on  the  20th  floor  of  100  %%££"** 
Broadway,1  and  the  instruments  were  on  the  23d  floor. 
In  Boston,  the  Weather  Bureau  is  located  in  the  main  tower  of  the 
post  office  building.  On  the  roof  will  be  found  a  thermometer  shelter, 
containing  a  maximum  and  a  minimum  thermometer,  wet  and  dry 
bulb  thermometers  which  can  be  whirled,  a  Richard  Freres  thermo- 
graph, and  sometimes  a  recording  hygrometer.  A  tipping  bucket 
rain  gauge,  a  Robinson  cup  anemometer,  an  electric  contact  sunshine 
recorder,  and  a  contact-making  wind  vane  will  also  be  found  suitably 
exposed.  If  the  station  has  but  three  rooms,  one  would  probably  con- 
tain the  desks  of  the  official  in  charge,  or  local  forecaster  official,  and 
his  first  assistant,  the  library,  and  perhaps  files  and  supplies.  A 
second  room  would  contain  the  apparatus  in  actual  use  and  additional 
apparatus  for  demonstration  purposes  and  to  replace  anything  which 
might  become  broken.  Here  would,  at  least,  be  found  the  "  triple 
register  "  for  recording  the  wind  velocity,  wind  direction,  sunshine, 
and  precipitation ;  a  Richard  Freres  barograph ;  and  two  good  mer- 
cury barometers.  The  third  room  would  be  a  work  room  for  prepar- 
ing the  weather  map  and  repairing  apparatus.  Here  would  be  found 
the  printing  presses  and  the  addressograph  for  addressing  the  daily 
weather  maps  and  other  publications.  At  the  section  centers  and  at 
stations  located  in  large  cities  more  room  is  necessary,  as  there  is  a 
larger  number  of  employees,  but  the  general  arrangement  is  the  same. 

At  a  cooperative  station,  only  a  maximum  and  minimum  thermometer 
in  a  thermometer  shelter  and  a  rain  gauge  are  necessary  so  that  the 
equipment    is    very   simple.     The    thermometer    shelter   is 
usually  located  in  the  open  over  sod,  and  some  five  or  six  ment  of  a 
feet  above  it.     It  may  be  fastened  to  the  north  side  of  some  cooperative 
unheated  building,  leaving  ample  space  for  ventilation  be- 
tween it  and  the  building.     The  rain  gauge  is  usually  located  in  the  open, 
a  few  feet  above  the  ground. 

361.   At  a  regular  station  of  the  U.  S.  Weather  Bureau  the  following 
observations  are  taken : 

1  May  1,  1911,  the  station  was  moved  to  the  Whitehall  Building,  17  Battery  Place. 
The  office  rooms  are  now  on  the  29th  floor  of  a  31-story  building. 


360 


METEOROLOGY 


Continuous 

Pressure  with  barograph.* 
Wind  direction! 
Wind  velocity  I  ( j^ecor(je(j  on  the  same  revolving  drum ;  instruments  in  the  open.) 

Precipitation    J 

Moisture  with  hygrometer,  f        Temperature  with  thermograph. f 

8  A.M.  and  8  P.M. 
Maximum  and  minimum  temperature. f      Pressure.* 


Temperature.! 

Moisture  (wet  bulb  thermometer).! 


Clouds  (upper  and  lower). 
Precipitation. 


Miscellaneous 
Fog,  frost,  hail,  thunder,  halo,  aurora,  sunset  and  sunrise  colors,  smoke,  haze. 

*  Instrument  in  the  office.  t  Instrument  in  the  thermometer  shelter. 

Form  No.  1083  —  Met'l. 

U.   S.    DEPARTMENT    OF    AGRICULTURE,    WEATHER    BUREAU. 

(Station} 

(Date) ,  19 

(75th  Meridian  Time.) 


8  A.M. 

8   P.M. 

Dry  thermometer                      .    . 

Wet  thermometer  

Maximum  thermometer  

Minimum  thermometer  

Precipitation.                          

{Upper  .                         

Lower  

Barometer  — 
Attached  thermometer  

Observed  reading  

Total  correction  

Station   

Reduced       

Compared  with  Form  No.  1001  — Met'l. 


NOTES: 


WEATHER  BUREAUS  AND   THEIR  WORK  .        361 

The  observations  at  8  A.M.  and  8  P.M.  are  taken  down  on  form  1083, 
which  is  here  reproduced.  The  data  on  this  form  and  values  obtained 
from  the  sheets  taken  from  the  recording  instruments  are 
copied  into  forms  1001  and  1014.  Form  1001  and  1083  to- 
gether  with  the  original  triple  register,  thermograph,  and 
barograph  sheets  are  sent  monthly  to  Washington.  The  tionand 
necessary  data  are  also  copied  into  the  "  Climatological 
Record,"  a  book  retained  by  the  local  station.  A  monthly 
summary  is  also  printed  by  each  regular  station.  A  pamphlet  entitled 
"  Instructions  for  Preparing  Meteorological  Forms  "  and  the  "  Station 
Regulations  of  the  Weather  Bureau  "  give  detailed  information  as  to 
the  taking  and  recording  of  the  observations.  The  Weather  Bureau 
also  publishes  a  large  number  of  blank  forms,  and  form  4024  is  a  check 
list  of  the  various  forms.  Samples  of  these  various  forms  may  be  se- 
cured from  the  regular  Weather  Bureau  stations  and  are  very  valuable 
illustrative  material  of  the  work  of  a  station. 

At  a  cooperative  station,  the  following  observations  are  required : 

Maximum  temperature.  Snow  on  ground. 

Minimum  temperature.  Duration  of  precipitation. 

Set  maximum.  Prevailing  wind. 

Precipitation.  Cloudiness. 

Snowfall.  Miscellaneous  phenomena. 

Only  a  monthly  report  is  required,  and  this  is  made  out  in  triplicate. 
One  copy  is  retained  by  the  observer  and  two  are  sent  to 
the   section   center.     One   of   these   is   eventually   sent    to 
Washington  for  filing.  taken  at  a 

At   a  special   station,  those  observations  are   taken  for  station? 

which  the  station  was  founded. 

*•• 

THE  DEVELOPMENT  OF  THE  DAILY  WEATHER  MAP 

362.   In  1780  the  Meteorological  Society  of  the  Palatinate,  with  its 
center  at  Manheim,  began  the  collection  and  publication  of  detailed 
meteorological    observations    for    Europe.     A    few    reports  The  first 
from  America  were  also  included  and  the  work  continued  weather 

map  was 

for  about  thirteen  years.     Based  upon  these  observations,  constructed 
the  first  synoptic   charts  or  weather  maps  were  made  by  by  Brandes- 
H.  W.  Brandes  in  1820.     He  constructed  them  for  the  whole  of  the 
year  1783,  and  described  the  results,  but  did  not   publish  the  maps. 
From  1821  until  his  death  in  1857,  William  Redfield   of    New  York 


362  METEOROLOGY 

collected  meteorological  observations  and  studied  storms  by  means  of 
weather  maps.  He  seems  to  be  the  first  one  in  this  country,  and 
Redfieid's  the  first  one  after  Brandes,  to  have  charted  simultaneous 
work-  observations  in  order  to  study  storms  and  atmospheric  move- 

ments and  changes.  In  this  country,  E.  Loomis  and  J.  Espy  also  made 
many  weather  maps  during  the  first  half  of  the  last  century.  In  Eng- 
land, Mr.  James  Glaisher  began  constructing  weather  maps  in  1848, 
and  during  the  London  World's  Fair  in  1851  the  first  weather  maps 
based  upon  observations  received  by  telegraph  were  produced.  After 
1851,  however,  the  work  was  discontinued.  In  1863  Leverrier  of  the 
The  first  ^aris  Observatory  began  the  publication  of  the  series  of 
regular  daily  weather  maps  for  Europe,  based  upon  telegraphic 
daily8  °  observations,  which  has  continued  unbroken  until  the  present 
weather  time.  The  series  of  daily  weather  maps  for  the  United  States 
was  begun  in  1871,  which  makes  the  United  States  the  fourth 
country  to  issue  regular  daily  maps,  as  the  Netherlands,  England,  and 
France  had  already  done  so.  At  first  the  maps  were  issued  in  this 
country  three  times  a  day,  at  7  A.M.,  2  P.M.,  and  9  P.M.  The  change 
was  made  from  three  maps  to  two  maps,  issued  8  A.M.  and  8  P.M., 
January  1,  1889.  Since  September  30,  1895,  only  one  daily  map,  the 
8  A.M.  map,  has  been  issued. 

In  1868  Leverrier  began  the  publication  of  a  daily  weather  map  for 
the  whole  globe,  but  the  work  was  discontinued  after  several  years. 

Weather  From  1875  to  1895>  the  U'  S'  Weather  Bureau  made  a 
maps  for  weather  map  for  the  whole  northern  hemisphere,  and  the 
maPs  were  published  for  the  first  ten  years  and  form  a  very 
valuable  collection.  This  map  is  still  made  in  manuscript 
at  Washington,  and  one  of  them  is  reproduced  as  Chart  L.  At  present, 
the  Deutsche  Seewarte,  at  Hamburg,  and  the  Danish  Meteorological 
Office,  at  Copenhagen,  publish  jointly  a  daily  map  of  the  North  Atlantic. 
This  together  with  the  maps  for  Europe  and  the  United  States  cover 
a  good  part  of  the  northern  hemisphere. 

THE    DAILY  WEATHER  SERVICE    OF    THE  U.  S.  WEATHER    BUREAU 

363.  The  taking  and  sending  of  the  observations.  —  At  all  the  regular 
The  obser-  stations  of  the  U.  S.  Weather  Bureau,  and  at  the  Canadian 
vations  are  and  Mexican  stations  as  well,  the  observations  are  taken  at 
eight!  East-  8  A>M-  an(*  $  P-M-  Eastern  Standard  Time,  that  is,  five  hours 
era  stand-  from  Greenwich.  This  means,  for  example,  that  they  are 
ard  une.  taken  in  the  morning  at  7  A.M.  in  Chicago,  at  6  A.M.  in 


WEATHER  BUREAUS  AND  THEIR  WORK  363 

Denver,  and  at  5  A.M.  in  San  Francisco.  In  fact,  the  observations 
are  commenced  a  little  before  eight,  so  that  the  work  may  be  finished 
and  the  telegram,  which  is  to  convey  them  to  other  stations,  prepared 
by  8  o'clock. 

The  observations  are  not  telegraphed  as  figures,  but  they  are  reduced 
by  means  of  an  elaborate,  yet  very  ingenious,  complete,  and  A  code  is 
satisfactory  code,  to  a  series  of  words.     According   to   the  ^k  trans 
code  book,  the  data  for  transmission  by  telegraph  may  con-  mission, 
sist  of  fifteen  words  and  they  are  to  be  enciphered  in  theVollowing  order:, 

1.  Name  of  station  (or  telegraphic  designation  therefor,  as  furnished  from 
the  central  office) . 

2.  Pressure  and  temperature  (corrected  readings) . 

3.  Precipitation. 

4.  Direction  of  wind,  state  of  weather,  and  maximum  temperature. 

5.  Current    wind    velocity    and  minimum  temperature,   or  current  wind 
velocity  and  maximum  temperature. 

6.  Minimum  or  maximum  temperature  (in  twenty-four  hours)  which  oc- 
curred more  than  twelve  hours  previous  to  observation. 

7.  Marked  rise  or  fall  of  pressure  (from  stations  specially  designated). 

8.  Report  on  the  river  observation  (from  stations  specially  designated  and 
in  certain  authorized  reports) . 

9.  Frost  (light,  heavy,  or  killing). 

10.  Thunderstorms. 

11.  Fog,  haze,  or  smoke. 

12.  Upper  clouds. 

13.  Lower  clouds  (when  sent). 

14.  Maximum  wind  velocity  and  direction  (in  accordance  with  special  in- 
structions) . 

15.  Special  monthly  or  weekly  reports. 

The  code  is  of  such  a  character  that  one  familiar  with  it  is  able  to  trans- 
late it  without  using  a  code  book  or  memorizing  words.  The  figures  are 
conveyed,  in  general,  by  the  first  vowel  and  the  consonant  which  pre- 
cedes it.  The  following  illustrations  of  the  second  word  in  the  telegram 
will  make  this  clear: 

PRESSURE  TEMPERATURE  WORD 

30.24  50  Demulsion 

30.24  58  De  mo  crat 

30.24  88  Desolate 

30.54  50  Mel  my 

30.54  58  Memory 

30.54  88  Me  so  type 

30.56  88  Misogamy 


364  METEOROLOGY 

Not  all  of  the1  fifteen  words  are  usually  sent.  Numbers  1,  2,  3,  4,  5,  14, 
are  nearly  always  sent;  number  6,  9,  10,  are  generally  sent;  and  the 
others  only  occasionally. 

The  purpose  of  using  a  code  instead  of  figures  is  primarily  to  secure 
brevity  and  thus  save  expense.  As  it  is,  the  telegraphic  tolls  of  the 
The  purpose  Weather  Bureau  amount  to  more  than  $100,000  annually 
of  the  code.  ancj  constitute  one  of  the  largest  bills  of  the  bureau.  The 
use  of  a  code  also  prevents  the  observations  from  being  tampered  with 
or  read  in  transit,  and  at  the  same  time  greater  accuracy  is  secured, 
as  words  are  always  transmitted  with  fewer  mistakes  than  figures. 

The  weather  telegram  containing  from  six  to  fifteen  words  is  thus 
filed  at  the  local  telegraph  office  by  each  regular  station  at  eight  o'clock. 
HOW  the  These  messages  have  precedence  over  all  others,  so  that 
telegrams  usually  before  9.30  o'clock  Washington,  New  York,  and 
other  large  stations  are  in  possession  of  the  observations 
made  at  all  regular  stations,  and  all  stations  which  produce  weather  maps 
have  received  a  sufficient  number  of  reports  to  construct  an  accurate 
weather  map.  This  exchange  of  telegrams  is  accomplished  by  a  circuit 
system.  The  telegrams  are  not  sent  individually  from  each  station  to 
every  other  station,  but  are  collected  at  some  center  and  then  sent  as  a 
unit  through  a  series  of  stations  connected  up  as  a  continuous  circuit.  It 
is  not  customary  to  wait  until  all  the  telegrams  have  been  collected  and 
then  form  the  circuit.  As  soon  as  a  sufficient  number  has  been  collected, 
they  are  sent  around  the  circuit.  At  map-producing  stations,  the  ob- 
servations thus  ordinarily  came  in  in  three  or  four  installments.  The 
last  ones  usually  get  in  before  10  o'clock.  The  system  of  circuits  is 
changed  from  time  to  time,  and  is  worked  out  with  the  greatest  care, 
so  as  to  make  the  amount  of  telegraphing,  and  thus  the  expense,  a 
minimum.  At  both  8  in  the  morning  and  8  at  night,  the  observations 
are  sent  out  as  just  described. 

364.  The  charting  of  the  observations.  —  The  central  station  at 
Washington  and  many  of  the  regular  stations  of  the  U.  S.  Weather 
Bureau  issue  daily  weather  maps  which  give,  both  in  the  form  of  a  chart 
and  in  tables,  the  observations  taken  at  8  in  the  morning.  A  weather 
map,  based  upon  the  8  P.M.  observations,  is  prepared  in  manuscript  at 
Washington  and  a  few  other  stations,  but  it  is  never  published  except 
by  the  newspapers. 

The  weather  map  (form  DD)  was  used,  prior  to  1910,  at  nearly  all  of 
the  map-producing  stations,  and  is  still  used  at  a  few  of  them.  It  is 
16  X  11  inches  and  contains  an  outline  map  of  the  United  States 


WEATHER  BUREAUS  AND   THEIR  WORK  365 

10  X  6^  inches.     This  outline  map  is  printed  in  brown  and  contains 
the  names  of  the  regular  stations.     On  this  map  are  charted  the  pres- 
sure, temperature,  wind  direction,  and  state  of  the  weather.  How  the 
The  pressure  is  indicated  by  a  series  of  solid  lines  called  iso-  observations 
bars,  drawn  through  places  having  the  same  pressure.      The  f^the 'daily 
lines  are  drawn  for  every  tenth  of  an  inch  difference  in  pressure  weather 
and  the  pressure  is  indicated  at  each  end  of  the  line.     These  map* 
lines  inclose  areas  of  high  and  low  pressure,  and  the  areas  of  highest  and 
lowest  pressure  are  marked  with  the  words  "  High  "  and  "  Low."     These 
are  the  extratropical  cyclones  and  anticyclones  which  have  been  studied 
in  a  previous  chapter.     Dotted  lines,  called  isotherms,  connect  places 
having  the  same  temperature.     These  lines  are  drawn  for  every  ten 
degrees,  and  the  temperature  of  each  line  is  indicated  at  both  ends  of  it. 
The  wind  direction  is  indicated  by  an  arrow  which  flies  with  the  wind. 
The  state  of  the  weather  is  indicated  by  a  set  of  symbols.     O  indicates 
a  clear  sky,  3  partly  cloudy,  •  cloudy,  ®  indicates  rain,  ©  snow, 
®  that  the  report  is  missing.     /?  indicates  that  a  thundershower  occurred 
at  the  station  during  the  preceding  twelve  hours.     These  explanations 
are  stated  briefly  in  the  "  Explanatory  Notes  "  which  are  printed  in  one 
corner  of  the  map  and  are  here  reproduced. 

EXPLANATORY  NOTES 

Observations  taken  at  8  A.M.,  seventy-fifth  meridian  time.  Air  pressure 
reduced  to  sea  level. 

ISOBARS,  or  continuous  lines,  pass  through  points  of  equal  air  pressure. 

ISOTHERMS,  or  dotted  lines,  pass  through  points  of  equal  temperature. 

SYMBOLS  indicate  state  of  weather :  O  clear ;  0  partly  cloudy ;  ©  cloudy ; 
R  rain ;  S  snow ;  M  report  missing ;  f&  thunderstorm.  ARROWS  fly  with  the 
wind.  i 

Shaded  areas  when  used  show  regions  of  precipitation  during  past  24  hours. 

"T"  in  table,  indicates  amount  too  small  to  measure. 

The  various  weather  maps  which  have  been  reproduced  in  the  chapter 
on  storms  will  illustrate  the  method  of  charting  the  observations. 
Further  information  is  also  conveyed  by  means  of  statistics  in  tabular 
form  in  the  lower  right-hand  part  of  the  map.  Here  are  given  tempera- 
ture at  8  A.M.,  75th  meridian  time,  temperature  change  in  24  hours, 
maximum  temperature  in  last  24  hours,  minimum  temperature  in  last 
12  hours,  wind  velocity  8  A.M.,  precipitation  in  last  24  hours.  On  the 
lower  left-hand  side  of  the  map  are  given  the  weather  predictions  for 


366  METEOROLOGY 

the  particular  region  where  the  station  is  located  and  a  general  discus- 
sion of  the  weather  conditions. 

Since  1910  most  of  the  map-producing  stations  issue  the  so-called 

"  commercial  weather  map,"  which  is  just  the  same  as  the  maps  which 

are  published  in  the  newspapers.     The  size  is  the  same  as 

commercial     before,  only  the  color  is  blue  instead  of  brown.     The  isobars, 

weather         ^ne  highs  and  lows,  the  method  of  indicating  the  direction 

filflP. 

of  the  wind,  and  the  method  of  indicating  the  state  of  the 
weather  are  the  same  as  before.  Only  four  isotherms  are  drawn;  for 
zero,  freezing,  90°  F.,  and  100°  F.  In  addition,  the  temperature  at 
8  A.M.,  the  amount  of  the  precipitation,  and  the  wind  velocity  are 
printed  on  the  face  of  the  map  near  each  station.  This,  of  course,  ne- 
cessitates a  change  in  the  statistical  material  given  in  tabular  form. 
The  following  legend  is  placed  on  these  maps  and  makes  the  slight 
differences  clear: 

Observations  taken  at  8  A.M.,  seventy-fifth  meridian  time. 

ISOBARS,  or  continuous  lines,  pass  through  points  of  equal  air  pressure. 

ISOTHERMS,  or  dotted  lines,  pass  through  point  of  equal  temperature;  they 
will  be  drawn  only  for  zero,  freezing,  90°,  and  100°. 

SYMBOLS  indicate  state  of  weather :  Q  clear ;  <J  partly  cloudy ;  •  cloudy  ; 
(R)  rain ;  ©  snow ;  (g)  report  missing. 

Arrows  fly  with  the  wind.  First  figure,  temperature ;  second,  24-hour  rainfall, 
if  it  equals  .01  inch ;  third,  wind  velocity  of  10  miles  per  hour  or  more. 

Many  newspapers  now  publish  a  map  of  this  kind,  together  with  sta- 
tistical material  in  tabular  form  and  the  predictions.  In  the  large  cities 
it  is  nearly  always  possible  to  get  an  evening  paper  which  contains  the 
morning  map  and  often  a  morning  paper  which  contains  the  evening 
map. 

365.  The  weather  map  (form  C)  published  at  Washington,  is  some- 
what larger  (24  X  16  inches)  and  contains  more  information.  The 
The  Wash-  outline  map  is  here  printed  in  blue  and  the  isotherms  are 
ington  map.  continuous  lines  and  are  printed  in  red.  On  these  maps 
three  additional  things  are  charted:  the  area  where  precipitation  has 
fallen,  areas  of  marked  rise  or  fall  in  temperature,  and  the  path  of  the 
"  lows."  The  area  where  precipitation  has  fallen  is  indicated  by  shad- 
ing. Areas  where  the  temperature  has  risen  or  fallen  20°  are  inclosed 
by  red  dots.  The  path  of  a  prominent  low  is  indicated  by  a  series  of 
arrows.  In  the  tables,  two  additional  columns  containing  the  pressure 
reduced  to  sea  level  and  standard  gravity,  and  the  change  in  pressure 


WEATHER  BUREAUS  AND  THEIR  WORK  367 

in  twelve  hours,  are  given.  The  predictions  issued  at  Washington 
and  at  the  various  forecast  centers,  viz.,  Chicago,  New  Orleans,  Denver, 
Portland,  Ore.,  and  San  Francisco  —  cover  the  whole  country. 

At  Washington,  about  1500  copies  of  the  weather  map  are  issued 
daily.  At  other  map-producing  stations,  from  a  few  hundred  to  3000 
maps  are  distributed  daily.  The  newspapers,  of  course,  make  the 
weather  map  widely  accessible. 

366.  The  construction  of  the  weather  map.  —  The  construction  of 
the  daily  weather  map  at  Washington  is  an  interesting  operation,  and 
usually  occupies  about  two  hours,  from  8.30  to  10.30  in  the 
morning.  As  soon  as  the  weather  telegrams  are  received 
they  are  sent  at  once  to  the  rooms  of  the  forecast  division,  is  con- 
Here  some  one  familiar  with  the  code  translates  the  messages  Washington, 
into  figures  and  reads  them  aloud.  As  the  telegrams  are 
being  read,  one  clerk  on  an  outline  map  of  the  United  States  and  ad- 
joining countries  constructs  a  chart  showing  the  change  which  has 
taken  place  in  the  temperature  during  the  past  twenty-four  hours.  A 
second  clerk  constructs  a  chart  showing  the  change  in  the  barometric 
pressure  during  the  past  twenty-four  hours.  A  third  clerk  constructs 
two  charts,  one  showing  the  humidity  and  the  other  showing  the 
amount,  kind,  and  direction  of  motion  of  the  clouds.  The  forecast 
official  himself,  or  some  one  under  his  direct  supervision,  constructs  a 
fourth  chart,  which  may  be  called  the  general  weather  chart,  and  is 
later  published  as  part  of  the  weather  map.  This  shows  the  pressure, 
temperature,  direction  and  velocity  of  the  wind,  the  rain  or  snow  which 
has  fallen,  and  the  state  of  the  weather  for  each  station.  As  soon  as  the 
weather  telegrams  have  all  been  received  and  read,  and  the  observations 
have  been  recorded,  the  isobars  and  isotherms  are  drawn  in,  the  areas 
of  high  and  low  pressure  are  so  marked,  the  direction  of  motion  of  the 
lows  is  indicated,  the  areas  where  precipitation  has  fallen  are  shaded, 
and  the  areas  where  the  temperature  has  risen  or  fallen  twenty  degrees 
are  inclosed.  The  map  is  then  hurried  to  the  lithographers  to  be  put 
upon  the  stone.  In  the  meantime,  a  force  of  typesetters  have  set  up, 
from  the  dictation,  the  material  for  the  tabular  form.  As  soon  as  the 
map  is  ready  to  be  lithographed,  the  forecast  official  immediately  begins 
to  dictate  his  forecasts  for  the  various  parts  of  the  country.  These  are 
immediately  set  up  in  type,  and  shortly  after  he  has  finished  the  proof  of 
the  chart,  tables,  and  forecasts  is  ready.  The  completed  weather  map 
is  then  printed  as  rapidly  as  possible,  and  the  whole  operation  does  not 
occupy  much  more  than  two  hours. 


368  METEOROLOGY 

At  the  regular  stations  of  the  U.  S.  Weather  Bureau,  where  daily 
weather  maps  are  produced,  very  much  the  same  thing  takes  place, 
The  chalk  usually  on  a  smaller  scale,  with  the  exception  that  the  chart 
plate  pro-  part  of  the  map  is  not  lithographed,  but  is  produced  by 

the  chalk  plate  process.  A  plate  of  steel  the  size  of  the 
chart  is  covered  with  a  layer  of  chalk  about  one  eighth  of  an  inch  thick. 
In  this  is  cut,  by  means  of  suitable  instruments,  the  various  symbols 
and  lines  for  indicating  the  various  things  which  are  depicted  on  the 
chart.  This  chalk  plate  is  then  locked  in  an  iron  frame  and  stereotyped 
in  the  usual  way  by  pouring  in  molten  type  metal.  The  plate  contain- 
ing the  impression  is  then  trimmed,  any  imperfections  are  remedied, 
and  necessary  corrections  are  made.  The  tables  and  forecasts  are  set 
up  in  type  as  before,  and  the  map  is  printed  in  the  regular  way.  At 
these  stations,  only  one  chart,  besides  the  one  which  is  reproduced,  is 
usually  constructed  as  the  observations  come  in. 

At  a  few  regular  stations  of  the  U.  S.  Weather  Bureau,  the  weather 
map  (chart,  tables,  and  forecasts)  is  still  mimeographed.  In  this  case, 

the  symbols  for  the  chart  are  usually  cut  into  the  waxed 

The  mimeo-  J 

graph  is         sheet  by  a  specially  constructed  typewriter.     The  isobars 
sometimes      an(j  isotherms  are  drawn  in  free  hand.     These  maps  are  just 

as  useful,  but  are  not  as  neat  in  appearance,  as  the  lithographed 
maps  or  those  prepared  by  the  chalk  plate  process. 

367.  The  distribution  of  the  map.  —  In  certain  cities,  a  few  weather 
maps  may  be  carried  by  special  messengers  to  particular  places,  but 

these  may  be  left  out  of  consideration,  and  it  may  be  said 

The  oistri~ 

butionofthe  that   they    are   practically    all    distributed    by   mail.     The 
weather         weather  maps  are  folded  and  placed  in  wrappers  which  have 
been  addressed  the  day  before,  by  means  of  an  addresso- 
graph.     The  outside  of  the  wrapper  carries  this  inscription: 

IMMEDIATE  — U.  S.  Weather  Report.   U.  S.  Department  of 

By  authority  of  the  Post  Office  Department,  Agriculture, 

June  18,  1881,  this  Report  will  be  Wpathpr  Rnrpan 

treated  in  all  respects  as  letter  mail.  Weather  Bureau. 

OFFICIAL  BUSINESS 


Penalty  for  Private  Use  $300.00 

This  insures  that,  anywhere  within  a  hundred  miles  of  a  map-producing 
station,  the  daily  weather  map  will  be  received  in  the  middle  or  later 
part  of  the  afternoon  at  the  latest. 


WEATHER  BUREAUS  AND  THEIR  WORK  369 

368.    Other  methods  of  distributing  the  maps,  forecasts,  data,  and 
warnings.  —  In   addition   to   issuing   daily   weather   maps, 

there  are  several  other  methods  used  by  the  U.  S.  Weather  tionai  way"s 

Bureau  in  distributing  its  meteorological  information,  fore-  of  distribut- 

.  .  „,  ,  ,.  , ,  ing  fore- 

Casts,  and  warnings.     These  are  by  means  of  the  newspapers,  casts< 

flags,  cards,  the  telephone,  and  special  messages. 

The  agents  of  the  various  press  associations,  and  the  reporters  for  the 
various  newspapers  visit  the  Weather  Bureau  stations  as  soon  as  the 
observations  have  been  received  and  charted.  They  are  The  work  of 
furnished  with  the  weather  maps,  forecasts,  and  all  the  data  the  news- 
which  they  care  to  use.  Nearly  all  the  daily  papers  print  p 
the  weather  forecast  at  the  top  of  the  first  page,  and  then  devote  more 
space  on  some  inside  page  to  weather  data  in  tabular  form,  a  general 
discussion  of  the  weather  conditions,  and  a  more  detailed  description 
of  the  impending  weather  changes.  The  probable  coming  of  any 
damage-causing  storm  or  weather  change  usually  receives  a  special 
write-up.  Since  1910  many  newspapers  are  publishing  the  weather 
map  complete,  i.e.  the  chart,  the  statistics  in  tabular  form,  a  dis- 
cussion of  the  weather  conditions,  and  the  forecast.  These  news- 
papers are  to  be  particularly  commended  in  this  respect,  for  they 
make  accessible  to  a  lar'ge  number  of  readers  the  full  meteorologi- 
cal material  in  complete  and  definite  scientific  form.  Whenever 
a  newspaper  undertakes  this  work,  the  nearest  Weather  Bureau 
station  usually  furnishes  the  stereotyped  plate  from  which  the  chart 
is  printed. 

Flags  are  also  used,  particularly  along  the  seacoast  and  in  the  lake 
regions,  to  make  known  the  weather  forecasts,  and  to  an-  The  various 
nounce  the  coming  of  storms.     The  various  flags  used  at  flags  and 
the  U.  S.  Weather  Bureau  display  stations  and  their  expla-  £jjr  mean- 
nation  are  here  given. 

The  weather  forecasts  are  also  printed  on  small  cards  and  these  receive 
a  wide  distribution,  particularly  through  the  mails.     About  25,000,000 
of  these  cards  are  distributed  annually.     They  are  placed  in  The  card 
small  metal  frames  and  exposed  in  public  places,  particularly  se™e- 
in  post  offices.     If  there  are  several  rural  free  delivery  lines  going  out 
from  any  post  office,  the  forecasts  will  be  telegraphed  from  the  nearest 
map-producing  station  of  the  Weather  Bureau  to  the  post  office  at  about 
10.30  in  the  morning.     The  forecast  is  then  stamped  by   means  of 
rubber  stamps  on  these  cards,  which  are  exposed  in  public  places  and 
also  distributed  by  the  rural  mail  carriers. 
2u 


370 


METEOROLOGY 


•••••ft 


RAIN   OR   SNOW 


LOCAL 
RAIN  OR  SNOW 


COI-D   WAVE. 


INFORMATION 


N.  £     WINDS 


S.   W.  WIN 


EXPLANATION   OF  THE   SIGNAL   FLAGS 
USED  AT 

WEATHER    BUREAU    DISPLAY   STATIONS. 

FAIR  SIGNAL.    The  White  Flag  alone  indicates  fair  weather,  stationary  temperature, 

RAIN  OR  SNOW  SIGNAL.    The  Bine  Flag  alone  indicates  rain  or  snow,  stationary  temperature. 

LOCAL  RAIN  OR  SNOW  SIGNAL     The  White  and  Blue  Flag  indicates  local  rain  or  snow,  station- 
ary temperature. 

COLD  WAVE  SIGNAL.    The  White  Flag  with  Black  Center  indicates  sudden  fall  in  temperature, 
usually  accompanied  by  stormy  weather. 

TEMPERATURE;  SIGNAL.    The  Black  Pennant  above  indicates  warmer  wteat her  of  th 
seuted  by  the  lower  flag;  below,  cooler  weather  of  the  type  represented  by  th 


INFORMATION  SIGNAL.    The  Red  Pennant  ind 

STORM  SIGNAL.    The  lied  Flag  with  Black  Ce 

expected. 
HURRICANE  SIGNAL.    Two  Storm  Signals  one 

a  tropical  hurricane  or  a  severe  and  dan« 
WIND  SIGNAL.     1'eimant.s  with  the.  Storm  Sitcn; 


cates  that  the 


ter  indicates  that  a  st 


ifher  indi 


indie 


1  dlsplayman  has  ret 

ve.ssHs  hound  for  eei 
!'  marked 


from  northeast  to  south;  white,  westerly,  from 
flag  indicates  r|1;tt  the  wind  is  expected  to  blow 
the  southerly  quadrants. 


By  night  a  Red  Light  indicates  Easterly  Winds,  and  a  White  Light  below  a  Red  Light 
Westerly  Winds.     No  night  Hurricane  Warnings  are  displayed. 


WEATHER  BUREAUS  AND  THEIR  WORK  371 

If  a  particularly  violent  or  damage-causing  storm  is  expected,  special 
warnings  are  often  sent  out  by  messengers  to  the  captains  of  all  vessels 
in  a  harbor,  and  to  those  who  will  be  particularly  inter-  special 
ested  in,  and  affected  by,  the  storm.  messages. 

369.  The  weather  service  in  the  evening  and  on  Sundays  and  holi- 
days. —  The  observations  taken  at  8  in  the  evening  are  telegraphed  to 
Washington    and  other  selected    stations   of  the  Weather  The  evening 
Bureau  in  the  regular  way.     At  these  stations,  a  manuscript  service- 
weather  map  is  made  and  the  forecasts,  data,  and  warnings  are  sent  out, 
but  no  weather  map  is  published,  except  by  some  newspaper. 

On  Sundays  and  holidays,  the  observations  are  taken  and  sent  by 
telegraph  in  the  usual  way.     At  Washington,  the  manuscript  weather 
maps  are  made,  but  the  one  for  Sunday  morning  is  not 
published  until  Monday  afternoon,  and  the  morning  map  On  Sundays 
for  a  holiday  is  published  a  day  or  two  later.     At  the  other  *nd  holi" 
map-producing  stations  of  the  Weather  Bureau  a  manuscript 
may  is  sometimes  constructed  for  the  Sundays  and  holidays,  but  they 
are  never  published. 

OTHER  WORK  AND  PUBLICATIONS  OF  THE  U.  S.  WEATHER  BUREAU 

370.  The  routine  work  at  a  regular  station  of  the  U.  S.  Weather 
Bureau  consumes   most  of  the  time  of  the  officials.     The  apparatus 
must  be  kept  in  good  working  condition.     The  observations 

must  be  taken  at  the  appointed  times,   and  the  weather  work  of  the 
telegrams  prepared  for  transmission.     The  charts   on  the  Weather 
revolving  drums  of  the  automatically  recording  instruments 
must  be  changed  at  stated  intervals.     The  observations  must  be  entered 
in  appropriate  forms  for  transmission  to  Washington  and  in  the  records 
which  are  retained  at  the  station.     If  it  is  a  map-producing  station,  all 
the  work  incident  to  the  construction  and  distribution  of  weather  maps 
must  be  performed.     The  forecasts,  data,  and  warnings  must  be  sent 
out.     The  monthly  report  must  be  prepared.     All  this  leaves  but  little 
time  for  other  work. 

Considerable  instructional  work  is  also  done  by  the  officials  at  Weather 
Bureau  stations.     If  the  station  is  located  in  a  city  where  there  is  a 
college  or  university,  it  is  often  the  local  forecast  official  who  Educational 
gives  the  course  or  courses  in  meteorology  in  the  institution,  and  research 
Many  public  lectures  are  also  given,  and  frequently  classes  v 
from  various  schools  visit  the  station  in  order  that  the  equipment  and 
work  may  be  explained  to  them. 


372  METEOROLOGY 

Research  work  is  also  encouraged  on  the  part  of  the  Weather  Bureau 
officials.  Very  little,  however,  is  done,  and  the  chief  reason  is  lack  of 
time. 

371.  The  publications  issued  by  the  U.  S.  Weather  Bureau,  in  addi- 
tion to  the  daily  weather  maps  and  forecast  cards,  may  be 
cations  of  divided  into  two  groups,  the  periodical  publications  and 
the  Weather  those  which  appear  at  irregular  intervals.  The  periodical 

Bureau. 

publications  are : 

Monthly  Weather  Review,  4°,  1872  to  date. 

Bulletin  of  the  Mount  Weather  Observatory,  8°,  1908  to  date. 

National  Weather  Bulletin  (weekly  during  the  summer,  monthly  during  the 

winter) . 

Snow  and  Ice  Bulletin  (weekly  during  the  winter). 
Meteorological  Charts  of  the  Great  Lakes. 
Annual  Report  of  the  Chief. 

.9 

The  publications  which  appear  from  time  to  time  are : 

Numbered  Bulletins  (nearly  40  have  now  appeared). 
Lettered  Bulletins  (Bulletin  V  has  recently  appeared). 
W.  B.  Publications  (these  include  many  of  the  others). 
Miscellaneous  Publications. 

Summary  of  the  Climatological  Data  for  the  United  States  by  Sections  (106 
are  to  be  issued). 

Prior  to  1909,  forty-four  section  centers  published  a  weekly  weather 
bulletin  during  the  summer  and  a  monthly  climatological  report,  but 
these  were  then  discontinued.  Each  regular  Weather  Bureau  station 
publishes  a  monthly  summary  of  a  single  page.  For  further  details  in 
connection  with  all  these  publications,  see  Appendix  IX. 

The  publications  of  the  U.  S.  Weather  Bureau  are  distributed  with  a 
free  hand.  The  daily  weather  maps  are  sent  free  of  charge  to  schools, 
The  distri-  Pos^  °ffices>  factories,  large  office  buildings,  in  fact,  where- 
butionofthe  ever  they  will  be  exposed  so  that  the  information  which 
they  contain  may  reach  quickly  a  large  number  of  people. 
The  other  publications  of  the  Weather  Bureau  are  sent  free  of  charge 
to  libraries  and  institutions  of  learning.  They  may  all  be  obtained  by 
any  one  on  payment  of  prices  which  are  not  excessive. 


WEATHER  BUREAUS  AND  THEIR  WORK  373 

THE  WEATHER  BUREAUS  OF  OTHER  COUNTRIES 

372.   Nearly  all  the  large  civilized  nations  of  the  world  now  maintain 
a  national  weather  service  and  publish  daily  weather  maps  The  coun_ 
and  forecasts.     This  is  true  of  all   the  larger   countries   of  tries  which 
Europe,  Japan,  China,  Canada,  India,  Australia,  New  Zealand,  ^01^  * 
Argentine  Republic,  and  Algeria.  weather 

The  construction  and  appearance  of  the  daily  weather 
map  is  much  the  same  in  all  countries.     In  the  details,  however,  there 
is  considerable  difference.     In  all  cases,  the  observations  are  taken  at 
the  same  time  at  many  stations.     These  are  transmitted  by 

The  weather 

telegraph,  using  a  code,  to  one  or  more  central  stations  where  maps  of 
the  daily  weather  maps  are  produced.     The  weather  map  other  . 

j.i-  L*  i_  •       i.   ,       j  £   countries. 

always  contains  an  outline  map  on  which  is  charted  some  01 
the  observational  material.  More  of  it  is  presented  in  tabular  forms 
and  forecasts  and  a  general  discussion  of  the  weather  conditions  is 
always  included.  The  observations  are  taken  at  different  hours  in 
different  countries,  but  always  in  the  early  morning.  The  code  used 
generally  consists  of  a  series  of  numbers,  and  does  not  make  use  of  words. 
Wind  velocity  is  often  charted  on  the  map  as  well  as  wind  direction. 
This  is  usually  done  by  placing  a  different  number  of  barbs  on  the  arrows. 
Sometimes  temperature  and  pressure  are  charted  on  separate  maps. 
Although  the  details  may  be  different,  one  familiar  with  weather  maps 
would  find  no  difficulty  in  interpreting  the  weather  map  of  any  other 
country.  The  weather  maps  of  Great  Britain,  France,  Germany,  Japan, 
and  India  are  particularly  interesting.  Samples  of  the  weather  maps 
issued  by  the  various  countries  may  occasionally  be  secured  from  the 
U.  S.  Weather  Bureau  at  Washington,  by  any  one  who  will  make  good 
use  of  the  illustrative  material. 

The  English  weather  service  is  managed  by  the  Meteorological 
Office  at  South  Kensington,  London,  S.  W.  There  are  upwards  of  200 
stations  scattered  over  the  British  Isles  and  divided  into  The  English 
five  classes.  In  addition,  the  Royal  Meteorological  Society  weather 
and  the  Scottish  Meteorological  Society  have  covered  the 
country  with  a  network  of  climatological  and  phenological  stations. 
The  weather  map  is  issued  daily,  and  is  based  on  the  observations 
received  by  telegraph  from  twenty-nine  home  and  forty-four  foreign 
stations.  The  home  observations  are  taken  at  7  A.M.  and  6  P.M. 
West  European  time.  The  foreign  observations  are  all  taken  in  the 
early  morning,  but  not  at  the  same  time.  The  telegram  which  conveys 


374  METEOROLOGY 

the  observations  consists  of  six  numbers  of  five  figures  each.  The 
daily  weather  report  consists  of  four  quarto  pages.  Pages  one  and  four 
contain  data  in  tabular  form,  while  pages  two  and  three  contain  charts 
of  the  various  meteorological  elements.  The  Meteorological  Office 
also  issues  a  weekly  and  a  monthly  weather  report. 

The  central  office  of  the  weather  service  for  the  Dominion  of  Canada  is 
located  at  Toronto.  Here  are  received  twice  daily,  by  telegraph,  the 

observations  from  forty-three  Canadian  stations  and 
dian  sixty-four  stations  in  the  United  States.  From  these,  the 

weather         daily   weather   forecasts    and    bulletins    are    prepared    and 

distributed  from  various  centers. 

The  preparation  of  the  daily  weather  map,  particularly  in  Europe, 
requires  international  cooperation.  This  has  lead  to  International 
Inter_  Meteorological  Congresses  and  to  uniformity  in  nomen- 

nationai  co-    clature,  time  of  observation,  and  the  like.     The  adoption  of 

the  international  system  of  cloud  classification  in  1891  is 
an  example  of  this.  There  is  also  a  set  of  international  meteorological 
symbols  for  the  various  things  observed.1  It  has  also  been  proposed 
to  have  a  set  of  international  storm  warnings.  The  system  shown  in 
Fig.  144  will  probably  be  universally  adopted. 


Day  warnings 


International  Storm  Warnings. 

Cones)/ 
/m.       For  a  gale  commencing  with  wind  in  the  NW.  quadrant. 

^J  For  a  gale  commencing  with  wind  in  the  SW.  quadrant. 
j^1  For  a  gale  commencing  with  wind  in  the  NE.  quadrant. 
^T  For  a  gale  commencing  with  wind  in  the  SE.  quadrant. 

££      For  a  hurricane. 

FIG.  144.  —  International  Storm  Warnings. 
»  See  Monthly  Weather  Review,  December,  1905,  p.  524. 


WEATHER    BUREAUS  AND   THEIR  WORK  375 

SOME   SPECIAL    METEOROLOGICAL  OBSERVATORIES;    THEIR  EQUIP- 
MENT AND  WORK 

373.   The  two  most  important  meteorological  observatories  in  this 
country  which  are  not  directly  connected  with  the  routine  work  of  a 
Weather  Bureau  station   are  the  Blue  Hill  Meteorological  Blue  Hill 
Observatory  near  Boston,  and  the  Mount  Weather  Research  and  Mount 
Observatory   at   Mount   Weather,  in   Virginia.     The   Blue  Weather- 
Hill  Observatory  is  a  private  observatory,  and  was  founded  by  Mr. 
A.  L.  Rotch  in  1885.     A  few  years  later  it  became  associated  with  the 
Astronomical  Observatory  of  Harvard   College   and  its  records   and 
results  are  printed  in  the  Annals  of  that  observatory.     Particularly 
noteworthy  has  been  the  work  of  this  observatory  in  the  use  of  kites  to 
explore  the  upper  air  and  in  cloud  observations.     The  Mount  Weather 
Research  Observatory  was  founded  by  the  U.  S.  Weather  Bureau,  and 
most  of  the  time  is  to  be  devoted  to  balloon  and  kite  work,  a  study  of 
the  solar  heat,  and  the  electrical  condition  of  the  atmosphere. 

Other  observatories,  some  of  them  connected  with  institutions  of 
learning,  are  making  valuable  series  of  observations  and  publishing 
results,  but  they  are  undertaking  no  elaborate  pieces  of  research  work. 
Among  these  may  be  mentioned  the  Experiment  Station  of  the  Massa- 
chusetts Agricultural  College  at  Amjierst,  Mass.  The  two  most  famous 
mountain  observatories  in  this  country  were  those  on  Pike's  Peak  in 
Colorado,  and  on  Mt.  Washington  in  New  Hampshire.  Both  have 
now  been  discontinued,  but  valuable  observations  were  secured. 

In  Europe,  the  most  famous  meteorological  observatories  are,  perhaps, 
those  at  Pottsdam  and  Lindenburg  near  Berlin ;  Pavlovsk 
near  St.  Petersburg;  Trappes,  Mont  Souris,  and  Pare  St.   meteoroiogi- 
Maur  in  France ;  and    Kew   and   Greenwich   in    England,   cai  obser- 
The  most    famous    mountain    observatories    are    probably  of  Europe. 
Sonnblick  and  Hoch  Obir  in  Austria,  Santis  in  Switzerland, 
Pic  du  Midi  and  Puy  de  Dome  in  France,  Wendelstein  and  Brocken 
in  Germany,  Etna  in  Italy,  and  Ben  Nevis  in  Great  Britain. 

There  are  now  so  many  observatories  in  addition  to  the  regular 
stations  of  the  weather  service  of  the  various  countries,  that  only  the 
more  important  ones  can  be  named.  To  attempt  to  describe  their 
equipment  and  work  in  small  space  is  out  of  the  question. 


376  METEOROLOGY 


QUESTIONS 

(1)  What  important  discovery  did  Benjamin  Franklin  make  in  connection 
with  storms?  (2)  State  in  outline  the  history  «f  the  U.  S.  Weather  Bureau. 
(3)  Name  the  chiefs  of  the  Weather  Bureau  and  t6e  characteristics  of  the  admin- 
istration of  each.  (4)  What  has  been  the  cost  ""of  the  Weather  Bureau?  (5) 
Describe  the  three  different  ways  in  which  the  United  States  is  divided  into 
districts  by  the  Weather  Bureau.  (6)  How  many  kinds  of  Weather  Bureau 
stations  are  there?  (7)  Describe  the  organization  of  the  Weather  Bureau  at 
Washington.  (8)  How  is  the  work  at  the  central  station  at  Washington  sub- 
divided ?  (9)  How  many  are  employed  by  the  U.  S.  Weather  Bureau  and  with 
what  salaries?  (10)  Describe  the  Weather  Bureau  station  at  Washington. 
(11)  Describe  a  typical  Weather  Bureau  station.  (12)  Describe  the  instru- 
ment equipment  of  a  Weather  Bureau  station.  (13)  What  observations  are 
taken  at  a  regular  Weather  Bureau  station  ?  (14)  What  observations  are  required 
at  a  cooperative  station?  (15)  Describe  the  development  of  the  daily  weather 
map.  (16)  Describe  the  development  of  the  weather  maps  for  the  northern 
hemisphere.  (17)  Describe  the  taking  and  sending  of  the  observations  for  the 
daily  weather  map.  (18)  Describe  the  code  for  sending  them.  (19)  How  are 
the  observations  charted  on  a  weather  map?  (20)  How  does  the  Washington 
weather  map  differ  from  those  issued  at  regular  stations?  (21)  Describe  the 
construction  of  the  weather  map  at  Washington.  (22)  Describe  the  chalk 
plate  process  of  producing  the  weather  map.  (23)  Describe  the  different  ways 
in  which  weather  forecasts,  data,  and  warnings  are  distributed.  (24)  De- 
scribe the  weather  service  in  the  evenings  and  on  Sundays  and  holidays.  (25) 
Describe  the  educational  and  research  work  of  the  Weather  Bureau  officials. 
(26)  What  are  the  periodical  publications  of  the  Weather  Bureau?  (27)  What 
publications  appear  from  time  to  time?  (28)  What  differences  are  found  in 
the  weather  maps  of  other  countries?  (29)  Name  and  describe  some  special 
meteorological  observatories  in  this  country  and  in  Europe. 

TOPICS  FOR   INVESTIGATION 

(1)  The  history  of  the  weather  map  to  1850. 

(2)  The  various  codes  used  to  transmit  weather  observations. 

(3)  The  processes  used  in  constructing  a  weather  map. 

(4)  The  history  and  present  organization  of  the  weather  service  of  some 
country. 

PRACTICAL   EXERCISES 

(1)  Construct  a  weather  map  for  the  United  States.  (The  necessary  observa- 
tions can  probably  be  secured  from  any  map-producing  station  of  the  U.  S. 
Weather  Bureau.  Material  for  six  maps  can  be  found  in  WARD'S  Practical 
Exercises  in  Elementary  Meteorology.) 

REFERENCES 

For  the  history  of  the  U.  S.  Weather  Bureau  and  its  present  organization,  consult : 
MOORE,   JOHN  W.,    Meteorology,    2d    ed.,    London,    1910.      (Chapter    V,  pp. 
44-68.) 


WEATHER  BUREAUS  AND  THEIR  WORK  377 

MOORE,  WILLIS  L.,   "Forecasting   the'  Weather  and  Storms/'  The  National 

Geographic  Magazine,  June,  1905. 
POLIS,  DR.  P.,  Der  Wetter  dienst  und  die  Meteorologie  in  den  Vereinigten  Staaten 

von  Amerika  und  in  Canada,  Berlin,  1908. 
Two  pamphlets  published  at  Washington  and  each  entitled  "  The  Weather 

Bureau." 
The  annual  "  Reports  of  the  Chief  of  the  Weather  Bureau." 

For  a  description  of  the  station  equipment  and  the  methods  of  taking  and  record- 
ing the  observations,  see : 

For  the  United  States  : 

Instructions  for  Cooperative  Observers.  Station  Regulations.  Instructions 
for  Preparing  Meteorological  Forms.  (All  published  by  the  U.  S.  Weather 
Bureau  at  Washington,  D.C.) 

For  Great  Britain : 

SCOTT,  R.  H.,  The  Observer's  Handbook. 

For  France : 

ANGOT,  ALFRED,  Instructions  Meteor ologiques. 

See  also  Appendix  IX. 

For  the  equipment  and  work  of  special  meteorological  observatories,  see : 

WALDO,  FRANK,  Modern  Meteorology,  London,  1893  (pp.  160-203). 

For  a  description  of  the  weather  services  of  other  countries,  see : 

CAMPBELL,   BAYARD,  "  Government  Meteorological  Organizations   in  Various 

Parts  of  the  World,"  Quart.  Jour.  R.  Met.  Soc.,  April,  1899. 
BEBBER,  W.  J.  VAN,  Lehrbuch  der  Meteorologie,  Stuttgart,  1890  (pp.  359-384). 
BORNSTEIN,  R.,  Leitfaden  der  Wetterkunde,  Braunschweig,  1906  (pp.  178-200). 
MOORE,  JOHN  W.,  Meteorology,  2d  ed.,  London,  1910  (pp.  26-43,  69-74). 
PERNTER,  J.  M.,  Wetterprognose  in  Osterreich,  Wien,  1907. 
Monthly  Weather  Review,  August,  1907,  p.  364. 


CHAPTER  VIII 

WEATHER   PREDICTIONS 
INTRODUCTION 

The  general  method  of  weather  forecasting,  374,  375. 
The  work  of  the  U.  S.  Weather  Bureau,  376-378. 

WEATHER  PREDICTION  CONSIDERING  THE  Low  AS  THE  DOMINATING 
FORMATION 

Locating  the  storm  center  twenty-four  hours  ahead,  379,  380. 
Determining  the  distribution  of  the  meteorological  elements,  381. 
The  prediction,  382,  383. 

WEATHER    PREDICTION    WHEN    V-SHAPED    DEPRESSIONS    AND    OTHER 
SECONDARY  ISOBARIC  FORMS  ARE  PRESENT,  384 

WEATHER  PREDICTION  CONSIDERING  THE  HIGH  AS  THE  DOMINATING 
FORMATION,  385 

WEATHER  PREDICTION  BY  SIMILARITY  WITH  PREVIOUS  MAPS,  386 
THE  PREDICTION  OF  PARTICULAR  AND  DANGEROUS  OCCURRENCES 

The  prediction  of  frost,  387. 

The  prediction  of  cold  waves,  388. 

The  prediction  of  tornadoes,  389. 

The  prediction  of  destructive  wind  velocities,  390. 

The  prediction  of  floods,  391. 

THE  ACCURACY  AND  VERIFICATION  OF  PREDICTIONS 

The  terms  used  in  official  predictions,  392. 

The  system  of  verification,  393. 

The  accuracy  attained,  394. 

The  popular  idea  of  the  accuracy  of  forecasts,  395. 

LONG  RANGE  PREDICTIONS 

Prediction  from  station  normals,  396. 
Weather  cycles,  397. 

Tendency  of  a  weather  type  to  continue,  398. 
Popular  superstitions  and  credulity,  399. 

FORECASTS  FROM  LOCAL  OBSERVATIONS  AND  APPEARANCE  OF  SKY 

Prediction  from  the  readings  of  instruments  and  appearance  of  the  sky,  400. 
Weather  proverbs  and  weather  rules,  401,  402. 

378 


WEATHER  PREDICTIONS  379 


INTRODUCTION 

374.  The  general  method  of  weather  forecasting.  —  Weather  has 
been  defined  as  the  condition  of  the  atmosphere  at  any  particular  time 
and  place.  The  condition  of  the  atmosphere  is  determined 
by  the  six  so-called  meteorological  or  weather  elements ;  and  weather 
namely,  temperature,  pressure,  wind,  moisture,  cloud, 
and  precipitation.  The  best  way,  then,  to  describe  the 
weather  or  to  depict  exactly  the  condition  of  the  atmosphere  is  to  state 
the  numerical  values -for  the  meteorological  elements.  To  forecast  or 
predict  the  weather  is  thus  to  foretell  the  values  which  the  meteoro- 
logical elements  are  expected  to  have. 

There  are  three  factors  which  determine  the  condition  of  the  atmos- 
phere. The  weather  may  thus  be  looked  upon  as  the  composite  or 
resultant  of  three  things :  (1)  the  typical  or  normal  condi-  „ 

The  weather 

tion  of  the  atmosphere  which  would  exist  if  there  were  no  is  a  corn- 
disturbances  or  local  influences  :  (2)  the  disturbances  caused  p°site  °* 

.  three  things. 

by  such  passing  meteorological  formations  as  extratropical 
cyclones,  anticyclones,  thundershowers,  tornadoes,  and  the  like ;  (3)  local 
influences  such  as  land  and  sea  breeze,  the  presence  of  large  bodies  of 
warm  water,  mountains,  etc. 

The  typical  or  normal  weather  which  would  exist  if  there  were  no 
disturbances  due  to  local  causes  or  the  passing  of  meteorological  forma- 
tions would  be  different  for  different  parts  of  the  country.  Nonnal  or 
For  the  northeastern  part  of  the  United  States,  it  would  be  typical 
something  like  this.  The  average  daily  temperature  would  wea 
be  highest  the  last  part  of  July,  and  then  decrease  steadily,  and  grad- 
ually, day  by  day,  until  the  last  of  January,  when  it  would  be  least.  It 
would  then  begin  to  steadily  and  gradually  increase.  Each  day  there 
would  be  the  regular  daily  oscillation  of  temperature  with  its  maximum 
in  the  early  afternoon  and  its  minimum  at  sunrise.  The  pressure  would 
be  very  constant,  slightly  higher  in  winter  than  in  summer.  There 
would  also  be  the  small  daily  oscillation  with  its  chief  maximum  at 
10  A.M.  and  its  chief  minimum  at  4  P.M.  The  wind  would  be  always 
moderate  in  velocity,  blowing  harder  in  the  winter  than  in  summer, 
and  by  day  than  by  night.  The  wind  direction  would  shift  from  north- 
west in  winter  to  southwest  in  summer  and  back  again.  The  absolute 
and  relative  humidity  would  also  show  a  regular  daily  and  annual 
variation.  There  would  be  very  few  clouds ;  perhaps  now  and  then  a 
cumulus  due  to  convection.  Precipitation  would  be  entirely  lacking. 


380  METEOROLOGY 

In  short,  the  regular  daily  and  annual  variations  in  the  meteorological 
elements  would  be  present,  but  there  would  be  no  irregular  fluctuations. 
It  would  be  difficult  to  give  numerical  values  to  this  theoretical  state 
of  things.  They  would  be  different  from  the  normals  found  from  obser- 
vation for  any  station,  as  cloud  and  precipitation  are  lacking.  How- 
ever, in  practical  forecasting,  one  is  not  concerned  with  this  theoretical 
state  of  things,  but  with  the  normals  as  derived  from  observation,  since 
cloud  and  precipitation  are  actually  present.  The  various  normals, 
then,  in  connection  with  the  meteorological  elements,  may  be  considered 
to  represent  normal  or  typical  weather.  • 

The  passing  meteorological  formations  which  exert  the  chief  influence 
in  the  United  States  are  the  lows  and  highs  (the  extratropical  cyclones 

and  anticyclones).     V-shaped  depressions  and  other  second- 
The  passing  J          .  '  *  K 

meteoroiogi-  ary  forms  of  isobars  should  perhaps   also    be  mentioned. 

cai  forma-  Occasionally,  a  tropical  cyclone  visits  the  Gulf  States. 
Thundershowers  and  tornadoes  should  also  be  included,  but 
thundershowers  always  accompany  a  low  or  V-shaped  depression,  and 
tornadoes  are  always  associated  with  thundershowers.  Thus,  extra- 
tropical  cyclones  and  anticyclones  are  by  far  the  most  important  forma- 
tions to  consider.  In  fact,  New  England  is  crossed  by  such  a  ceaseless 
procession  of  these  two  formations,  that  the  weather  is  nearly  always 
dominated  by  one  or  the  other,  and  can  hardly  ever  be  said  to  be  typical. 
All  of  these  formations  have  already  been  critically  studied  in  the 
chapter  on  storms. 

Local  influences  are  not  usually  very  numerous  or  of  great  importance. 
If  a  place  is  located  on  the  seacoast,  then  land  and  sea  breeze  must  be 
Local  in-  taken  into  account.  In  forecasting  the  weather  for  New 
fluences.  York  State,  the  presence  of  the  Great  Lakes  play  an  impor- 
tant part,  particularly  in  the  late  autumn  and  early  winter,  when  they 
are  still  much  warmer  than  the  land  and  are  putting  large  quantities  of 
moisture  into  the  atmosphere.  A  large  river  also  sometimes  influences 
weather  conditions. 

375.  The  various  normal  values,  when  once  determined  for  the 
different  meteorological  elements,  hold  for  all  time.  Thus  normal 
The  lows  weather  is  known  for  weeks  or  years  ahead.  The  local 
and  highs  influences  are  usually  unimportant,  and  can  be  estimated 
portant  **  with  fair  precision.  The  chief  difficulty,  and  practically  the 
things  in  only  difficulty,  is  thus  to  determine  the  influence  of  the 
tmg<  passing  meteorological  formation.  If  the  lows  and  highs 
were  even  always  typical,  weather  forecasting  would  be  an  easy  matter. 


WEATHER  PREDICTIONS  381 

That  is,  if  they  all  followed  typical  courses  with  known  velocities  and 
were  typical  as  regards  the  distribution  of  the  meteorological  elements 
about  them,  their  influences  could  be  readily  estimated.  As  it  is, 
weather  forecasting  is  by  no  means  an  easy  matter,  as  the  lows  and 
highs  are  seldom  typical  as  regards  path,  velocity,  or  characteristics. 
Weather  forecasting,  then,  really  turns  on  this  one  thing,  the  estimation 
of  the  influence  of  the  passing  meteorological  formations.  The  general 
method  to  be  followed  in  predicting  the  weather  is  thus  apparent.  The 
first  thing  to  do  is  to  estimate  the  influence  which  the  passing  meteoro- 
logical formation  is  going  to  exert  one  or  two  days  in  the  future.  The 
local  influences  must  next  be  estimated.  When  these  two  have  been 
combined  with  normal  weather,  the  resultant  is  the  expected  or  pre- 
dicted weather.  The  details  in  this  process  will  be  considered  a  little 
later. 

376.    The  work  of  the  U.  S.  Weather  Bureau.  —  For  the  purpose  of 
weather  forecasting,  the  United  States  is  divided  by  the  U.  S.  Weather 
Bureau  into  six  forecast  districts  with  centers  at  Washing-  where  the 
ton,  Chicago,  Denver,  San  Francisco,  Portland,  and  New  forecasts  of 
Orleans.     These  districts  are  shown  on  the  map  in  figure 


142.  Based  on  the  8  A.M.  observations,  the  officials  in  charge  Bureau  are 
of  each  forecast  district  issue  weather  forecasts  and  warnings 
for  the  respective  districts.  As  soon  as  made,  these  are  forwarded  by 
telegraph  to  the  Central  Office  at  Washington,  and  forecasts  for  all 
districts  appear  on  the  Washington  weather  map.  Based  on  the  8  P.M. 
observations,  forecasts  and  warnings  are  issued  from  Portland  and  San 
Francisco  only  for  the  respective  districts.  The  forecasts  and  warnings 
are  issued  from  Washington  for  all  the  other  districts.  A  local  forecast 
official  at  a  map-producing  station  issues  forecasts  for  the  immediate 
vicinity  of  the  station  only.  On  the  weather  map,  this  forecast  appears, 
and  also  the  forecasts  for  the  state,  and  perhaps  adjoining  states,  which 
have  come  from  the  respective  forecast  centers.  The  prediction  based 
on  the  8  A.M.  observations  are  for  36  hours,  while  those  based  on  the 
8  P.M.  observations  are  for  the  following  48  hours. 

The  forecasts  consist  of  predictions  of  temperature,  wind,  and  state 
of  the  weather.  This  last  includes  the  amount  of  cloud  and  the  kind 
and  quantity  of  precipitation.  Pressure  and  moisture  are  Of  what  the 
never  forecasted.  Cold  waves,  frosts,  and  high  wind  ve- 
locities  (storms)  are  also  predicted  and  the  appropriate 
warnings  sent  out.  A  local  forecast  official  at  a  map-producing  station 
forecasts  temperature,  wind,  and  state  of  the  weather  only.  All  fore- 


382  METEOROLOGY 

casts  and  warnings  in  connection  with  high  wind  velocities  (storms), 
cold  waves,  and  frosts  come  from  the  forecast  centers. 

These  forecasts  and  warnings  of  the  Weather  Bureau  are  not  only 
printed  on  the  daily  weather  map,  but  they  receive  a  wide  distribution 
by  means  of  newspapers,  cards,  flags,  and  special  messengers.  These 
various  methods  of  distribution  have  already  been  discussed  in  section 
368. 

377.  Long  training  is  required  on  the  part  of  the  Weather  Bureau 
officials  before  they  are  allowed  to  issue  forecasts.     An  official  in  charge 
The  training  °^  a  station  where  forecasts  are  not  issued  and  first  assistants 
of  a  fore-       at  large  stations  are  permitted  to  make  practice  forecasts. 

After  this  has  been  continued  with  fair  success  for  a  year, 
the  authority  to  issue  forecasts  may  be  given.  Each  official  who  is 
authorized  to  make  local  forecasts  is  still  required  to  make  practice 
forecasts  for  the  state  in  which  the  station  is  located.  He  must  also 
make  practice  storm,  cold  wave,  and  frost  forecasts  for  the  local  station 
where  he  is.  All  of  these  practice  forecasts  are  immediately  mailed  to 
Washington  for  verification. 

Ability  to  forecast  well  depends  upon  characteristics  of  mind  as  well 
as  careful  training.  Greely  in  his  American  Weather  says :  "  The 
Th  skill  of  a  weather  predictor  arises  largely  from  his  alert 

teristics  of  a  comprehensiveness  of  mind,  accurate  and  retentive  memory, 
successful  phlegmatic .  but  confident  temperament,  and  long  experi- 
ence." A  weather  forecaster  must  take  in  many  details  at 
a  glance ;  he  must  recollect  past  occurrences ;  he  must  not  lose  confi- 
dence in  himself  or  his  ability,  even  if  an  occasional  prediction  goes 
wrong. 

378.  The  general  method  of  weather  prediction  in  other  countries 
is  the  same  as  in  this ;  although  the  details  may  be  quite  different.     Most 
The  countries  issue  but  one  set  of  forecasts  and  warnings  each 
methods  of     day.     This  is  often  in  the  early  afternoon,  instead  of  the 
forecastlfin    mornmg-     Some    countries    get    supplementary    telegrams 
other  coun-    from  a  few  stations  just  before   the  forecasts  are  issued. 

The  forecasts  are  usually  issued  from  a  central  station  for 
the  whole  country,  although  this  is  not  true  for  the  larger  countries. 
The  signals  used  and  the  methods  of  distributing  the  forecasts  are  often 
quite  different. 


WEATHER  PREDICTIONS  383 

WEATHER  PREDICTION  CONSIDERING  THE  Low  AS  THE  DOMINATING 

FORMATION 

379.    Locating  the  storm  center  twenty-four  hours  ahead.  —  Since 
the  estimation  of  the  disturbances  caused  by  passing  me- 
teorological formations  is  the  thing  of  chief  importance  in 
forecasting  the  coming  weather,  weather  prediction  may  be  tions  must 
considered  under  three  heads :     (1)  when  a  passing  low  is  ^ered 
the  dominating  formation;   (2)  when  a  V-shaped  depression 
or  some  secondary  isobaric  form  is  exerting  the  chief  influence ;  (3)  when 
a  passing  high  is  exerting  the  dominating  influence. 

If  it  is  a  low  which  is  going  to  dominate  the  weather  for  the  immediate 
future,  the  first  thing  to  do  is  to  determine  where  the  center  is  expected 
to  be  twenty-four  and  forty-eight  hours  ahead.  A  fairly 
exact  estimation  of  where  the  center  will  be  twenty-four  hours  Jt^e  £[  iocat_ 
ahead  must  be  made ;  the  position  forty-eight  hours  ahead  ing  a  low 
need  be  only  roughly  estimated.  The  general  rule  is  to  hours  ahead 
determine  the  track  which  the  low  seems  to  be  following,  and 
then  put  it  ahead  on  this  track  the  distance  normally  covered  in  twenty- 
four  and  forty-eight  hours.  If  a  low  has  just  formed,  or  has  just  come 
into  the  field  of  observation,  the  past  is  no  guide  as  to  the  track.  The 
safest  rule  in  these  cases  is  to  assume  that  the  low  probably  will  follow 
the  most  frequented  track  which  passes  through  the  locality  where  it 
appeared.  For  this  the  Van  Cleef  system  of  tracks  (see  section  300) 
will  probably  be  the  most  satisfactory.  If  the  forecasting  is  being  done 
for  the  northeastern  part  of  the  United  States,  the  lows  have  usually 
been  observed  and  charted  on  the  weather  map  a  day  or  two  before  they 
become  the  dominating  influence.  If  this  is  the  case,  note  which  track 
the  low  has  been  following  and  put  it  ahead  on  this  track.  Either  the 
Bigelow,  Russell,  or  Van  Cleef  system  of  tracks  may  be  used.  It 
probably  would  be  better  to  use  that  system  which  contains  the  track 
which  the  low  seems  to  be  following  most  exactly.  The  normal  velocity 
of  motion  is  about  900  miles  a  day  in  winter  and  about  600  in  summer 
and  intermediate  values  in  other  seasons.  (See  section  302.) 

After  it  has  been  determined  where  the  center  of  the  low  would  be 
twenty-four  and  forty-eight  hours  ahead,  if  it  followed  its  track  with 
normal  velocity,  it  is  customary  for  the  skilled  forecaster  to  modify 
this  estimation  by  taking  account  of  other  considerations.  This  simply 
means  that  as  a  result  of  his  long  experience,  he  has  formed  certain 
empirical  rules  for  his  own  guidance.  This  shows  the  necessity  of  long 


384  METEOROLOGY 

experience  before  reliable  forecasts  can  be  made  and  also  the  difficulty 
in  explaining  exactly  how  a  weather  forecast  is  made.  Some  of  these 
Someempir-  rlues  are  the  following:  (1)  Lows  near  each  other  tend  to 
ical  rules  coalesce.  This  is  particularly  true  if  one  low  is  in  the 
ing  the  7"  northern  Mississippi  Valley  and  the  other  in  the  southern, 
normal  posi-  or  if  one  low  is  coming  up  the  Atlantic  Coast  and  the  other 
is  coming  eastward  across  the  Great  Lakes.  They  usually 
coalesce  at  an  intermediate  point  and  become  more  intense  than  either 
component.  Often  the  appearance  of  coalescing  may  be  given  by  the 
fading  away  of  one  low,  while  the  other  becomes  more  intense  and  dom- 
inant. (2)  Lows  and  highs  repel  each  other.  If  a  high  is  directly  on 
the  track  of  a  coming  low,  the  low  is  likely  to  be  retarded  or  to  be  de- 
flected from  its  normal  track.  (3)  Lows  tend  to  follow  the  record  of 
previous  days.  This  means  that  if  a  low  is  a  slow-moving  one,  it 
tends  to  retain  that  characteristic  all  through  its  life  history. 
(4)  Lows  tend  to  move  toward  the  areas  of  greatest  rainfall  during  the 
preceding  twenty-four  hours.  (5)  Lows  tend  to  move  toward  the  areas 
of  highest  dewpoint.  (6)  Lows  tend  to  move  toward  areas  of  least  wind 
velocity.  (7)  Lows  tend  to  grow  more  intense  as  they  approach  bodies 
of  water  as  the  Great  Lakes  or  Atlantic  coast.  (8)  Lows  tend  to  move 
faster  as  the  pressure  grows  less.  (9)  Lows  with  above  normal  winds 
weaken,  while  lows  with  below  normal  winds  develop  lower  pressure. 
(10)  Lows  on  a  curved  track  tend  to  move  faster  after  they  begin  to  move 
northeastward.  This  set  of  rules  makes  no  pretense  at  being  complete. 
Only  the  well-recognized  and  more  important  ones  have  been  given. 
Each  forecaster,  as  he  acquires  experience,  will  prefer  to  formulate  his 
own  experience.  They  will  be  found  very  useful,  however,  by  a  be- 
ginner. 

The  method,  then,  of  determining  where  the  center  of  a  low  is  expected 
to  be  twenty-four  and  forty-eight  hours  ahead  is  to  put  the  low  forward 
The  method  on  ^ne  track  which  it  seems  to  be  following  the  normal  dis- 
briefly  tance,  and  then  to  modify  this  estimation  by  taking  account 

of  certain  general  principles  which  have  been  learned  from 
experience  and  perhaps  formulated  as  rules.  As  a  result  of  this  process, 
the  forecaster  will  come  to  a  very  definite  conclusion  as  to  where  the 
center  of  the  low  is  expected  to  be  twenty-four  hours  ahead,  and  he  will 
also  have  a  general  idea  of  where  he  expects  it  to  be  forty-eight  hours 
ahead. 

380.  In  this  connection,  the  question  can  be  well  raised  as  to  what 
really  determines  the  track  which  a  low  is  to  follow  and  its  velocity 


WEATHER  PREDICTIONS  385 

of  motion.     The  path  and  velocity  are  probably  completely  determined 
by  the  general  drift  of  the  atmosphere,  the  characteristics  of  the  dis- 
tribution of  the  meteorological  elements  about  the  formation 
itself,  the  location  and  characteristics  of  the  surrounding  Whfch  deter- 
meteorological  formations,  the  surface  topography  of  the  mine  the 
country,  and  the  meteorological  condition  of  the  country.     It  velocity  of 
will  be  seen  at  once  that  the  factors  which  enter  in  are  large  motion  °f  * 
in  number  and  very  complex.     If  the  value  and  relative  im- 
portance of  all  of  those  factors  could  be  determined  from  a  study  of 
many  weather  maps,  it  then  would  be  possible  in  any  particular  case  to 
determine  the  amount  of  motion,  and  the  direction  of  motion,  which 
each  factor  would  cause,  and  the  resultant  for  all  the  factors  would  give 
the  direction  of  motion  and  the  velocity  of  motion  of  the  low.     This 
can,  however,  probably  never  be  done  with  precision.     These 
factors  are,  however,  of  very  different  importance.     Some 
are  of  prime  importance,  and  many  have  such  insignificant  the  problem 
influence  that  they  can  almost  be  neglected.     The  general  2bkP°S" 
drift  of  the  atmosphere  and  the  distribution  of  pressure  about 
the  low  itself  are  probably  the  most  important  factors  in  determining  the 
path  and  velocity  of  motion  of  a  low.     The  distribution  of  the  tempera- 
ture over  a  country  is  probably  the  next  most  important   factor,  while 
the  distribution  of  the  winds  about  the  low  as  regards  direction  and 
velocity,  and  whether  the  low  is  accompanied  by  unusually  high  or  low 
wind  velocities,  are  also  very  important  considerations.     The  location  of 
the  rain  area  during  the  preceding  twenty-four  hours  and  the  direction 
and  intensity  of  the  surrounding  highs  and  lows  perhaps  stand  next 
in  importance. 

As  an  approximation  to  the  solution  of  the  general  problem,  the  path 
and  velocity  of  motion  of  a  low  might  be  worked  out,  using  the  two 
most  important  factors  only.     This  has  been  done  by  Mr. 
Edward  H.  Bowie,  who  was  then  local    forecast  official  at  method  of 
St.  Louis,  Mo.,  and  has  been  recently  transferred  to  Wash-  locating  a 
ington  to  be  forecaster  there.     His  investigation  will  be  found 
in   the   Monthly  Weather  Review  for  February,  1906  (Vol.  XXXIV, 
p.  61),  and  the  reader  must  be  referred  to  this  article  for  a  full  discussion 
of  his  method.     He  first  determines,  for  the  various  months  in  the  year, 
the  twenty-four  hour  drift  of  the  atmosphere  for  all  parts  of  the  United 
States.     Then,  in  the  case  of  the  low  in  question,  the  pressure  gradient 
toward  the  center  from  the  north,  northeast,  east,  southeast,  etc.,  is 
found  from  the  weather  map.     The  resultant  of  these  eight  pressure 
2c 


386  METEOROLOGY 

gradients  is  then  found,  and  this  represents  the  direction  and  magnitude 
of  the  unbalanced  pressure  which  is  forcing  the  low  to  move.  The  com- 
bination of  this  unbalanced  pressure  with  the  general  drift  of  the  atmos- 
phere gives  the  direction  of  motion  and  the  velocity  of  motion  of  the  low. 
Only  two  of  the  general  factors  are  taken  account  of  in  this  method,  but 
the  high  degree  of  accuracy  which  Mr.  Bowie  has  attained  in  forecasting, 
by  the  use  of  his  methods,  attests  the  prime  importance  of  these  two 
factors  and  the  value  of  his  method.  Even  when  the  Bowie  system  is 
not  used  in  full,  the  underlying  principle  is  of  great  value  to  the  fore- 
caster. One  can  notice  very  easily  in  which  quadrant  of  a  low  the  iso- 
bars are  most  closely  packed  together.  Here  the  pressure  gradient  will 
be  largest,  and  the  low  will,  in  general,  move  away  from  tlnVarea'. 

Trabert 1  has  recently  called  attention  to  the  fact  that  the  distribution 
of  temperature  over  a  country  may  determine  the  direction  of  motion  of  a 
low.  Usually  the  temperature  increases  from  the  north  toward  the 
south.  In  this  case,  the  winds  on  the  eastern  side  of  a  low  are  bringing 
warmer,  moisture-laden  air  from  the  south,  while  the  winds  on  the 
western  side  are  bringing  cold,  dry  air  from  the  north.  The  pressure 
will  thus  be  falling  on  the  eastern  side  and  rising  on  the  western  side, 
and  the  low  will  thus  move  towards  the  east.  If  the  temperature  dis- 
tribution over  a  country  were  different,  a  different  direction  of  motion 
might  result.  The  motion,  in  general,  would  be  at  right  angles  to  the 
temperature  gradient. 

In  1905  the  Belgian  Astronomical  Society  instituted  an  international 
competition  in  forecasting  at  Liege,  and  the  first  prize  was  awarded  to 
The  Guil-  Gabriel  Guilbert  of  Caen.  Later  (1909)  Guilbert  published 
bert  rules.  hjs  method  of  forecasting  in  book  form  under  the  title 
"  Nouvelle  methode  de  prevision  du  temps."  His  system  is  really 
based  upon  three  rules,  all  of  which  relate  to  the  winds  which  surround 
the  low.  These  rules  may  be  summarized  and  stated  as  follows  : 2 

(1)  Every  depression  that  gives  birth  to  a  wind  stronger  than  the 
normal  will  fill  up  more  or  less  rapidly.     On  the  other  hand,  every  de- 
pression that  forms  without  giving  rise  to  winds  of  corresponding  forqe 
will  deepen,  and  often  depressions  that  are  apparently  feeble  will  be 
transformed  into  true  storms. 

(2)  When  a  depression  is  surrounded  by  winds  having  varying  degrees 
of  excess  or  deficiency,  as  compared  with  the  normal  wind,  it  moves 

1  See :    "Die   Zugrichtung   der   Depressionen,"    by  WILH.  TKABERT,  in   Das   Wetter, 
February,  1911. 

2  See  Monthly  Weather  Review,  May,  1907,  p.  210. 


WEATHER  PREDICTIONS  387 

towards  the  region  of  least  resistance.  These  favorable  areas  are  made 
up  of  regions  in  which  the  winds  are  relatively  light,  and  especially  of 
such  as  have  divergent  winds  with  respect  to  the  center  of  the  depression. 

(3)  The  rise  of  pressure  takes  place  along  a  direction  normal  to  the 
wind  that  is  relatively  too  high,  and  it  proceeds  from  right  to  left ;  an 
excessive  wind  causes  a  rise  of  pressure  on  its  left. 

Here,  again,  we  have  a  whole  system  of  forecasting  built  upon  one  of 
the  more  important  of  the  many  factors  which  determine  the  path  and 
velocity  of  motion  of  a  low.  The  wind  direction  and  its  velocity  and  the 
pressure  gradient  are  very  closely  related,  so  that,  in  a  way,  the  JBowie 
system  and  Guilbert's  rules  rest  on  the  same  foundation.  These  rules 
are  of  great  value  to  the  forecaster,  as  they  can  be  easily  applied,  and 
help  to  guide  one's  estimation  as  to  the  probable  direction  and  velocity 
of  motion  of  a  low. 

It  will  also  be  noticed  that  all  of  the  empirical  rules  which  were 
stated  in  section  379  for  locating  the  center  of  a  low  twenty-four  hours 
ahead  depend  on  the  influence  of  one  or  several  of  the  various  factors 
which  determine  the  path  of  a  low. 

381.    Determining  the  distribution  of  the  meteorological  elements. — 
The  method  of  determining  the  probable  location  of  the  center  of  the 
dominating  low  twenty-four  and  forty-eight    hours  ahead  First  study 
has  just  been  fully  discussecf.     The  next  step  is  to  determine  critically  the 
the   probable   distribution   of  the   meteorological   elements  J^^^eie^ 
about  the  low  in  its  predicted  location.     In  order  to  do  this,  ments  about 
first  study  critically  the  distribution  of  the  elements  about  * 
the  low  as  indicated  on  the  weather  map  which  is  serving  as  the  basis 
for  the  forecast.     Notice  in  detail  just  what  the  values  of  temperature, 
pressure,  wind,  moisture,  cloud,  and  precipitation  are  in  different  parts 
of  the  area  covered  by  the  low.     If  there  is  any  departure  from  the 
normal  distribution  about  a  low,  try  to  explain  each  departure  from 
normal.     It  might  be  well  in  this  connection  to  emphasize  the  neces- 
sity of  noting  carefully  whether  the    low  has  a  wind  shift  line,  or  is 
accompanied  by  the  winter  or  summer  type  of  cloud  area.     Next  decide 
whether  the  predicted  location  of  the  center  of  the  low  twenty-  Next  esti. 
four  hours  ahead  is  going  to  change  the  previous  distribution  mate  the 
in  any  way.     Nearness  to  a  body  of  water  or  a  range  of  causedlby 
mountains  and  the  meteorological  condition  of  the  country  the  new 
probably  exert  the  greatest  modifying  influence.     Finally 
assume  that  the  distribution  of  the  elements  about  the  low,  twenty- 
four  hours  ahead,  will  be  the  same  as  on  the  previous  weather  map  with 


388  METEOROLOGY 

the  exception  of  any  changes  which  the  new  location  of  the  low  may 
be  expected  to  cause.  The  rule,  then,  briefly  expressed,  is  to  assume 
The  rule  that  the  distribution  of  the  elements  will  be  what  it  was, 
briefly  ex-  with  the  exception  of  the  changes  caused  by  the  new  sur- 

pressed.  roundingS. 

382.  The  prediction.  —  The  various  steps  which  must  be  taken  be- 
fore a  weather  forecast  can  be  made  have  been  discussed  individually 
and  in  detail.  It  has  been  seen  that  the  weather  is  the  corn- 
factors  must  posite  or  resultant  of  three  factors :  normal  weather,  local 
be  summed  influence,  and  the  disturbances  caused  by  passing  meteoro- 
logical formations.  Normal  weather  is  known  for  days  and 
months  in  advance.  The  local  influences  are  usually  insignificant. 
No  rules  can  be  laid  down  for  these.  They  must  be  learned  from 
experience  for  each  individual  place.  In  the  case  of  a  low,  the  method 
of  locating  its  center  and  determining  the  distribution  of  the  elements 
about  it,  and  thus  its  influence  twenty-four  or  forty-eight  hours  ahead, 
has  been  explained.  It  now  remains  simply  to  sum  up  these  three 
factors  in  order  to  make  a  weather  prediction. 

It  is  the  best  of  practice  for  a  beginner,  or  even  for  one  who  has  ac- 
quired considerable  skill  in  forecasting,  to  force  oneself  to  form  an  exact 
The  making  picture  of  what  the  weather  is  expected  to  be.  This  exact- 
of  exact  ness  can  well  be  carried,  in  some  Cases,  to  the  point  of  estimat- 
givesavaiu-  m&  ^ne  numerical  value  which  each  of  the  elements  is  expected 
able  train-  to  have.  It  must  not  be  expected,  however,  that  such  a  fore- 
mg*  cast  will  verify  in  every  detail.  Something  will  almost 

always  be  wrong,  but  fortunately  the  forecaster  is  not  left  to  helplessly 
wonder  what  went  wrong.  As  soon  as  the  weather  has  occurred,  and 
the  next  weather  map  has  been  received,  it  is  always  possible  to  see 
exactly  what  was  overestimated  or  underestimated,  and  these  very 
mistakes  become  stepping  stones  for  the  acquirement  of  more  skill,  ac- 
curacy, and  confidence  in  forecasting.  It  will  bear  repetition  that  these 
exact  forecasts  have  great  value  as  a  training.  If  such  an  exact  forecast 
is  to  be  made,  the  best  order  in  which  to  predict  the  values  of  the  mete- 
orological elements  is  perhaps  this :  (1)  pressure ;  (2)  wind  direction 
and  velocity ;  (3)  clouds  —  amount  and  kind ;  (4)  moisture ;  (5)  tem- 
perature ;  (6)  precipitation  —  kind  and  amount. 

The  forecasts  of  temperature,  wind,  and  state  of  the  weather  issued 
by  the  U.  S.  Weather  Bureau  are  definite  and  explicit,  but  do  not  usually 
contain  estimates  of  the  numerical  values  that  the  elements  are  ex- 
pected to  have.  The  various  phrases  which  may  be  used,  and  their 


WEATHER  PREDICTIONS  389 

exact  meaning,  are  laid  down  in  the  "  Station  Regulations  "  of  the 
U.  S.  Weather  Bureau.     Some  of  these  regulations  are  here  quoted. 
"  Forecasts  based  on  the  8  A.M.  reports  will  be  for  the  night  The  fore_ 
of  the  current  day,  8  P.M.  to  8  A.M.,  and  for  the  following  casts  as 
day,  8  A.M.  to  8  P.M.     They  may  cover  the  afternoon  of  the  ™& 
current  day  only  when  marked  changes  are  expected.     Am-  Weather 
biguous  expressions  will  be  avoided  in  the  preparation  of  Bureau- 
forecasts.      When  conditions  are  so  uncertain  as  to  make  the  accuracy 
of  a  forecast  doubtful,  the  word   '  probably '   or  '  possibly '  may"  be 
used. 

"  Forecasts  of  temperature  changes  will-  be  made  when  the  24-hour 
changes  at  8  A.M.  and  8  P.M.  of  the  following  day  are- expected  to  equal 
or  exceed  6°  in  the  months,  June,  July,  August,  and  September ;  8°  in 
April,  May,  October  and  November;  and  10°  in  December,  January, 
February,  and  March.  The  terms  '  warmer,  colder,  decidedly  warmer, 
decidedly  colder '  will  be  used  in  describing  the  corresponding  changes. 
Forecasts  of  fair,  partly  cloudy,  or  cloudy,  will  be  made  when  pre- 
cipitation to  the  amount  of  0.01  inch  or  more  is  not  expected. 
When  precipitation  of  0.01  inch  or  more  is  expected,  its  character 
will  be  indicated  by  the  use  of  such  terms  as  '  rain,  snow,  local 
rain,  showers,  local  snow,  snow  flurries,  thundershowers,  thunder- 
storms/ etc." 

383.  The  following  example  will  serve  to  illustrate  the  method  of 
making  an  exact  prediction  and  also  a  more  general  one,  such  as  would 
be  issued  by  a  local  forecast  official.  The  weather  maps  for 
8  A.M.,  Sunday,  Dec.  8,  1907,  and  for  8  A.M.,  Monday, 
Dec.  9,  are  reproduced  as  Charts  XXXVIII  and  XXXIX.  which  the 
The  first  table  contains  various  observations  made  at  many  j^based. 
regular  weather  bureau  stations  at  8  A.M.  on  Monday,  while 
the  second  table  contains  the  detailed  observations  made  at  Albany, 
N.Y.,  at  8  A.M.  and  8  P.M.  on  Sunday  and  at  8  A.M.  on  Monday,  The 
problem  is,  on  the  basis  of  this  material,  to  form  a  definite  picture  of  the 
weather  changes  expected  at  Albany  during  the  next  thirty-six  hours,  and 
then  to  formulate  such  a  forecast  as  would  be  issued  by  a  local  forecast 
official.  The  forecast  must  be  made  separately  for  Monday  night 
(8  P.M.  Monday  to  8  A.M.  Tuesday)  and  for  Tuesday  (8  A.M.  to  8  P.M. 
of  that  day),  thus  covering  a  period  of  thirty-six  hours  from  the  time  of 
the  last  observations  which  can  be  utilized.  Similar  observational 
material  and  the  weather  maps  are  always  available  whenever  a  fore- 
cast is  made. 


390 


METEOROLOGY 


OBSERVATIONS  8  A.M.  MONDAY,  DEC.  9,  1907,  AT  VARIOUS  WEATHER  BUREAU 

STATIONS 


a 

o 

TEMPERATURE 

1  * 

»  g 

TEMPERATURE 

P» 

1  1 

S  i 

OH 

O     Q 

hJ     W 

w  ^ 

O    O 

h-J     O 

S    ^ 

w  M 

fc    *5 

—  ~~ 

fc 

STATIONS 

00    ^ 

a  S 

O  hH 

STATIONS 

P§  00 

«esi 

o1"1 

FH 

Min.  in 

Max.  in 

•H    < 

& 

Iss 

Min.  in 

Max.  in 

!§  ^3 

|g 

£  | 

Last  12 

Last  24 

^  K 

M 

S  ^ 

Last  12 

Last  24 

*   02 

^  j 

5  o 

Hours 

Hours 

a  g 
z  ° 

*   h-J 
•<J     £ 

§K 

Hours 

Hours 

Si 

^      M 

w 

£ 

£n  C^l 

gW 

£cS 

Abilene    .     . 

46 

70 

20 

Marquette  .     . 

36 

42 

10 

T 

ALBANY    . 

26 

44 

4 

Memphis    . 

56 

60 

12 

.42 

Alpena     .     . 

34 

38 

8 

.12 

Miles  City  .     . 

Amarillo  .     . 

30 

60 

24 

Milwaukee  .     . 

42 

44 

4 

.22 

Asheville  .     . 

50 

60 

12 

.36 

Modena  .     .     . 

24 

50 

4 

Binghamton 

30 

42 

6 

Montgomery  . 

58 

66 

4 

.16 

Bismarck 

16 

40 

16 

32 

.04 

Montreal 

36 

38 

6 

T 

Boston     .    . 

40 

48 

4 

Moorhead   .     . 

12 

32 

14 

Buffalo     .    . 

44 

32 

8 

New  Orleans  . 

58 

66 

6 

.64 

Cairo   .     .     . 

54' 

58 

10 

.22 

New  York  .     . 

40 

52 

4 

Canton    .     . 

36 

44 

4 

T 

Norfolk  .     .     . 

42 

64 

6 

Charleston  . 

56 

64 

14 

.02 

North  Platte  . 

24 

52 

24 

36 

Chattanooga 

52 

60 

4 

.02 

Oklahoma  .     . 

38 

68 

30 

38 

.02 

Chicago   . 

44 

52 

12 

.02 

Omaha    .     .     . 

36 

48 

38 

40 

.06 

Cincinnati    . 

48 

62 

6 

.30 

Oswego   .     .     . 

36 

40 

14 

T 

Cleveland     . 

42 

54 

24 

.04 

Philadelphia    . 

38 

54 

4 

Columbus    . 

42 

58 

14 

.24 

Phoenix  .     .     . 

40 

66 

4 

Davenport  . 

46 

52 

4 

.02 

Pierre      .     .     . 

22 

44 

24 

Denver 

22 

52 

6 

36 

Pittsburg    .     . 

42 

60 

6 

T 

Detroit 

40 

52 

18 

.22 

Portland,  Me. 

30 

42 

4 

Dodge 

30 

50 

30 

36 

.01 

Portland,  Ore. 

40 

46 

4 

.26 

Duluth 

32 

38 

12 

.04 

Rapid  City      . 

El  Paso 

36 

62 

4 

Rochester    .     . 

42 

50 

4 

T 

Erie     . 

46 

48 

14 

T 

St.  Louis 

52 

56 

10 

T 

Escanaba 

36 

40 

10 

.18 

St.  Paul  .     .     . 

38 

46 

18 

Fort  Smith  . 

48 

70 

8 

Salt  Lake    .     . 

26 

42 

4 

Galveston    . 

62 

68 

10 

T 

San  Diego   .     . 

52 

64 

4 

Grand  Rapids 

44 

52 

6 

.38 

San  Francisco  . 

58 

58 

4 

Green  Bay  . 

34 

44 

12 

.28 

S.  S.  Marie      . 

36 

38 

4 

.14 

Havre      .     . 

-2 

24 

0 

.06 

Scranton      .     . 

32 

52 

4 

Helena     .     . 

14 

28 

4 

T 

Spokane 

30 

36 

4 

Houghton    . 

32 

38 

18 

.02 

Springfield,  111. 

50 

56 

4 

.01 

Huron      .     . 

18 

42 

22 

T 

Springfield,  Mo. 

48 

60 

6 

Indianapolis 

48 

50 

12 

.40 

Syracuse      .     . 

36 

48 

8 

T 

Jacksonville 

64 

72 

10 

Tampa    .     .     . 

62 

76 

8 

.02 

Kansas  City 

46 

58 

28 

28 

.02 

Toledo     .     .     . 

42 

58 

14 

.42 

Key  West 

72 

80 

12 

Vicksburg   .     . 

58 

64 

6 

.26 

Knoxville 

46 

54 

4 

Washington     . 

32 

62 

4 

La  Crosse 

40 

46 

4 

.20 

White  River    . 

— 

— 

Lander     . 

6 

36 

4 

.22 

Williston     .     . 



— 

— 

Los  Angeles 

50 

66 

6 

Winnemucca  . 

32 

48 

4 

Louisville 

52 

62 

8 

.16 

Winnipeg    .     . 

-6 

20 

12 

Macon     . 

58 

66 

6 

.06 

Yellowstone    . 

4 

30 

6 

.04 

WEATHER  PREDICTIONS 

OBSERVATIONS  AT  ALBANY 


391 


• 

DEC.  8,  1907 

DEC.  9,  1907 

Temp. 

(F.) 

8  A.M                                

25.4 
32.4 
23.6 
35.0 
44.0 
25.4 

28.0 

35.4 
26.7 

Max.  previous  12  hours  

Min.  previous  12  hours  

8  P.M  

Max.  previous  12  hours  

Min  previous  12  hours 

Pressure 
reduced 
to 
sea  level 

SAM 

30.25 
30.25 
30.20 
30.19 
30.25 
30.15 

30.19 
30.19 
30.16 

Max.  previous  12  hours  
Min.  previous  12  hours  

8  P.M  

Max.  previous  12  hours  

Min.  previous  12  hours  

Wind 

f  Direction  SAM 

S.W. 

N. 
2 
2 

N. 
2 

Direction  8  P.M. 

1  Velocity  8  A.M.            

Velocity  8  P.M  

Moisture  - 

fRel.  humid.  8  A.M  

94 
91 

100 

tRel.  humid.  8  P.M  

Clouds 

Kind  and  direction  of  motion  8  A.M. 
Cloudiness  8  A.M.       

Alto-stratus 
from  W. 
Pt.  cloudy  (6) 
light  fog 
Light  fog 
Clear 

Dense  fog 
Foggy 

Kind  and  direction  of  motion  8  P.M. 
Cloudiness  8  P.M  

Precipi- 
tation    • 
in  inches 

Snow  on  g 
Snow  on  g 

Amount  at  8  A.M.  during  previous 
12  hours 

0 
0 

0.5 
0.3 

0 
Trace 

Amount  at  8  P.M.  during  previous 
12  hours      

Kind  

round  SAM 

'round  8  P  M 

As  soon  as  the  two  weather  maps  are  consulted,  it  will  be  seen  at  once 
that  it  is  an  area  of  low  pressure  which  is  going  to  dominate  the  weather 
of  the  Middle  Atlantic  States  during  Monday  night  and  The  motion 
Tuesday.      On  the  Sunday  map  it  will  be  seen  that  an  area  Of  the  highs 


of  high  pressure  of  only  moderate  intensity  is  controlling  the  J"1. 
weather  of  the  Atlantic  seaboard,  that  an  area  of  low  pres-  preceding 
sure  of  marked  intensity  and  regular  form  with  its  center  twenty-four 
just  north  of  Texas  is    dominating    the    weather    of    the 
whole  Mississippi  Valley,  and  that  an  area  of  high  pressure  is  just 
pushing  its  way  in  from  the  extreme  northwest.     By  8  A.M.  on  Mon- 


392  METEOROLOGY 

day,  as  indicated  on  the  map  of  that  day,  the  area  of  high  pressure  has 
moved  farther  out  over  the  Atlantic  Ocean  and  has  almost  released  its 
control  of  the  weather  of  the  Atlantic  seaboard.  The  area  of  low  pres- 
sure has  moved  southeast,  recurved,  and  is  now  pursuing  its  course  up 
the  Mississippi  Valley.  It  has  become  a  strong,  well-marked  formation, 
and  will  surely  dominate  the  weather  of  the  Middle  Atlantic  states 
during  the  next  two  days.  The  northwestern  area  of  high  pres- 
sure has  pushed  its  way  in  and  become  a  well-marked,  nearly  typical 
high. 

The  first  step  is  to  form  one's  opinion  as  to  the  path  which  the  low  will 
probably  follow,  and  the  location  of  its  center  twenty-four  hours  ahead, 

that  is,  at  8  A.M.  on  Tuesday.  In  this  case  either  the  Bigelow, 
of  toe°center  Russeu<>  °r  Van  Cleef  system  of  tracks  may  be  used.  Each 
of  the  low  contains  a  well-marked  track  which  corresponds  with  the 
tours^ahead.  m°tion  of  the  low.  One  would  expect  it  to  move  up  the 

Mississippi  Valley,  across  the  Great  Lakes,  and  down  the  St. 
Lawrence  Valley.  The  rule  for  locating  the  center  twenty-four  hours 
ahead  is  to  put  it  along  on  the  track  which  it  seems  to  be  following  the 
normal  amount,  and  then  to  modify  the  estimation  by  other  considera- 
tions. For  December  the  normal  amount  is  something  less  than  900 
miles  in  twenty-four  hours.1  One  would  thus  expect  the  center  to  be 
just  north  of  lake  Erie  at  8  A.M.,  Tuesday.  Now,  are  there  any  modify- 
ing circumstances  ?  Near-by  lows  tend  to  coalesce  and  lows  and  highs 
repel  each  other.  There  is  no  near-by  low  with  which  to  coalesce,  and 
the  Atlantic  high  is  too  weak  and  too  far  away  to  retard  it.  The  west- 
ern high  is  pushing  in  vigorously,  and  might  tend  to  push  the  low  east- 
ward a  little.  Lows  tend  to  follow  the  record  of  previous  days.  The 
low  has  hardly  covered  the  900  miles  during  the  previous  twenty-four 
hours,  so  that  it  seems  to  be  one  moving  with  a  little  less  than  normal 
speed .  Lows  tend  to  move  faster  after  they  recurve,  and  they  also  tend  to 
move  faster  and  grow  a  little  deeper  as  they  approach  the  Atlantic  Ocean. 
It  will  thus  be  seen  that  the  slow  motion  of  the  previous  twenty-four  hours 
has  been  explained,  and  one  would  expect  greater  speed  during  the  next 
twenty-four  hours.  If  the  Bowie  system  of  forecasting  is  used,  it  will  be 
seen  at  once  that  the  pressure  lines  are  crowded  together  in  the  north- 
western and  western  portion.  This  means  that  the  formation  would  be 
pushed  eastward.  It  will  also  be  noticed  that  the  region  of  high  winds  is 
between  the  low  and  the  following  high.  If  the  Guilbert  rules  are  held 
in  mind,  one  would  expect  again  that  the  formation  would  move  more 

1  For  the  numerical  data  about  highs  and  lows,  see  Chapter  VI,  part  B. 


WEATHER  PREDICTIONS  393 

than  the  normal  amount  eastward.  As  a  result  of  all  these  modifying 
considerations,  one  would  put  the  center  a  little  farther  along  its  course 
and  somewhat  eastward.  As  a  final  conclusion,  then,  Lake  Ontario 
might  be  chosen  as  the  probable  location  of  the  center  of  the  low,  8  A.M. 
on  Tuesday. 

The  Atlantic  high  will  have  moved  far  out  over  the  ocean,  and  the 
western  high  will  follow  eastward  behind  the  low;  but  neither 
of  these  formations  will  exert  any  influence  on  the  weather  able  motion 
at  Albany  during  the  time  interval  in  'question.  of  the  other 

The  next  question  is  this.  What  will  be  the  probable  dis- 
tribution of  the  meteorological  elements  about  the  low  on  this  Tuesday 
morning,  when  its  center  is  over  Lake  Ontario.  To  answer  this  one  must 
first  study  critically  the  present  distribution  and  then  esti-  The  ^stri, 
mate  the  probable  changes  caused  by  the  new  location.  The  bution  of 
central  pressure  is  29.6  inches.  This  is  just  about  normal  JJogSaiTie- 
for  a  fully  developed  typical  low,  and  the  form  of  the  isobars  ments  about 
and  the  direction  of  the  longer  axis  of  the  oval  are  also  very  e  low* 
typical.  The  wind  directions  form  a  very  perfect  counterclockwise  spiral 
about  the  area  of  low  pressure.  At  only  a  very  few  stations  there  are  un- 
usual or  unexpected  wind  directions.  In  the  northern,  eastern,  and  south- 
ern quadrants  the  wind  velocity  is  light  or  moderate.  In  the  belt,  at  the 
west,  between  the  low  and  the  high,  the  wind  velocities  are  large,  reach- 
ing the  velocity  of  a  storm  wind,  forty  or  more  miles  per  hour,  at  a  few 
stations.  The  temperature  lines  are  much  distorted.  The  low  is  char- 
acterized by  a  very  marked  rise  of  temperature  in  front  of  it,  and  a  sharp 
drop  on  the  western  side  of  the  center.  The  cloud  area  extends  far  out 
to  the  east,  reaching  to  the  coast.  On  the  western  side,  the  nimbus 
cloud  area  quickly  gives  place  to  a  clear  sky.  The  precipitation  has 
taken  the  form  of  rain,  and  has  been  general  in  the  northern,  eastern, 
and  southern  portions  of  the  storm.  The  quantity  has  not  been  large, 
as  it  varies  from  a  mere  trace  to  not  more  than  half  an  inch  at  any  sta- 
tion. What  changes  may  be  expected  in  this  distribution  of  the  elements 
due  to  the  new  location  of  the  center?  As  a  low  crosses  the  Great 
Lakes  and  nears  the  Atlantic  coast,  it  comes  into  regions  of  greater 
moisture  and  usually  becomes  a  little  deeper.  One  would  thus  expect, 
Tuesday  morning,  to  find  the  central  pressure  a  little  less,  and  the 
amount  of  precipitation  somewhat  larger,  but  in  other  respects  without 
change. 

During  Tuesday,  one  would  expect  the  low  to  pass  down  the  St. 
Lawrence  Valley  and  past  the  Albany  station. 


394  METEOROLOGY 

The  definite  picture  of  the  expected  changes  in  each  element  may 
now  be  formed. 

Pressure.  —  At  8  A.M.,  Monday,  at  Albany,  the  pressure  was  30.19, 
the  wind  direction  was  north,  and  its  velocity  was  two  miles  per  hour. 
The  definite  ^.11  this  means  that  the  departing  high  was  just  on  the  point 
picture  of  of  passing  over  the  weather  control  to  the  coming  low. 
weather*1  ^ne  would  thus  expect  the  pressure  to  drop  steadily  during 
changes  at  Monday  and  Tuesday.  If  the  estimated  central  pressure 
of  the  low  at  8  A.M.  -on  Tuesday  is  placed  at  29.4,  then  one 
would  expect  the  pressure  at  Albany  to  be  about  29.5  or  29.6  at  this 
time.  It  should  continue  to  fall  during  Tuesday,  reaching  its  lowest, 
perhaps  29.4  or  even  a  little  lower,  Tuesday  afternoon  or  night.  After 
the  center  of  the  low  had  passed  nearest  to  Albany,  the  pressure  would, 
of  course,  begin  to  rise. 

Wind.  —  At  8  A.M.,  Monday,  the  wind  was  north  with  a  velocity  of 
only  two  miles  per  hour.  It  will  be  remembered  that  the  wind  velocities 
on  the  eastern  side  of  the  low  were  light  or  moderate.  One  would  thus 
expect  only  light  or  moderate  winds  until  the  center  of  the  low  had 
passed.  The  direction  should  change  to  the  east  or  southeast  by  Mon- 
day night.  It  would  then  shift  more  to  the  south,  and  should  continue 
from  some  southerly  quarter  during  most  of  Tuesday.  Later  on  Tues- 
day the  wind  should  shift  to  the  southwest  and  west,  and  eventually 
northwest.  The  Hudson  River  Valley,  however,  runs  north  and  south. 
Thus,  due  to  a  local  influence,  one  would  expect  the  easterly  and  westerly 
components  to  be  lessened  and  the  winds  to  be  mostly  southerly  or 
northerly. 

Cloud..  —  At  8  A.M.,  Monday,  it  was  foggy  almost  to  the  point  of 
rain,  but  no  rain  had  fallen.  One  would  expect  it  to  remain  totally 
cloudy  during  the  whole  of  Monday  and  Tuesday.  The  nimbus  cloud 
area  ought  to  be  reached  before  Monday  night,  and  it  ought  to  continue 
during  the  whole  of  Tuesday.  If  it  should  rain  intermittently,  the 
cloud  would,  of  course,  be  called  some  form  of  stratus,  instead  of  nimbus 
when  there  was  no  precipitation. 

Moisture.  —  At  8  A.M.,  Monday,  it  was  foggy  with  a  relative  humidity 
of  100  per  cent.  With  the  rising  temperature,  the  relative  humidity 
would  grow  somewhat  less,  but  it  should  remain  high  all  during  the 
storm. 

Temperature.  —  At  8  A.M.,  Monday,  the  temperature  was  28.0°. 
The  maximum  during  the  previous  day  had  been  44°.  The  coming 
storm  was  characterized  by  a  marked  rise  in  temperature.  One  would 


WEATHER    PREDICTIONS  395 

thus  expect  a  maximum  on  Monday  well  above  44°,  say  50° ;  but  little 
drop  during  the  night ;  a  maximum  on  Tuesday  even  higher  than  on 
Monday,  say  55°  to  60°.  By  Tuesday  night,  the  temperature  should 
begin  its  rather  sharp  drop. 

Precipitation.  —  Up  to  8  A.M.,  Monday,  there  had  been  no  precipi- 
tation. With  the  rapidly  rising  temperature,  the  kind  of  precipitation 
would,  without  doubt,  be  rain.  The  amount  at  various  stations  had  been 
moderate,  a  trace  to  -  a  half  inch.  The  quantity  should  grow  larger. 
One  would  thus  expect  some  during  Monday  night  and  a  half  inch  or 
more  during  Tuesday. 

The  definite  picture  of  the  expected  weather  changes  during  Monday 
night  and  Tuesday  has  now  been  given.     It  remains  to  formulate  the 
forecast  such  as  a  local  forecast  official  would  issue.     There  The  official 
are  no  predictions  of  a  cold  wave,  frosts,  or  destructive  winds  forecast- 
to  be  made.     The  forecast  will  thus  be  concerned  with  temperature 
and   state    of  the  weather.     A  forecast  in   accord  with  the  definite 
picture  which  has  just  been  formed  would  be  :  For  Albany  and  vicinity, 
rain  and  warmer  to-night ;  Tuesday,  rain  with  continued  high  tempera- 
tures. 

The  weather  map  for  8  A.M.,  Tuesday,   Dec.   10,   1907,  is  repro- 
duced as  Chart  XL,  and  the  accompanying  table  contains  the  detailed 
observations  at  Albany  at  8  P.M.  Monday,  and  at  8  A.M.  Theverifica- 
and  8  P.M.  Tuesday.     By  studying  this  map  and  the  table  tion- 
it  becomes  evident  at  once  to  what  extent  the  predictions  verify. 

The  illustration  which  has  just  been  given  will  serve  to  elucidate  the 
method  of  forecastings.     There  is  no  royal  road  to  becoming  a  skillful 
forecaster.     Practice  is  the  essential  thing.     At  the  begin-  Experience 
ning  many  mistakes  will  be  made.     Influences  will  be  over-  is  essential- 
estimated  or  underestimated,  or  overlooked.     When  the  next  map  is 
issued  and  the  weather  occurs,  the  forecaster  is  not  left  to  wonder  help- 
lessly what  went  wrong.     It  is  nearly  always  evident  what  was  incor- 
rectly estimated,  and  these  very  mistakes  are  the  most  valuable  things 
in  the  acquirement  of  skill  and  experience. 


396 


METEOROLOGY 
OBSERVATIONS  AT  ALBANY 


DEC.  9,  1907 

DEC.  10,  1907 

Temp. 

(F.) 

8  A.M  

45.5 
51.0 
27.0 

53.0 
53.0 
45.5 

48.2 
55.0 

48.2 

Max.  previous  12  hours. 

Min.  previous  12  hours  

8  P.M  

Max.  previous  12  hours  

Min.  previous  12  hours.  .  .  .  

Pressure 
reduced 
to 
sea  level 

8  A.M  

29.95 
30.19 
29.95 

29.52 
29.95 
29.52 
29.40 
29.52 
29.40 

Max.  previous  12  hours  

Min.  previous  12  hours  

8  P.M. 

Max.  previous  12  hours. 

Min.  previous  12  hours. 

Wind 

[Direction  8  A.M  

S. 
15 

S. 
.    N. 
13 
3 

Direction  8  P.M  

Velocity  8  A.M  

Velocity  8  P.M  

Moisture  • 

f  Rel.  humid.  8  A.M  

70 

90 
92 

(Rel.  humid.  8  P.M 

Clouds 

IKind  and  direction  of  motion  8  A.M. 
Cloudiness  8  A.M  

Stratus  from  S. 
Cloudy  (10) 

Nimbus  from  S. 
Cloudy  (10) 
STimbus  from  N  . 
Cloudy  (10) 
light  fog 

Kind  and  direction  of  motion  8  P.M. 
Cloudiness  8  P.M  

Precipi- 
tation 
in  inches 

Snow  on  g 
Snow  on  g 

Amount  at  8  A.M.  during  previous 
12  hours. 

Trace 
Rain 

Trace 

.10 

.58 
Rain 

Trace 
Trace 

Amount  at  8  P.M.  during  previous 
12  hours  

Kind  

round  8  A.M  

round  8  P.M.  . 

WEATHER  PREDICTION  WHEN  V-SHAPED  DEPRESSIONS  AND  OTHER 
SECONDARY  ISOBARIC  FORMS  ARE  PRESENT 

384.   Whenever  a  weather  map  is  to  serve  as  the  basis  of  a  weather 
forecast,  the  form  of  the  isobaric  lines  must  be  critically  studied.    If  these 

All  second-  ^nes  nave  anv  otner  ^orm  tnan  tnat  of  ova^s  surrounding 
ary  isobaric  areas  of  low  or  high  pressure,  they  must  be  given  particular 
b°e™otcdUSt  attenti°n>  anc*  this  is  especially  the  case  if  the  general  east- 
ward motion  of  all  meteorological  formations  will  bring  them 
near  enough  to  the  station  in  question  to  exert  an  influence  during  the 


WEATHER  PREDICTIONS  397 

following  thirty-six  or  forty-eight  hours.  Two  of  these  isobaric  forms, 
other  than  the  oval  lows  and  highs,  have  received  special  names  and 
merit  consideration.  These  are  V-shaped  depressions,  and  secondary 
lows.  Other  isobaric  forms  have  been  named,  and  have,  perhaps, 
individual  characteristics,  but  their  influence  on  weather  is  so  slight 
that  they  do  not  merit  consideration. 

A  V-shaped  depression  is  a  pocket-like  bulge  or  projection  in  an  iso- 
baric line,  and  may  have  several  different  meanings.  If  two  areas  of 
low  pressure  are  quite  near  together,  the  isobaric  lines  in  The  various 
crossing  the  ridge  of  higher  pressure  between  them,  must  kinds  of  v- 
have  this  V-shaped  bend.  The  same  is  true  of  the  saddle  pre-Sons*5" 
which  separates  two  areas  of  high  pressure  which  are  near  and  what 
together.  In  these  two  cases,  the  V-shaped  bulge  has  no  they  Slgmfy> 
particular  significance  or  importance.  If  the  isobaric  lines  in  the 
southern  quadrant  of  a  low  all  have  this  V-shaped  bulge,  it  will  prob- 
ably be  found  that  the  low  has  a  well-marked  wind  shift  line.  Lows  with 
this  peculiarity  have  already  been  fully  discussed,  and  the  significance 
of  this  V-shaped  bulge,  as  far  as  coming  weather  is  concerned,  is  evident. 
If  an  isobaric  line  surrounding  an  extensive  high  shows  such  a  bulge, 
it  often  means  that  an  area  of  low  pressure  is  about  to  form,  that  is,  it 
marks  the  origin  of  an  extratropical  cyclone.  This  is  particularly  liable 
to  be  the  case  if,  in  addition  to  this  bulge,  the  wind  is  blowing  from  dif- 
ferent directions  at  near-by  stations  and  a  small  cloud  area  has  formed. 
Sometimes  a  V-shaped  depression  which  does  not  become  a  definite  low 
will,  however,  cau^e  thundershowers  in  summer  or  a  snow  flurry  in 
winter.  A  V-shaped  bulge  in  connection  with  an  area  of  low  pressure 
may  mean  that  a  secondary  low  is  about  to  form. 

The  general  rule  for  weather  forecasting,  in  connection  with  V-shaped 
depressions  is,  then,  to  note  carefully  if  they  exist  and,  if  so,  to  remember 
that  they  may  signify  simply  the  dividing  line  between  near-by 
lows  or  highs,  the  presence  of  a  wind-shift  line  in  connection  with 
a  low,  the  formation  of  a  new  area  of  low  pressure,  or  the  formation 
of  a  secondary  low. 

A  secondary  low  is  an  area  of  low  pressure  of  much  smaller  extent 
but  sometimes  of  greater  violence,  which  may  exist  in  the  southern  quad- 
rants of  a  much  larger  low.     They  are  not  very  common  in  secondary 
this  country,  but  quite  common  in  Europe.     They  usually  lows- 
move  eastward  and  northward,  faster  than  the  big  low,  so  that  they 
appear  to  circle  around  it  in  a  counterclockwise  direction.     Sometimes 
the  big  low  fades  away  and  the  secondary  assumesN^t^f  importance. 


398  METEOROLOGY 

In  weather  prediction,  it  is  best  to  consider  the  secondary  low  simply  as 
another  low,  and  forecast  accordingly.  The  explanation  of  the  popular 
expression  that  "  the  weather,  instead  of  clearing  off  as  it  gave  promise, 
has  had  a  relapse  "  is  often  to  be  found  in  the  existence  of  these  second- 
ary lows.  Charts  XLI  and  XLII,  which  represent  the  daily  weather 
maps  for  Dec.  27  and  28,  1904,  illustrate  well  the  formation  of  a  second- 
ary low  from  a  V-shaped  bulge.  On  the  27th  only  a  slight  V-shaped 
bulge  is  in  evidence,  while  on  the  28th  the  secondary  low  is  already 
fully  formed.  The  weather  maps  for  Jan.  27  and  28,  1908 ;  May  20 
and  21,  1908;  April  30  and  May  1,  1909;  March  6  and  7,  1910;  Oct. 
27  and  28,  1910 ;  Nov.  27  and  28,  1910,  also  illustrate  well  the  forma- 
tion of  secondaries. 

WEATHER  PREDICTION   CONSIDERING  THE  HIGH  AS  THE  DOMINATING 

FORMATION 

385.  If  it  is  a  passing  high  instead  of  a  low  which  is  going  to  dominate 
the  weather  for  the  coming  twenty-four  or  forty-eight  hours,  the  general 

method  of  procedure,  in  making  a  forecast,  is  just  the  same 
oAhe  as  *f  **  were  a  low.  Determine  first  where  the  center  of  the 

method  of  high  will  be  twenty-four  and  forty-eight  hours  ahead.  This 
predictions.  *s  done  by  noting  the  path  which  it  seems  to  be  following, 

and  then  putting  it  ahead  on  this  path  the  normal  distance 
covered  in  the  given  time.  This  estimated  position  is  then  modified  by 
taking  into  account  other  considerations,  which  may  be  simply  the 
general  result  of  experience  or  may  have  taken  the  form  of  definite  rules. 
Next  assume  that  the  distribution  of  the  meteorological  elements  about 
it  will  be  what  it  was,  with  the  exception  of  any  changes  which  its  new 
location  might  cause.  It  is  important  in  this  connection  to  hold  in 
mind  the  two  chief  ways  in  which  an  area  of  high  pressure  builds  and 
becomes  more  intense.  These  are  through  the  discharge  of  air  from 
lows  and  through  growing  colder  due  chiefly  to  radiation  to  a  clear 
sky.  After  the  probable  location  of  the  high  and  the  distribution  of 
the  elements  have  been  determined,  the  method  of  arriving  at  the  fore- 
cast is  just  the  same  as  that  for  the  low,  which  has  been  treated  in  detail. 

WEATHER  PREDICTION  BY  SIMILARITY  WITH  PREVIOUS  MAPS 

386.  Prediction  by  similarity  with  previous  maps  is  an  entirely  dif- 
ferent method  Ji^weather  forecasting  from  that  which  has  just  been  de- 


WEATHER  PREDICTIONS  399 

scribed  in  detail.     It  might  well  be  called  the  mechanical  or  auto- 
matic method  of  forecasting,  because  individual  judgment  plays  no 
part.  1  The  first  step  is  to  find,  in  the  series  of  weather  This  is  the 
maps,  that  one  which  is  the  most  similar  to  the  map  which  mechanical 
is  to  be  the  basis  of  the  forecast.     Weather  maps   have  method- 
been  issued  regularly  in  this  country  since   1871,  so  that  The  method 
the  file  covers  nearly  forty  years.     In  order  to  use   this  stated* 
method  of  forecasting,  it  presupposes  that  the  maps  have  been  classi- 
fied and  indexed  so  that  the  similar  map  can  be  found.     In  this  long 
series,  there  probably  would  always  be  at  least  one  map  which  would  be 
fairly  similar  to  the  map  under  consideration.     The  method  of  making 
the  forecast  is  simply  to  assume  that  the  weather  changes  which  took 
place  in  the  previous  instance  will  again  occur  in  the  present  case. 

The  six  daily  weather  maps  which  are  reproduced  as  Charts  XLIII- 
XLVIII  illustrate  well  this  method  of  forecasting.  The  three  maps  for 
Thursday,  Jan.  9,  1908,  for  Tuesday,  Jan.  14,  1908,  and  for  An  aiustra_ 
Monday,  Jan.  27,  1908,  are  extremely  similar.  In  each  tionofthe 
case,  there  is  a  high  with  a  very  moderate  central  pressure  method- 
(30.1-30.3)  with  its  center  over  the  Gulf  States,  and  this  high,  in  each 
case,  has  a  lip  or  projection  extending  over  the  Great  Lakes.  In  each 
case,  there  is  a  low  of  considerable  intensity  which  is  departing  by  way 
of  the  lower  St.  Lawrence  Valley,  and  another  low  of  considerable  inten- 
sity which*  is  pushing  in  from  the  extreme  northwest.  The  high  is 
accompanied  in  each  case  by  a  decided  drop  in  temperature  over  the 
Great  Lakes.  Further  points  of  similarity  can  be  detected  by  studying 
critically  the  distribution  of  the  elements  in  the  three  cases.  The  three 
maps  which  follow  are  extremely  similar.  In  each  case,  the  high  drifted 
eastward  the  same  amount  and  the  extension  over  the  Great  Lakes 
became  more  pronounced  and  was  accompanied  by  a  still  further  drop 
in  temperature.  In  each  case  the  coming  low  moved  eastward  about 
the  same  amount  and  developed  a  double  center,  becoming  really  a 
trough  of  low  pressure  extending  from  the  Great  Lakes  to  the  Gulf. 
Weather  prediction  for  the  Middle  Atlantic  and  New  England  states 
based  upon  the  principle  of  similarity  would  have  received  very  com- 
plete verification. 

THE  PREDICTION  OF  PECULIAR  AND  DANGEROUS  OCCURRENCES 

387.    The  prediction  of  frost.  — The  method  of  predicting  the  damage- 
causing  frosts  of  the  late  spring  and  early  autumn  was  described  in 


400  f  METEOROLOGY 

Chapter  V,  B,  (sections  206-210).  A  frost  is  predicted  by  the  officials 
of  the  U.  S.  Weather  Bureau  in  exactly  the  same  way  as  any  other 
The  method  temperature.  On  the  basis  of  the  weather  map  the  fore- 
of  frost  caster  must  estimate  the  probable  minimum  temperature 
on'  (real  air  temperature  in  the  thermometer  shelter)  on  the  fol- 
lowing morning.  In  making  this  estimation  he  will  be  guided  largely 
by  the  probable  clearness  of  the  sky  during  the  night  and  the  prob- 
able wind  velocity.  It  will  be  remembered  that  a  clear  sky  and  ab- 
sence of  wind  are  essential  for  a  large  drop  in  temperature  and  a  con- 
sequent frost. 

If,  after  the  probable  minimum  temperature  in  the  thermometer 
shelter  has  been  estimated,  it  is  desired  to  determine  the  probable 
temperature  of  low-growing  vegetation  in  the  open  at  various 
The  ca"^s  points  in  a  limited  area  surrounding  the  station  in  question, 
ence  be-  three  things  must  be  taken  into  account :  (1)  Plant  tempera- 
tween  the  tures  go  below  the  real  air  temperature  because  they  are  not 
in  a  ther-  sheltered  and  are  free  to  radiate  their  heat.  (2)  Vegetation 
mometer  js  iocated  near  the  ground  and  not  at  the  height  of  the  ther- 

shelter  and 

of  vegetation  mometer  shelter.     (3)  The  variation  in  temperature  over  a 
under  dif-      limited  area  is  often  considerable.     Thus  the  temperature 

ferentcondi-  . 

tions.  of  vegetation  in  the  open,  near  the  ground,  and  in  the  coldest 

part  of  a  limited  area  may  be  expected  to  be  from  5°  to  even, 
in  extreme  cases,  20°  lower  than  the  estimated  minimum  in  the  ther- 
mometer shelter.  These  facts  are  of  vital  importance  and  must  not  be 
overlooked. 

Frost  predictions  and  warnings  are  not  issued  by  local  forecast  officials, 
but  are  issued  only  by  the  district  forecasters  from  the  respective  centers. 
What  con-  Warnings  of  light  and  heavy  frost  will  be  verified  by  the 
stitutes  a  occurrence  of  light  and  heavy  frost  respectively ;  and  also 
Weather*6  ^v  a  reP°rted  minimum  temperature  of  40°  and  32°  respec- 
Bureau  tively,  accompanied  by  clear  or  partly  cloudy  weather  and 
light  winds  or  calm  during  the  period  for  which  frost  is  fore- 
cast. Thus  what  constitutes  a  frost  has,  in  a  certain  sense,  been  defined 
by  the  Weather  Bureau. 

388.  The  prediction  of  cold  waves.  —  The  meaning  of  the  term 
What  con-  "  co^  wave  "  has  been  made  definite  by  the  U.  S.  Weather 
stitutes  a  Bureau.  Nos.  299-304  of  the  "  Station  Regulations,"  which 

vave*     are  reproduced  here,  contain  the  definition. 
"  Cold  wave  warnings  will  be  ordered  when  it  is  expected  that  a  24- 
hour  fall  in  temperature,  equalling  or  exceeding  that  specified  for  the 


WEATHER  PREDICTIONS  401 

district,  will  occur  within  the  36  hours  following  the  observation  upon 
which  the  order  is  based,  accompanied  by  a  minimum  temperature  of 
the  required  degree,  or  lower.  The  districts  and  the  respective  temper- 
ature falls  and  minimum  temperatures  required  to  verify  cold  wave 
warnings  during  the  different  seasons  are  as  follows : 

"  In  northern  Maine,  northern  New  Hampshire,  northern  Vermont, 
northeastern  New  York,  western  Wisconsin,  western  Iowa,  Minnesota, 
North  Dakota,  South  Dakota,  Nebraska,  Montana,  Wyoming,  Idaho, 
eastern  Washington,  and  eastern  Oregon :  a  24-hour  fall  of  20°,  with  a 
minimum  of  zero  in  December,  January,  and  February,  and  a 
minimum  of  16°  from  March  to  November  inclusive. 

"In  southern  Maine,  southern  New  Hampshire,  southern  Vermont, 
Massachusetts,  Rhode  Island,  Connecticut,  New  York,  (except  north- 
eastern part),  northern  New  Jersey,  Pennsylvania,  Ohio,  Indiana, 
Michigan,  Illinois  (except  Cairo),  western  Maryland,  West  Virginia 
northern  Kentucky,  Missouri,  eastern  Iowa,  eastern  Wisconsin,  Kansas, 
Colorado,  the  Texas  panhandle,  northern  New  Mexico,  northern  Ari- 
zona, Utah,  and  Nevada;  a  24-hour  fall  of  20°  with  a  minimum  of  10° 
in  December,  January,  and  February,  and  a  minimum  of  24°  from  March 
to  November,  inclusive. 

"  In  southern  New  Jersey,  Delaware,  eastern  Maryland,  the  District 
of  Columbia,  Virginia,  western  North  Carolina,  the  northwestern  quar- 
ter of  South  Carolina,  northern  Georgia,  northern  Alabama,  northern 
Mississippi,  Tenessee,  Cairo,  southern  Kentucky,  Arkansas,  Oklahoma, 
Indian  Territory,  northern  Texas  (except  the  panhandle),  southern 
New  Mexico,  western  Washington,  and  western  Oregon  :  a  24-hour  fall 
of  20°,  with  a  minimum  of  20°  in  December,  January,  and  February, 
and  a  minimum  of  28°  from  March  to  November,  inclusive. 

"  In  eastern  North  Carolina,  central  South  Carolina,  central  Georgia, 
central  Alabama,  central  Mississippi,  northern  Louisiana,  and  central 
Texas :  a  24-hour  fall  of  18°  with  a  minimum  of  25°  in  December, 
January,  and  February,  and  a  minimum  of  32°  from  March  to  November, 
inclusive. 

"In  the  coast  region  of  South  Carolina  and  Georgia,  extreme  southern 
Georgia,  Florida,  extreme  southern  Mississippi,  southern  Louisiana, 
the  Texas  coast,  California,  and  southern  Arizona :  a  24-hour  fall  of  15°, 
with  a  minimum  of  32°  in  December,  January,  and  February,  and  a 
minimum  of  36°  from  March  to  November,  inclusive." 

The  best  precept  to  follow  in  making  a  cold  wave  forecast  is  perhaps 
the  following,  which  is  a  slight  modification  of  the  one  advocated  by 

2D 


402 


METEOROLOGY 


The  area  for 
which  a  cold 
wave  is  to 
be  pre- 
dicted. 


LOW 


the  U.  S.  Weather  Bureau.  Through  the  coming  area  of  high  pres- 
sure (a  cold  wave  is  always  caused  by  a  coming  high)  draw  two  axes  at 
right  angles  to  each  other  extending  north  and  south,  and 
east  and  west.  The  area  for  which  to  predict  a  cold  wave 
will  be  an  oval  area  located  entirely  in  the  southeast  quad- 
rant. A  cold  wave  is  particularly  sure  to  occur  if  a  passing 
low  is  located  to  the  east  of  the  high,  and  the  high  has  an 
oval  form  with  the  larger  axis  extending  northeast-southwest.  The  ac- 
companying figure  makes  clear  the 
area  for  which  the  cold  wave  is  to 
be  predicted.  The  brief  rule  is 
then  to  determine  first  if  the  prob- 
able drop  in  temperature  will  be 
sufficient  to  cause  a  cold  wave, 
and  then  to  predict  it  for  an  area 
located  as  stated  above.  Great 
caution  should  be  used  in  predict- 
ing it  for  any  other  area,  as  many 
failures  have  resulted  from  so 
doing. 

Cold  wave  forecasts  and  warn- 
ings are  issued    only  by  district 
forecasters,  and  not  by  local  fore- 
cast officials.      On  the  Washing- 
ton weather  map,  these  forecasts  are  indicated  by  the  letters    CW 
printed  near  each  station  for  which  a  cold  wave  is  predicted, 
•^reliction .     Chart  XLIX,  which  reproduces  the  daily  weather  map  for 
indicated.       Jan.  6,  1909,  illustrates  well  the  area  for  which  a  cold  wave 
is  predicted,  and  the  method  of  indicating  the  prediction  on 
the  weather  map. 

Bulletin  P  (W.  B.  publication  355)  of  the  U.  S.  Weather  Bureau,  by 
Edward  B.  Garriot,  entitled  "  Cold  Waves  and  Frosts  in  the  United 
illustrations  States,"  contains  328  charts  which  reproduce  the  daily 
of  cold  weather  maps  and  exhibit  the  meteorological  conditions 

which  attended  the  principal  cold  waves  from  1888  to  1902 
inclusive.  This  invaluable  publication  also  contains  a  brief  account  of 
the  origin  and  cause  of  cold  waves,  and  presents  a  chronological  account 
of  the  historic  cold  periods  in  the  United  States  prior  to  1888.  No  one 
wishing  to  make  cold  wave  forecasts  could  do  better  than  study  criti- 
cally this  exhaustive  treatise. 


FIG.   145.  —  Diagram    Illustrating   the  Area 
for  which  to  Predict  a  Cold  Wave. 


WEATHER  PREDICTIONS  403 

389.  The  prediction  of  tornadoes.  —  Since  a  tornado  usually  covers 
an  area  only  a  few  hundred  feet  wide  and  a  few  miles  lo»gf it  is  decidedly 
undesirable  to  alarm  a  whole  state  or  several  states  witli  the  . 
forecast  that  a  tornado  could  occur.     For  this  reason,  tor-  Of  tornadoes 
nadoes  are  never  predicted  by  the  U.  S.  Weather  Bureau.  are  not 
No.  244  in  the  "  Station  Regulations  "  covers  the  point. 

"  Forecasts  of  tornadoes  are  prohibited.  When  conditions  are  favor- 
able for  the  occurrence  of  destructive  local  storms,  the  term  severe 
thunderstorms  '  or  '  severe  local  storms '  may  be  used  by  district  fore- 
casters. The  phrase  '  conditions  are  favorable  for  the  destructive  local 
storms  '  will  be  used  only  by  the  Chief  of  Bureau,  or,  in  his  absence,  from 
the  Central  Office,  by  the  chief  of  the  Forecast  Division." 

In  a  previous  chapter  (sections  343  to  346)  there  was  given  a  full 
discussion  of  the  portions  of  the  country  most  frequented  by  tornadoes, 
the  time  of  day  and  season  of  occurrence,  the  type  of  low  whentor_ 
which  most  frequently  gives  rise  to  the  violent  thunder-  nadoes  are 
showers  which  are  accompanied  by  tornadoes,  and  the  loca-  posslble- 
tion  of  the  tornado  with  reference  to  the  low.     In  forming  one's  own 
opinion  from  a  weather  map  as  to  whether  a  tornado  might  occur,  there 
are  three  things  to  note  :  First,  is  the  type  of  low  one  which  is  suggestive 
of  tornadoes,  secondly,  is  it  the  season  of  the  year  when  they  are  likely 
to  occur,  thirdly,  is  the  place  where  they  could  be  expected  to  occur 
one  much  frequented  by  tornadoes. 

390.  The  prediction  of  destructive  wind  velocities  (storms).  —  Storm 
warnings  are  displayed  when  the  wind  is  expected  to  attain  a  velocity 
for  a  period  of  five  minutes,  equaling  or  exceeding  the  verify-  ^^  con_ 
ing  velocity  within  the   24  hours  following  the  time  that  stitutes  a 
the  warning  is  ordered  hoisted.     The  above  may  serve  as  a  s 
definition  of  what  is  meant  by  a  destructive  or  storm  wind.     There  are 
at  present  nearly  60  Weather  Bureau  stations  on  the  Atlantic  coast  and 
Gulf  coast,  on  the  Great  Lakes,  and  on  the  Pacific  coast  in  addition  to 
the  many  adjacent  storm  warning  display  stations,  where  these  warn- 
ings are  displayed.     The  verifying  velocity  is  different  for  The  veru«y_ 
each  station,  and  varies  from  roughly  20  to  60  miles  per  ingveioci- 
hour.     For  example,  at  New  York  it  is  44  miles  per  hour,  * 

at  Boston  32,  at  Portland,  Me.,   28,  at  Norfolk  26,  at  Buffalo  26,  at 
Chicago  46,  at  Duluth  40,  at  Seattle  35,  and  at  San  Francisco  36. 

The  probable  occurrence  of  a  verifying  velocity  is  pre- 
dicted in  the  same  way  as  any  other  wind  velocity.  A  storm 
warning  should  contain  the  location  of  the  storm  center,  tion. 


404  METEOROLOGY 

and  the  probable  direction  in  which  it  will  move,  with  a  forecast  of  the 
force,  direction,  and  shifts  of  the  wind.  These  warnings  are  issued  only 
by  a  district  forecaster,  and  the  flags  used  to  announce  them  are 
described  in  section  348. 

Bulletin  K  (W.  B.  publication  288)  of  the  U.  S.  Weather  Bureau,  by 

E.  B.  Garriott,  entitled  "  Storms  of  the  Great  Lakes/'  contains  952 

.        charts,  which  reproduce  the  daily  weather  maps  and  exhibit 

the  meteorological   conditions  which   attended  the  storms 

which  were  accompanied  by  a  wind  velocity  of  the  verifying  amount 

from  1876  to  1900  inclusive.     This  exhaustive  treatise  on  the  storms  of 

the  Great  Lakes  must  be  studied  critically  by  any  one  who  would  predict 

wind  velocities  for  this  region. 

391.  The  prediction  of  floods.  —  The  method  of  predicting  floods 
will  be  given  in  Chapter  X,  which  treats  of  floods  and  river  stages. 
The  reguia-  The  regulations  concerning  river  and  flood  forecasts  are 

ce°rrinC°river   the  followmg: 

and  flood  "  Flood  warnings  and  forecasts  of  river  stages  will  be  issued 

forecasts.  from  forecast  centers  that  are  river  centers,  and  from  specially 
designated  river  centers.  Copies  of  all  river  forecasts  and  warnings  will 
be  promptly  transmitted  to  the  Central  Office ;  regular  forecasts  on  the 
Daily  River  Forecast  card  (Form  No.  1086  —  Met'l),  and  special  fore- 
casts on  the  Special  River  Forecast  card  (form  No.  1087  —  Met'l). 
Flood  warnings  will  be  immediately  transmitted  by  telegraph  to  the 
Central  Office." 


THE  ACCURACY  AND  VERIFICATION  OF  PREDICTIONS 

392.  The  terms  used  in  official  predictions.  —  The  terms  which 
may  be  used  by  the  officials  of  the  U.  S.  Weather  Bureau  in  making 

predictions  are  all  prescribed  and  each  has  a  definite  mean- 
useVin^ffi-  m&-  This,  to  a  certain  extent,  hampers  individuality 
cial  predic-  and  a  free  expression  of  opinion  as  to  the  coming 
prescribed  weather,  but  it  is  necessary  in  order  to  prevent  am- 
ty  the  biguity  and  hedging,  and  to  make  possible  a  systematic 

Bureau*  verification  of  the  predictions.  The  terms  which  may  be 

used,  and  the  meaning  of  these  terms  in  connection  with 
forecasts  of  temperature  and  state  of  the  weather,  have  been  given  in 
section  382.  What  constitutes  a  dangerous  wind  velocity  (storm), 
a  cold  wave,  and  a  frost  has  been  stated  in  sections  390,  388,  387, 
respectively. 


WEATHER  PREDICTIONS  405 

393.    The  system  of  verification.  —  All  forecasts  and  warnings  are 
sent  to  the  Central  Office  of  the  Weather  Bureau  for  verification,  and 
the  only  official  verification  takes  place  there.     In  the  case  All  forecasts 
of  the  forecast  districts,  the  forecasts  and  warnings  are  sent  and  warn- 
each  day  to  Washington  by  the  forecasters  for  the  district.  ^w^shing- 
All  local  forecasts  of  temperature  and  state  of  the  weather  ton  for  veri- 
made  at  regular  stations,  and  all  practice  forecasts  in  connec-     cation- 
tion  with  temperature,  state  of  the  weather,  storm  winds,  cold  waves, 
and  frosts  are  entered  on  the  appropriate  form  (No.  1069  —  Met'l)  and 
forwarded  to  Washington.     At  the  central  office  of  the  U.  S.  Weather 
Bureau  at  Washington,  these  forecasts  and  warnings  are  verified  in 
accordance  with  a  definite  set  of  rules,  and  the  percentage  of  The  ^ 
accuracy  of  each  forecaster  can  be  determined.     The  fore-  verified  on 
casts  and  warnings  are  not  verified  as  a  whole,  but  on  five  five  differ- 

ent  counts. 

different   counts  —  that   is,    the   forecasts   of   temperature, 
state  of  the  weather,  storm  winds,  cold  waves,  and  frosts  are  all  verified 
separately.     These  rules  for  verifying  can  best  be  given  by  quoting 
from   the   "  Station   Regulations." 

"  Forecasts  and  warnings  and  practice  forecasts  and  warnings  will  be 
verified  by  the  following  rules : 

"  Rain  or  snow  forecasts  and  all    modifications  thereof,  The  rules 
containing  the  terms  l possibly/  'probably/  etc.,  will  be  veri-  jngpre-7" 
fied  by  the  occurrence  of  0.01  inch  or  more  of  precipita-  dictions, 
tion. 

"  Precipitation  to  the  amount  of  0.01  inch  or  more  which  is  not  fore- 
cast, or  which  is  forecast  and  does  not  occur,  will  be  charged  as  a 
failure. 

"  A  forecast  of  precipitation  will  be  considered  to  be  neither  a  success 
nor  a  failure  when  a  trace  of  precipitation  is  recorded  within  the  period 
specified. 

"  In  determining  the  percentage  of  verification,  the  number  of  fore- 
casts verified  will  constitute  the  dividend;  the  divisor  will  be  the  sum 
of  the  number  of  forecasts  verified,  the  number  of  forecasts  not  verified, 
and  the  number  of  occurrences  of  0.01  inch  of  precipitation  or  more 
which  were  not  forecast. 

"  Forecasts  of  temperature  change,  and  all  modifications  thereof  con- 
taining the  terms  '  possibly/  '  probably/  etc.,  will  be  verified  by  the 
occurrence  of  a  temperature  change,  of  the  kind  forecast,  equaling  or 
exceeding  the  stationary  limit  for  the  season. 

"  Temperature  changes  equaling  or  exceeding  the  stationary  limit 


406  METEOROLOGY 

which  are  not  forecast,  or  which  are  forecast  and  do  not  occur,  will  be 
charged  as  failures. 

"  In  determining  the  percentage  of  verification,  the  number  of  forecasts 
verified  will  constitute  the  dividend ;  the  divisor  will  be  the  sum  of  the 
number  of  forecasts  verified,  the  number  of  forecasts  not  verified,  and 
the  number  of  occurrences  of  temperature  changes  equaling  or  exceed- 
ing the  stationary  limit  not  forecast. 

"  A  storm  warning  will  be  verified  by  the  occurrence  of  a  verifying 
velocity  for  a  period  of  5  minutes  at  the  station  where  the  warning  is 
displayed,  or  by  the  occurrence  of  a  verifying  velocity  within  150  miles 
of  the  station,  within  24  hours  after  the  warning  was  ordered  hoisted, 
and  without  regard  to  changes  in  the  original  order  that  may  be  made 
during  the  period. 

"  The  continuance  of  a  verifying  velocity  for  a  period  of  20  minutes 
without  a  warning  will  be  considered  a  storm  without  a  warning. 

"  Not  more  than  one  verification,  or  one  storm  without  warning,  will 
be  counted  for  any  one  station  during  any  24-hour  period. 

"  In  determining  the  percentage  of  verification,  the  number  of  warn- 
ings verified  will  constitute  the  dividend ;  and  the  sum  of  the  number 
of  warnings  verified,  the  number  of  warnings  not  verified,  and  the 
number  of  storms  without  warnings  will  constitute  the  divisor 

"  Cold  wave  warnings  will  be  verified  by  the  occurrence  of  the  re- 
quired 24-hour  fall  in  temperature,  accompanied  by  the  required  mini- 
mum within  the  36  hours  following  the  regular  observation  on  which 
the  warning  is  based. 

"  The  occurrence  within  a  24-hour  period  of  the  temperature  change 
and  minimum  temperature  required  to  verify  a  cold  wave  warning,  for 
which  a  warning  was  not  issued,  will  be  counted  a  cold  wave  without 
warning. 

"  Not  more  than  one  verification,  or  one  cold  wave  without  warning, 
will  be  counted  at  any  one  station  during  any  24-hour  period. 

"  In  determining  the  percentage  of  verification,  the  number  of  cold 
wave  warnings  verified  will  constitute  the  dividend ;  and  the  sum  of  the 
number  of  warnings  verified,  the  number  of  warnings  not  verified,  and 
the  number  of  cold  waves  without  warnings  will  constitute  the  divisor." 

"  In  determining  the  percentage  of  verification,  the  number  of  frost 
warnings  verified  will  constitute  the  dividend;  and  the  sum  of  the 
number  of  frost  warnings  verified,  the  number  of  warnings  not  verified, 
and  the  number  of  frosts  and  verifying  temperatures  without  frost 
for  which  warnings  were  not  issued  will  constitute  the  divisor." 


WEATHER  PREDICTIONS  407 

394.    The  accuracy  attained.  —  The  statement  is  usually  made  that 
the  accuracy  attained  by  the  official  forecasters  of  the  U.  S.  Weather 
Bureau  is  between  80  and  85  per  cent.     This  is,  however, 
a  general  average  for  all  forecasters,  all  sections  of  the  coun- 
try,  and  all  five  of  they  lines  along  which  forecasts  and  warn-  is  above  8o 
ings  are  verified.     The  accuracy  which  can  be  attained  in 
different  parts  of  the  country  is  very  different,  (it  is  probably  easier 
to  forecast  for  California  than  any  other  part  of  the  country,  and  the 
hardest  part   is   perhaps   New  England.^  The  forecasts  of 
cold  waves  probably  verify  least  often,  as  the  accuracy  is  J^cy  dc-U" 
only  about  70  per  cent.     Some  forecasters  are  more  skillful  pendsonthe 
than  others,  but  the  accuracy  of  different  forecasters  pre-  ^e^er  e 
dieting  for  the  same  locality  would  probably  not  differ  as  and  the 
much  as  10  per  cent.     An  accuracy  of  85  per  cent  means  that  Jcte/'6" 
on  the  average,  one  day  in  seven  will  see  a  complete  failure 
of  the  weather  predictions.     It   must   be   held  in  mind  in  this  con- 
nection, however,  that  a  mere  guess  ought  to  yield  the  accuracy  of  but 
50  per  cent. 

The  accuracy  attained  in  Europe  is  about  the  same  as  in  this  country, 
although  the  terms  used  in  forecasting,  and  the  method  of  verification, 
are  quite  different  in  different  countries. 

Weather  forecasting,  as  regards  accuracy,  may  be  considered  on  about 
the  same  level  as  the  practice  of  medicine.     The  forecaster  can  diagnose 
the  present  condition  of  the  atmosphere  with  as  much  pre-  weather 
cision  as  a  physician  can  determine  the  bodily  condition  of  forecasting 
a  patient^  He  can  predict  the  coming  weather  certainly  with 

certainty  as  a  physician  can  predict  the  exact  course!  practice  of 
and   outcome   of   any  well-known   disease.     Weather    fore- 


casting can  probably  never  hope  to  attain  the  accuracy  of  astronomy 
in  predicting  celestial  occurrences. 

The  increase  in  accuracy  during  the  last  fifteen  years  has  been  small, 
and  this  would  seem  to  indicate  that  present  methods    can  yield  no 
greater  accuracy.     This  certainly  does  not  mean  that  no  elab-  The  accu 
orate  investigations  have  been  undertaken  to  determine  if  racy  has  not 
a  new  system  could  be  devised  or  the  present  methods  im-  increased 
proved.     The  weather  map  for  the  whole  northern  hemi- 
sphere has  been  constructed  daily,  and  the  permanent  highs  and  lows 
have  been  critically  studied  in  order  to  determine  if  changes  in  them 
exert  an  influence  on  the  passing  highs  and  lows   and  thus  determine 
their  path,  velocity,  and  characteristics.     The  weather  maps  for  differ- 


408  METEOROLOGY 

ent  levels  above  the  earth's  surface  have  been  constructed  in  order  to 
determine  if  the  passing  meteorological  formations,  as  portrayed  at  these 
.  levels,  are  less  erratic  in  their  behavior  than  at  the  earth's 
vestigations  surface.  The  areas  of  greatest  and  least  pressure  and  tern- 
have  been  perature  change  have  been  studied  critically  to  determine 

undertaken      f*.  .  J 

to  improve  if  these  are  any  more  regular  in  their  behavior  tha^n  highs 
or  lows.  The  variations  in  the  energy  received  from  the  sun 
have  been  determined  and  studied  in  order  to  ascertain  if 
these  influence  the  highs  and  lows.  Of  these,  the  study  of  the  per- 
manent highs  and  lows  has  yielded  the  greatest  results,  but  none  of  these 
investigations  has  revolutionized  the  methods  of  forecasting  or  added 
much  to  the  accuracy. 

395.    The  popular  idea  of  the  accuracy  of  weather  forecasts.  —  The 

newspapers  and  periodicals  are  essentially  just,  fair,  and  considerate 

in  their   attitude  towards  the   Weather  Bureau,   and  the 

papers  and     failure  of  some  predictions  to  verify.     It  is  very  seldom  that 

periodicals      a  hostile  editorial  is  seen  or  a  vindictive,  sarcastic  attitude 

are  fair. 

or  comment  is  met  with.  There  are,  of  course,  many  jokes 
at  the  expense  of  the  Weather  Bureau  and  its  failures.  There  are  also 
many  jokes  about  the  repairs  necessary  to  an  automobile,  but  that  does 
not  prove  that  the  automobile  is  not  both  a  useful  and  pleasure-giving 
vehicle.  There  are  a  very  few  who  sneer  at  the  Weather  Bureau  and 
The  declare  that  it  should  be  done  away  with.  Those  who  have 

Weather  this  attitude  are  usually  those  who  really  know  very  little 
very  profit-  about  meteorology,  the  work  of  the  Weather  Bureau,  or  the 
able  in-  value  of  its  predictions.  The  Weather  Bureau  certainly  saves 

to  this  country  annually  at  least  three  and  probably  ten  times 
its  cost.  It  thus  gives  very  handsome  returns  for  what  is  expended, 
and  any  one  who  sneers  at  its  work  is  purely  a  useless,  destructive 
critic,  unless  he  can  propose  some  plan  which  would  lead  to  greater 
returns. 

It  has  often  been  said  that  it  would  be  very  desirable  if  the  weather 
map  could  receive  such  wide  distribution  that  it  would  come  to  the 
notice  of  every  one,  and  at  the  same  time  the  public  could  be  educated 
to  interpret  the  map.  Every  person  would  then  make  his  own  predic- 
tions, and  the  attitude  of  a  person  towards  what  he  does  himself,  even  if 
imperfect,  is  always  very  different  from  what  it  is  towards  the  same 
thing  done,  perhaps  somewhat  better,  by  some  one  else. 


WEATHER  PREDICTIONS  409 


LONG-RANGE  PREDICTIONS 

396.   Prediction   from   station   normals.  —  The   definite   predictions 
made  by  the  officials  of  the  U.  S.  Weather  Bureau  on  the  basis  of  the 
daily  weather  map  are  for    only  thirty-six  or  forty-eight 
hours  ahead,  and  it  is  impossible  to  extend  the  period  with  meant  by 
any  certainty  to  more  than  three  days,  even  in  the  case  of  ^ng-Taxige 
the  most  typical  maps.     Recently  the  attempt  has  been  f 
made  to  issue  a  general  prediction  for  a  whole  week,  and  the  basis  of  this 
will  be  discussed  later.     The  popular  desire  is,  however,  not  for  predic- 
tions for  a  few  hours  ahead  or  at  most  a  week  ahead,  but  for  predictions 
for  a  month,  or  several  months,  or  a  whole  year  in  advance.     These  are 
called  long-range  predictions. 

The  attempt  has  sometimes  been  made  to  use  the  station  normals 
for  the  various  meteorological  elements  as  the  basis  of  long-range  pre- 
dictions.    For  example,  for  the  northeastern  part  of  New  No  Definite 
York  State,  it  could  be  stated  with  certainty  on  the  basis  of  predictions 


these   various   normals   that     the   temperature   would   fall          6  made 


below  zero  at  least  once  during  the  winter,  that  there  would  station 
be  at  least  one  thundershower  during  the  summer,  that  at  E 
least  two  feet  of  snow  would  fall  during  the  winter,  etc.     These  state- 
ments are,  however,  not  really  long-range  predictions,  but  merely  a 
description  of  the  climate  of  the  locality.     Furthermore,  they  give  no 
indication  as  to  what  the  weather  is  going  to  be  on  any  definite  date,  or 
even  whether  the  season  is  going  to  be  too  hot,  or  too  cold,  too  wet,  or 
too  dry. 

The  attempt  has  also  been  made  to  use  departures  from  normal  as 
the  basis  of  long-range  predictions.     There  is  a  strong  popular  belief 
that,  for  example,  if  a  certain  month  is  too  hot  or  too  dry,  Departure 
the  next  month  will  be  too  cold  or  too  wet,  in  order  to  com-  from  no*mai 

cannot  be 

pensate.     Now  these  departures  from  normal,  particularly  used  as  a 
in  the  case  of  temperature  and  precipitation,   have  been  {^sisr^ 
studied  critically  for  many  stations  and  for  many  years,1  weather 
and  the  conclusion  drawn  from  such  research  work  has  always  predictions. 
been  that,  from  the  departure  from  normal  on  the  part  of  any  particular 
month,  no  conclusion  whatever  can  be  drawn  as  to  whether  the  follow- 
ing month  will  be  too  hot  or  too  cold,  too  wet  or  too  dry. 

Much  research  work  has  also  been  done  on  the  probable  character- 
istics of  one  season  as  determined  by  the  departure  from  normal  of  a 

i  See  Bulletin  U  of  the  U.  S.  Weather  Bureau. 


410  METEOROLOGY 

previous  season.  Koppen  has  found,  for  the  middle  of  Europe,  that 
Seasonal  the  probability  of  a  departure  from  normal  in  the  other  di- 
sequence.  rection  in  the  case  of  temperature,  from  one  season  to  an- 
other, is  as  follows  : 

winter  to  spring        0.49  spring  to  summer  0.45 

winter  to  summer     0.44  spring  to  autumn  0.40 

summer  to  autumn  0.38  autumn  to  winter  0.45 

summer  to  winter     0.50  autumn  to  spring  0.52 

This  means,  for  example,  that  the  probability  of  a  cold  autumn  follow- 
ing a  warm  summer  is  38  out  of  100.  It  will  be  seen  at  once  that  the 
preponderance  of  probability  is  too  slight  to  make  this  of  much  value  in 
forecasting.  At  any  rate,  no  definite  forecasts  can  be  made.  Hellmann 
has  found,  for  Berlin,  that  there  is  the  greatest  probability  of  the  follow- 
ing successions,  but  the  preponderance  of  probability  was  always  small  : 

moderately  mild  winter cool  summer 

very  mild  winter warm  summer 

moderately  cold  winter cool  summer 

very  cold  winter       very  cool  summer 

moderately  warm  summer moderately  mild  winter 

very  warm  summer .  cold  winter 

Similar  probable  sequences  could  be  worked  out  for  any  station  on  the 
basis  of  the  observations,  and  the  preponderance  of  probability  could 
be  determined  for  each  sequence.  It  is  of  too  little  value  in  forecasting, 
however,  to  make  it  worth  while.  If  it  has  been  done,  one  might  say, 
for  example,  after  a  cold  winter  had  passed,  that  the  probability  of  a 
warm  summer  was  perhaps  46  out  of  the  100.  This  would  be  very  far 
from  a  satisfying  long-range  prediction. 

397.  Weather  cycles.  —  The  daily  and  annual  change  in  the  values  of 
the  meteorological  elements  is,  of  course,  very  marked.  There  is  always 
a  pronounced  diurnal  and  annual  variation,  and  the  charac- 
cycies  have  teristics  and  magnitude  of  these  variations  in  the  case  of  each 
been  de-  element  have  been  fully  discussed  in  previous  chapters.  Inves- 
tigators have  also  thought  they  have  found  several  minor 
periods  or  cycles  which  underlie  the  changes  in  the  values  of  the  elements. 
Cycles  of  3  days,  5.5  days,  11  years,  and  35  years  without  doubt  exist.1 

Cycles  of  26.7  days,  corresponding  to  the  sun's  rotation,  7  years, 
A  criterion  19  years,  corresponding  to  the  nutation  period,  and  many 
of  value.  others  have  been  announced  by  different  investigators.  In 
fact,  the  number  of  cycles  has  become  so  large  that  it  is  a  serious  ques- 

1  See  Monthly  Weather  Review,  April,  1899,  p.  156. 


WEATHER  PREDICTIONS  411 

tion  if  some  criterion  should  not  be  used  to  determine  if  a  cycle  is  worthy 
of  consideration.  It  could  well  be  said  that  a  cycle  is  not  worth  con- 
sidering, unless  (1)  it  is  very  well  marked,  if  it  occurs  at  only  a  few  sta- 
tions ;  (2)  it  occurs  at  many  stations,  if  it  is  of  small  magnitude ;  (3)  it 
corresponds  to  something  else  which  has  the  same  period  and  could  be 
its  cause. 

The  three-day  cycle  is  probably  the  average  interval  between  the 
passing  of  a  low  and  the  coming  of  the  following  high.  The  5.5-day 
cycle  is,  in  a  certain  sense,  a  double  three-day  cycle.  It  is  The  s.s-day 
probably  the  average  time  interval  between  the  passage  of  cycle- 
well-marked  lows  near  any  given  station.  It  is  evident  to  any  one  who 
follows  the  daily  weather  maps,  day  after  day,  that  these  two  cycles 
may  be  said  to  exist,  but  they  are  very  far  from  definite  and  could  not 
be  projected  into  the  future  more  than  two  weeks  at  the  very  most,  and 
only  then  with  great  uncertainty. 

The  eleven-year  cycle  corresponds  to  the  sunspot  period,  and  is  very 
pronounced  in  terrestrial  magnetism  and  atmospheric  electricity.  It 
is  easily  found  in  certain  of  the  meteorological  elements,  The  eieven- 
but  the  magnitude  of  the  variation  is  extremely  small,  and  vear  cycle- 
the  observations  must  be  carefully  averaged  at  many  stations  fora 
long  time,  to  detect  it  with  certainty.  Its  presence  has  been  detected 
chiefly  in  connection  with  the  number  of  tropical  cyclones,  the  amount 
of  precipitation,  and  the  tracks  followed  by  lows.1 

The  cycle  of  35  years  is  due  to  Bruckner,  and  is  particularly  noticeable 
in   connection  with  temperature   and   precipitation.     Not   only   have 
direct  observations  been  used  in  the  discussion,  but  such  The  thirty- 
indirect  data  as  the  dates  of  harvests,  the  opening  and  clos-  five-year 
ing  of  navigation,  the  height  of  inclosed  seas,  the  severity  of  cycle* 
winters,  etc.     In  fact,  even  the  size  of  the  circle  of  annual  growth  in 
the  case  of  very  old  trees  has  been  used. 

European  observations  show  that  during  the  last  two  centuries  — 
1746-1755,  1791-1805,  1821-1835 ;    1851-1870  were  relatively  warm. 
1731-1745,   1756-1790,   1806-1820,   1836-1850,    1871-1885,  were  rela- 
tively cold. 

1756-1770,  1781-1805,  1826-1840,  1856-1870  were  relatively  dry. 
1736-1755,  1771-1780,  1806-1825,  1841-1855,  1871-1885  were  relatively 
wet. 

The  amount  of  the  oscillation  in  the  case  of  the  precipitation  is  very  dif- 
ferent in  different  countries,  and  is  usually  greater  in  the  interior  than  near 

1  See  W.  J.  HUMPHREYS,  Astrophysical  Journal,  September,  1910. 


412  METEOROLOGY 

the  coast.     On  the  average,  it  amounts  to  about  20  per  cent  of  the  total 

amount.     The  period  is  not  strictly  35,  but  varies  from  34  to  36  years. 

It  will  be  seen  at  once  that  all  of  these  cycles  are  too  indefinite  and 

of  too  little  importance  to  play  any  part  in  weather  fore- 
are  of  no        casting  or  serve  as  a  basis  for  long-range  predictions, 
forecastin  398'    Tendency  of  a  weather  type  to   continue.  —  If  the 

ceaseless  changes  in  the  values  of  the  meteorological  elements 
are  carefully  studied,  it  will  be  apparent  that  the  number  of  changes  in 
the  weather  from  one  day  to  the  next  is  smaller  than  the  number  of 
There  is  a  continuations  of  the  same  kind  of  weather.  That  is,  after  a 
decided  ten-  cold  day,  there  is  a  greater  probability  of  another  cold  day 
fhTexisting  tnan  °^  a  warm  day,  and  after  a  rainy  day  there  is  a  greater 
weather  to  probability  of  another  rainy  day  than  of  a  fair  day. 

There  is  thus  a  decided  tendency  for  the  existing  weather  to 
continue.  At  Brussels,  for  example,  the  probability  of  a  change  after 
a  certain  number  of  days  with  a  certain  character  as  regards  tempera- 
ture and  rain  is  given  in  the  following  table : 

After  a  continuance  of  1       2       3       4        5       6       7      10      15  days 

Temperature  .25    .24    .22    .21     .17    .17    .15    .15    .13 

Rain  .37    .32    .30    .26    .27    .24    .25    .23    .23 

This  means  that  if  there  have  been  four  rainy  days  in  succession,  the 
probability  of  the  next  day  being  fair  is  only  26  out  of  100.  A  similar 
table  could  be  prepared  for  any  station,  and  the  same  decided  tendency 
of  existing  weather  to  continue  would  be  found.  This  table  also  shows 
that  if  a  weather  forecaster  in  Brussels  should  each  day  systematically 
predict  that  the  next  day  would  be  the  same  as  regards  temperature  and 
rainfall,  he  would  attain  an  accuracy  in  forecasting  of  about  75  per  cent. 
If  longer  periods  of  time  are  considered,  it  will  be  found  that,  as  soon 
as  a  definite  type  of  weather  has  become  established,  there  is  a  decided 
tendency  for  this  type  to  continue  several  days  or  perhaps 
weather  also  even  several  weeks.  A  warm  rainy  period  is  apt  to  continue 
tends  to  fOr  a  week  or  two,  and  cold  dry  weather  usually  lasts  equally 
long.  During  the  types,  the  highs  and  lows  tend  to  follow 
the  same  tracks  and  have  the  same  characteristics.  This  is  a  matter 
This  is  due  of  great  practical  importance  in  forecasting,  and  more  and 
manen?er"  more  attention  is  being  paid  to  it.  This  tendency  of  a 
areas  of  weather  type  to  continue  is  probably  the  result  of  what  Teis- 
iowhpres-  serene  de  Bort  calls  the  "  centers  of  activity  "  of  the  atmos- 
sure.  phere.  If  the  chart  which  exhibits  the  isobaric  lines  for 

January  is  studied,  it  will  be  seen  that  in  the  northern  hemisphere  there 


WEATHER  PREDICTIONS  413 

are  four  permanent  highs,  an  immense  one  over  Asia,  a  smaller  one 
central  over  North  America,  and  two  still  smaller  ones  over  the  Atlantic 
and  Pacific  oceans.  There  are  also  two  permanent  areas  of  low  pressure 
with  their  centers  over  the  north  Atlantic  near  Iceland  and  over  the  north 
Pacific  south  of  Alaska.  These  areas  are  the  so-called  centers  of  activity 
and  remain  practically  fixed.  They  are  the  permanent  landmarks  of 
pressure,  and  stand  in  sharp  contrast  to  the  moving  areas  of  low  and 
high  pressure.  If  any  change  occurs  in  the  location  or  intensity  of  these 
permanent  areas,  the  moving  areas  of  high  and  low  pressure  have  dif- 
ferent characteristics  and  move  over  different  tracks,  and  thus  give  rise 
to  a  different  type  of  weather.  Since  these  permanent  areas  of  pressure 
change  their  characteristics  slowly  and  reciprocally,  there  is  a  tendency 
for  a  given  weather  type  to  continue  for  several  days,  or  even 'several 
weeks.  The  general  forecasts  which  have  lately  been  attempted  at 
Washington  for  a  week  ahead,  are  based  upon  a  study  of  these  perma- 
nent areas  of  pressure  and  this  tendency  of  a  weather  type  to  continue.  ' 
These  centers  of  action  change  with  the  time  of  year.  A  series  of  iso- 
baric  charts,  exhibiting  the  normal  pressure  for  every  month  in  the  year, 
would  be  necessary  for  the  full  discussion  of  the  subject.  The  exact 
type  of  weather  which  exists  for  given  characteristics  on  the  part  of  these 
areas  has  not  yet  been  fully  determined.  In  fact,  not  much  more  has 
been  done  than  to  recognize  the  importance  of  these  areas  and  the  fact 
that  the  type  of  weather  probably  does  depend,  to  a  large  extent  on  their 
characteristics.1  The  pressure  distribution  over  the  whole  northern 
hemisphere  for  January  28,  1910,  is  given  as  chart  L.  This  is  an  illus- 
tration of  the  daily  pressure  map  as  made  at  Washington  for  the  whole 
northern  hemisphere  (see  section  362).  The  Siberian  high  has  an  un- 
usual intensity.  The  low  over  Europe  is  the  strong  persistent  one  which 
caused  the  excessive  precipitation  over  France,  which  resulted  in  the 
great  floods  at  Paris  in  January,  1910. 

399.    Popular  superstitions  and  credulity.  —  In  the  foregoing  para- 
graphs (sections  396,  397,  398),  the  science  of  meteorology 
has  presented  all  that  it  has  to  offer  in  connection  with  long- 
range  weather  predictions.     On  the  basis  of  the  daily  weather  what  can  be 
map,  exact  predictions  for  the  coming  36  or  48  hours  can  be  JJ?^ 
made,  and  these  verify  in  about  80  to  85  per  cent  of  the  along  the 
cases.     Based  on  the  tendency  of  a  weather  type  to  con-  ^thtr 
tinue,   and  on  a  study  of  the  centers  of  activity  in    the  prediction. 

1  For  a  short  bibliography  of  articles  on  this  subject  see  Bulletin  of  the  Mount  Weather 
Observatory,  vol.  Ill,  part  4,  p.  237. 


414  METEOROLOGY 

northern  hemisphere,  it  is  possible  to  make  a  general  forecast  for  per- 
haps a  week  ahead.  It  was  seen  that  long-range  weather  predictions 
could  not  be  made  with  any  desirable  definiteness  or  certainty  based  on 
station  normals,  departures  from  normal,  or  weather  cycles.  Since 
this  is  the  state  of  the  case,  the  U.  S.  Weather  Bureau  and  scientific 
meteorologists  are  frank  and  honest  enough  to  admit  that,  at  present, 
long-range  weather  predictions  are  an  impossibility.  Long-range 
weather  predictions  are  very  desirable,  and  every  means  of  making  them 
is  being  thoroughly  investigated ;  but  simple  honesty  demands  that  the 
admission  be  made  that,  at  present,  there  is  no  way  known  of  making 
them. 

The  popular  desire,  however,  is  for  long-range  weather  predictions, 
and,  this  being  the  case,  it  is  not  remarkable  that  various  attempts  are 
The  various  made  to  satisfy  this  desire  in  one  way  or  another.  Some 
things  which  try  to  make  long-range  predictions  for  themselves  and 
basis  for a  friends,  using  as  a  basis  the  action  of  some  animal,  or  the 
long-range  weather  on  some  definite  date,  or  something  connected  with 
recasts.  ^  moon  Others,  bolder  and  usually  with  a  desire  for 
financial  gain,  publish  these  forecasts  in  almanacs  and  the  like.  Some 
newspapers  even  publish  these  predictions,  and  worse  than  this,  they 
actually  pay  for  the  privilege  of  publishing  these  worse  than  useless  pre- 
dictions. Now  if  these  long-range  predictions  are  not  mere  guesses,  the 
various  things  which  serve  as  a  basis  may  be  grouped  under  three 
heads  :  (1)  the  influence  of  the  moon,  the  sun,  or  the  planets  —  in  short, 
astronomical  control ;  (2)  the  actions  of  animals,  birds,  and  plants ; 
(3)  the  weather  during  certain  days,  months,  or  season.  Now,  in  the 
case  of  the  sun  and  moon,  as  was  stated  in  connection  with  weather 
cycles,  there  may  be  a  very  slight  influence  on  some  phases  of  the  weather, 
but  the  influence  is  so  slight  that  it  is  almost  impossible  to  find  it  with 
certainty.  It  is  certainly  so  small  that  it  can  play  absolutely  no  part  in 
forecasting.  As  for  the  rest,  there  is,  in  the  first  place,  absolutely  no 
scientific  reason  why  they  should  have  any  connection  with  the  coming 
weather ;  and,  in  the  second  place,  the  observations  at  many  stations  have 
been  averaged  for  many  years  to  determine  if  they  do  have  any  influence, 
and  the  result  has  always  been  to  find  that  they  do  not.  Thus  all  long- 
range  forecasts  built  upon  such  things  are  mere  superstitions  and  have 
no  foundation  whatever.  They  are  no  better  than  mere  guesses,  and  it 
should  be  remembered  that  a  mere  guess  should  be  correct  half  the  time, 
so  that  there  should  be  no  surprise  at  some  chance  verifications.  It 
might  perhaps  be  agreed  that  those  who  make  these  long-range  forecasts 


WEATHER  PREDICTIONS  415 

simply  for  themselves  and  friends,  should  be  allowed  the  possible  pleas- 
ure of  this  probably  harmless  self-delusion.  It  is  a  very  different 
matter,  however,  in  the  case  of  those  who  publish  their  forecasts  and 
particularly  those  who  publish  them  for  financial  gain.  They  are  not 
only  making  gain  at  the  expense  of  the  superstition  and  credulity  of  the 
public,  but  they  are  making  the  public  believe  that  something  can  be 
done  which  cannot  be  done. 

These  published  forecasts  are  of  all  degrees  of  definiteness.     Some- 
times,  in  some  almanacs,  the  statement  "  Snow  may  be  expected  at  this 
time  "  will  be  found  on  the  margin,  and  extending  over  a  A  typjcai 
third  of  a  winter  month.     Since  this  would  be  verified  by  long-range 
snow  occurring  anywhere  in  the  United  States  at  any  time  f 
during  the  period,  it  is  needless  to  add  that  it  is  certain  of  verification. 
Sometimes  the  more  elaborate  forecasts  run  something  like  this  :  Febru- 
ary 21  to  the  end  of  the  month,  constitutes  a  storm  period  of  marked 
energy.     There  will  be  snow  in  the  northwest,  gales  over  the  Great 
Lakes,  and  freezing  weather  on  the  Atlantic  coast.     A  storm  of  marked 
energy  will  move  from  west  to  east  across  the  country.     Its  coming  will 
cause  warmer  weather  with  rain  or  snow.     It  will  be  followed  by  north- 
west winds  and  colder  with  snow  flurries  in  New  England.     There  will 
be  thundershowers  in  the  Southern  states  and  the  month  which  closes 
with  this  period  will  average  too  warm.     The  above  is  copied  from  no 
source,  has  no  basis  whatever,  and  can  be  applied  to  any  year.     It  is 
simply  a  mixture  of  meteorological  information,  meteorological  statis- 
tics, glittering  generalities,  and  plain  guesses  :  and  any  forecaster  of  the 
U.S.  Weather  Bureau,  and  any  scientific  meteorologist  could  write  predic- 
tions like  the  above  by  the  page  and  volume  if  he  were  willing  to  cheapen 
and  degrade  his  science  to  that  extent.     There  is  never  an  in- 
terval of  seven  days  at  the  end  of  February  without  a  low  of  analysis  of 
fair  intensity  crossing  the  United  States  somewhere.     Lows  such  a 
always  move  from  west  to  east,  and  are  preceded  by  south 
winds  and  warmer  weather  and  are  followed  by  northwest  winds  and 
colder  weather.     All  this  is  simply  meteorological  information.     Now, 
as  regards  the  gales  on  the  Great  Lakes,  snow  flurries  in  New  England, 
and  thundershowers  in  the  Southern  states,  if  a  series  of  weather  maps 
covering  these  seven  days  for  the  last  twenty  years  were  carefully  studied, 
it  would  be  seen  that  these  occurrences  took  place  about  18  out  of  the 
20  years.     The  chance  is  thus  ten  to  one  in  favor  of  these  things  happen- 
ing.    It  is  therefore  an  extremely  safe  prediction  considering  statistics. 
Freezing  weather  on  the  Atlantic  coast  is  a  glittering  generality,  as  the 


416  METEOROLOGY 

32°  F.  line  always  intersects  the  Atlantic  coast  somewhere.  That  the 
month  would  average  too  warm  is  a  plain  guess,  and  thus  stands  an  even 
chance  of  being  right  for  any  station.  Furthermore,  there  are  certain 
to  be  many  localities  where  it  will  verify. 

The  widespread  belief  in  the  existence  of  an  equinoctial  storm  and 

Indian  summer  comes,  to  a  certain  extent,  under  the  head  of  popular 

.        superstitions.     If  the  equinoctial  storm  is  defined  as  a  rain- 

noctiai  storm  storm,  lasting  at  least  three  days,  and  occurring  within  two 

and  Indian     or  three  days  of  the  21st  of  September,  then  there  is  seldom  a 

summer. 

year  when  one  occurs.  If,  however,  the  equinoctial  storm 
is  defined  as  any  rainstorm  lasting,  say  a  day  or  longer,  and  occurring 
within  two  weeks  of  the  21st  of  September,  then  there  is  very  seldom  a 
year  when  several  equinoctials  do  not  occur.  The  reason  for  the  belief 
in  an  equinoctial  storm  is  probably  the  fact  that  about  this  time  of  year 
the  first  storms  of  the  winter  type,  with  steadily  falling  precipitation, 
make  their  appearance.  They  stand  in  sharp  contrast  to  the  summer 
type  with  the  sultry  weather  and  thundershowers.  Storms  of  the 
winter  type  can  occur,  however,  during  any  month  of  the  summer. 
The  amount  of  the  precipitation  near  the  21st  has  been  shown  by  averag- 
ing the  observations  at  many  stations  to  be  no  greater  than  before  or 
after  that  date.1 

The  case  is  similar  with  Indian  summer.  If  Indian  summer  is  defined 
as  a  spell  of  peculiar  weather  in  the  autumn,  characterized  by  great 
warmth,  smokiness,  and  haziness,  and  lasting  for  several  weeks,  then 
Indian  summer  seldom  occurs.  If,  however,  Indian  summer  is  defined  as 
a  few  days  of  slightly  greater  warmth  and  haziness,  which  only  serve 
to  emphasize  our  otherwise  delightful  autumn  weather,  then  Indian 
summer  nearly  always  occurs.2 

FORECASTS  FROM  LOCAL  OBSERVATIONS  AND  APPEARANCES  OF  SKY 

400.  Prediction  from  the  readings  of  instruments  and  the  appear- 
ance of  the  sky.  —  The  question  is  often  raised,  if  it  is  possible  to  predict 
A  general  the  weather  from  the  sky  appearance  and  the  indications 
cornin*  the  °^  meteorological  instruments  without  using  the  daily  weather 
weather  can  maps.  It  can  readily  be  shown  that  a  good  general  idea  of 
rrom^sk6*1  ^e  commS  weather  can  probably  be  formed  in  this  way, 
appearance  but  that  exact  and  definite  prediction  requires  the  use  of  the 

1  See  Monthly  Weather  Review,  November,  1901,  p.  508. 

2  See  Monthly  Weather  Review,  January,  1902,  p.  19. 


WEATHER  PREDICTIONS  417 

daily  weather  map.  Suppose,  for  example,  that  it  is  January, 
and  the  wind  has  just  changed  to  the  southeast.  Suppose,  observa- 
furthermore,  that  the  temperature  is  rapidly  rising,  that  tions- 
the  moisture  is  increasing,  that  the  barometer  is  falling,  that  the 
sky  is  hazy,  and  the  cirrus  clouds  are  visible,  perhaps  thickening  to 
cirro-stratus  or  cirro-cumulus.  It  is  evident  that  an  area  of  low  pressure 
is  about  to  dominate  the  weather.  The  normal  sequence  of  weather 
changes  during  the  next  two  days  would  be  precipitation  probably  in  the 
form  of  snow,  winds  shifting  to  the  northwest,  and  then  clearing  and 
colder.  A  good  general  idea  of  the  coming  weather  has  thus  been  formed. 
If  such  questions  as  the  probable  amount  of  snowfall,  the  probable  dura- 
tion of  the  snowfall,  the  probable  rise  in  temperature,  the  possibility  of 
the  snow  turning  to  rain,  the  probable  drop  in  temperature  after  the 
storm  passes,  are  to  be  answered,  the  daily  weather  maps  are  indispens- 
able. As  a  second  illustration,  suppose  it  is  again  January,  and  the 
wind  has  just  gone  to  the  northwest.  Suppose,  furthermore,  that  the 
barometer  is  rising,  the  moisture  is  decreasing,  the  temperature  is  falling 
rapidly,  the  clouds  are  breaking  up  into  strato-cumulus  and  cumulus, 
and  the  air  is  becoming  clear.  It  is  evident  that  the  weather  control  is 
passing  to  a  coming  high.  The  normal  weather  sequence  would  be 
two  or  more  days  with  northwest  winds,  low  temperature  at  night,  and 
plenty  of  sunshine. 

The  sky  appearance  and  the  indications  of  local  instruments  indicate 
whether  a  coming  or  departing  low,  a  coming  or  departing  high,  is  domi- 
nating the  weather.  As  soon  as  this  is  determined,  the  probable 
sequence  of  weather  changes  for  the  next  day  or  two  is  at  once  ap- 
parent. To  make  a  definite  prediction,  however,  weather  maps  must 
be  used. 

The  readings  of  local  instruments  are  of  such  value  in  determining  the 
location  and  direction  of  motion  of  a  low,  that  the  following  is  printed  on 
the  face  of  all  weather  maps  issued  by  the  U.  S.  Weather  Bureau: 

"  When  the  wind  sets  in  from  points  between  south  and  southeast  and 
the  barometer  falls  steadily,  a  storm  is  approaching  from  the  west  or 
northwest,  and  its  center  will  pass  near  or  to  the  north  of  the  observer 
within  twelve  to  twenty-four  hours,  with  winds  shifting  to  northwest  by 
way  of  southwest  and  west.  When  the  wind  sets  in  from  points  between 
east  and  northeast,  and  the  barometer  falls  steadily,  a  storm  is  approach- 
ing from  the  south  or  southwest,  and  its  center  will  pass  near  or  to  the 
south  or  east  of  the  observer  within  twelve  to  twenty-four  hours,  with 
winds  shifting  to  northwest  by  way  of  north.  The  rapidity  of  the 

2E 


418  METEOROLOGY 

storm's  approach  and  its  intensity  will  be  indicated  by  the  rate  and 
amount  of  the  fall  in  the  barometer." 

401.    Weather  proverbs  and  weather  rules.  —  Weather  proverbs  are 
as  old  as  written  language.     The  generalization  of  weather  experience 

into  proverbs  and  weather  rules  seems  to  have  been  one  of 
many  the  first  acts  of  civilized  man.  Weather  proverbs  dating 

*overbs        ^ac^  to  at  least  4000  B.C.  have  been  found  on  the  clay  tablets 

of  Babylonia ;  there  are  many  of  them  scattered  through  the 
oldest  manuscripts;  their  number  has  increased  through  centuries; 
and  at  present  there  are  hundreds  of  them.  The  following  collection, 
Dr.  jenner's  in  versified  form,  usually  ascribed  to  Dr.  Jenner,  the  dis- 
coiiection.  coverer  of  vaccination,  is  probably  the  most  famous  and 
interesting : 

"The  hollow  winds  begin  to  blow, 
The  clouds  look  black,  the  glass  is  low, 
The  soot  falls  down,  the  spaniels  sleep, 
And  spiders  from  their  cobwebs  creep; 
Last  night  the  Sun  went  pale  to  bed, 
The  Moon  in  halos  hid  her  head, 
The  boding  shepherd  heaves  a  sigh, 
For  see  !  a  rainbow  spans  the  sky ; 
The  walls  are  damp,  the  ditches  smell, 
Closed  is  the  pink-eyed  pimpernel  ; 
Hark  how  the  chairs  and  tables  crack ! 
%    Old  Betty's  joints  are  on  the  rack; 

Her  corns  with  shooting  pains  torment  her 
And  to  her  bed  untimely  sent  her ; 
Loud  quack  the  ducks,  the  peacocks  cry, 
The  distant  hills  are  looking  nigh ; 
How  restless  are  the  snorting  swine  ! 
The  busy  flies  disturb  the  kine, 
Low  o'er  the  grass  the  swallow  wings  ; 
The  cricket,  too,  how  sharp  he  sings  ! 
Puss  on  the  hearth,  with  velvet  paws, 
Sits  wiping  o'er  her  whiskered  jaws  ; 
The  smoke  from  chimneys  right  ascends, 
Then  spreading  back  to  earth  it  bends ; 
The  wind  unsteady  veers  around, 
Or  setting  in  the  South  is  found ; 
Through  the  clear  stream  the  fishes  rise, 
And  nimbly  catch  th7  incautious  flies  ; 
The  glowworms,  num'rous,  clear,  and  bright, 


WEATHER  PREDICTIONS  419 

Illumed  the  dewy  dell  last  night ; 

At  dusk  the  squalid  toad  was  seen 

Hopping  and  crawling  o'er  the  green ; 

The  whirling  dust  the  wind  obeys, 

And  in  the  rapid  eddy  plays ; 

The  frog  has  changed  his  yellow  vest, 

And  in  a  russet  coat  is  dressed ; 

The  sky  is  green,  the  air  is  still, 

The  merry  blackbird's  voice  is  shrill, 

The  dog,  so  altered  is  his  taste, 

Quits  mutton  bones  on  grass  to  feast ; 

And  see  yon  rooks,  how  odd  their  flight ! 

They  imitate  the  gliding  kite, 

And  seem  precipitate  to  fall, 

As  if  they  felt  the  piercing  ball. 

The  tender  colts  on  back  do  lie, 

Nor  heed  the  traveler  passing  by. 

In  fiery  red  the  Sun  doth  rise, 

Then  wades  through  clouds  to  mount  the  skies. 

'Twill  surely  rain,  —  I  see  with  sorrow 

Our  jaunt  must  be  put  off  to-morrow." 

Several  fairly  complete  compilations  of  weather  proverbs  have  been 
made,  and  in  the  references  to  the  literature  given  at  the  end  of  this 
chapter  some  of  them  are  mentioned.  Various  countries  have  different 
weather  proverbs  and  sometimes  those  well  known  in  one  country  will 
be  unknown  or  will  not  apply  at  all  in  another.  Weather  proverbs, 
particularly  those  referring  to  the  sky  appearance  and  the  meteorological 
elements,  are  often  called  weather  rules,  or  prognostics. 

402.   By  far  the  most  important  question  in  connection  with  weather 
proverbs  is  whether  they  have  any  foundation  or  not ;  that  is,  whether 
they  are  mere  superstitions  or  whether  they  have  a  basis  Thefive 
in  fact.1     In  this  regard,  weather  proverbs  may  be  divided  classes  of 
into  five  classes.     The  first  two   are  fairly  well  founded,  weathf 

.  .  proverbs. 

while  the  last  three  are  mere  superstitions. 

The  first  class  includes  those  which  infer  an  impending  weather  change 
from  the  sky  appearance  and  something  connected  with  the  meteoro- 
logical elements  —  for  example,  "  Rainbow  in  the  morning, 
sailor  take  warning;  rainbow  at  night,  sailor's  delight." 
Now  a  rainbow  in  the  morning,  means  that  the  sun  is  shining  classes. 

1  See  "  Some  Weather  Proverbs  and  their  Justification,"  by  W.  J.  HUMPHREYS,  Popular 
Science  Monthly,  1911. 


420  METEOROLOGY 

in  the  east,  which  is  clear,  and  that  it  is  raining  in  the  west.  Since 
thundershowers  and  storms  move,  in  general,  from  west  to  east,  this 
means  that  rainy  weather  is  impending.  Similarly  a  rainbow  at  night 
means  that  the  sun  is  setting  clear  in  the  west  while  it  is  raining  to  east- 
ward. This  indicates  a  departing  storm  and  that  a  period  of  good 
weather  is  at  hand.  A  second  example  of  this  same  class  of  proverbs 
is  the  following :  "  Mackerel  sky  and  mares'  tails  make  lofty  ships 
carry  low  sails."  Now  mackerel  sky  and  mares'  tails,  as  popularly 
expressed,  mean  technically  that  cirro-cumulus  clouds  are  present  in  the 
sky.  This  is  a  transition  cloud  form  from  cirrus  to  the  coming  nimbus. 
The  storm  cloud  is  usually  accompanied  over  the  ocean  by  high  winds, 
and  this  will  cause  a  vessel  to  carry  but  little  canvass.  It  is  both  inter- 
esting and  instructive  in  connection  with  weather  proverbs  to  try  to 
trace  out  the  scientific  basis  for  them. 

The  second  class  of  weather  proverbs  are  those  which  infer  the  coming 
weather  from  the  behavior  of  animals,  plants,  and  inanimate  things. 
The  coming  of  a  low  with  its  rain  area  and  high  shifting  winds  is  usually 
heralded  by  an  increase  of  temperature  and  moisture  and  a  decrease  of 
pressure.  Drains  are  said  to  smell  before  rain.  This  may  simply  mean 
that  the  lower  pressure  causes  some  of  the  air  to  escape,  thus  causing 
the  odor  to  become  noticeable.  The  increase  in  temperature  and  mois- 
ture often  causes  a  change  in  the  behavior  of  animals.  Their  cries  and 
actions  are  different,  but  this  does  not  mean  in  any  sense  that  the 
animals  are  endowed  with  prophetic  vision.  They  are  simply  reacting  to 
a  changed  present  condition  which  is  the  forerunner  of  the  rain  period. 

The  three  classes  of  weather  proverbs  which  have  no  scientific  basis 
whatever,  are  (l)  those  which  infer  the  future  weather  at  some  distant 
date  from  the  actions  of  animals  or  plants ;  (2)  those  which  infer  the 
future  weather  from  the  weather  at  some  previous  time;  (3)  those 
which  infer  the  weather  from  some  astronomical  body.  No  credence 
whatever  is  to  be  placed  in  these  sayings,  because  there  is  no  reason  why 
they  should  be  true ;  and  statistics  show  that  they  are  not  true.  The 
following  will  serve  as  examples  of  these :  Squirrels  gather  more  nuts 
before  a  hard  winter ;  If  it  rains  St.  Swithin's  day,  it  will  rain  forty  days ; 
The  moon  and  the  weather  change  together. 

A  running  commentary  on  Dr.  Jenner's  doggerel  verses,  quoted  above, 

will  illustrate  the  scientific  basis  for  many  of  these  prognos- 

A  commen-    tics.     These  are  all  rain  prognostics,  and  belong  to  the  first 

T'nner'?1      ^wo  c^asses  °^  wea^her  proverbs.     Since  rain  is  expected,  it 

collection.       can  be  inferred  at  once  that  a  low  is  approaching.     This 


WEATHER  PREDICTIONS  421 

means  that  the  temperature  and  moisture  are  increasing,  the  pressure  is 
lessening,  the  wind  is  in  the  southeast,  and  haze  and  cirriform  clouds 
are  prevalent.  The  glass  is  low  refers,  of  course,  to  the  falling  barometer. 
The  red  color  of  the  sun  at  sunrise,  the  presence  of  clouds,  the  halo 
around  the  moon,  the  pale  appearance  of  the  sun,  all  indicate  the  hazy 
condition  of  the  atmosphere  and  the  presence  of  cirriform  clouds.  The 
shifting,  rising  wind  indicates  that  the  storm  is  coming  steadily  nearer. 
The  falling  of  soot  in  chimneys  and  the  dampness  of  walls  simply  indi- 
cate that  the  moisture  is  larger.  To  the  same  cause,  joined  with  higher 
temperatures,  may  be  attributed  the  closing  of  sensitive  flowers,  the 
rheumatic  pains,  the  shooting  of  corns,  the  low  flight  of  insects  and  of 
birds  in  search  of  them,  the  restlessness  of  animals,  and  the  other  changes 
in  their  cries  and  movements. 

QUESTIONS 

(1)  Define  weather  and  weather  prediction.  (2)  Of  what  three  things  is 
weather  the  composite?  (3)  Describe  the  normal  or  typical  weather  for  the 
northeastern  part  of  the  United  States.  (4)  Name  the  passing  meteorological 
formations  which  exert  the  chief  influence  on  the  weather.  (5)  State  what  is 
meant  by  local  influences.  (6)  State  in  outline  the  general  method  of  weather 
forecasting.  (7)  How  is  the  United  States  divided  for  the  purpose  of  fore- 
casting? (8)  What  forecasts  are  issued  by  a  local  forecast  official?  (9)  Of. 
what  does  a  forecast  consist?  (10)  What  training  do  the  weather  bureau  fore- 
casters receive?  (11)  What  are  the  characteristics  of  a  successful  forecaster? 

(12)  How  do  the  methods  of  making  weather  forecasts  differ  in  other  countries? 

(13)  Describe  in  full  the  method  of  locating  the  center  of  an  area  of  low  pressure  24 
hours  ahead.  (14)  What  are  some  of  the  rules  for  modifying  the  estimated  position  ? 
(15)  What  really  determines  the  track  of  a  low  ?    (16)  Which  are  the  more  impor- 
tant factors  in  determining  the  track  of  a  low  ?     (17)  Describe  the  Bowie  method 
of  locating  a  low.     (18)  Describe  in  detail  the  method  of  determining  the  distri- 
bution of  the  meteorological  elements  about  a  low  24  hours  ahead.     (19)  What 
are  the  advantages  of  making  an  exact  forecast?     (20)  What  terms  may  be 
used  in  the  forecasts  made  by  the  U.  S.  Weather  Bureau?     (21)  What  is  meant 
by  secondary  isobaric  forms?     (22)     Describe  in  detail  the  various  kinds  of 
V-shaped  depressions  and  their  significance.      (23)  What  are  secondary  lows? 
(24)  Describe  in  outline  the  method  of  making  a  weather  prediction  when  a  high 
is  the  dominating  formation.     (25)  Describe  the  method  of  weather  prediction 
by  similarity  with  previous  maps.     (26)  How  are  frost  predictions  made  ?     (27) 
How  is  the  temperature  of  low-growing  vegetation  itself  determined?      (28) 
What  constitutes  a  frost  in  the  weather  bureau  sense?     (29)  What  is  meant 
by  a  cold  wave  ?     (30)  How  is  the  area  for  which  a  cold  wave  is  predicted  to  be 
determined?     (31)  How  is  the  cold  wave  prediction  indicated  on  the  weather 
map?     (32)  Why  are  tornadoes  not  predicted?     (33)  When  is  a  tornado  likely 
to  occur?     (34)     What  is  meant  by  a  destructive  wind  velocity  or  storm? 
(35)  What  are  the  regulations  concerning  river  and  flood  forecasts  ?     (36)  What 
terms  may  be  used  in  the  official  weather  bureau  predictions?     (37)  How  are 
weather  predictions  verified?      (38)  On  what  counts  are  weather  predictions 
verified?     (39)  What  accuracy  is  attained  by  official  forecasters?     (40)  What 


422  METEOROLOGY 

investigations  have  been  undertaken  to  improve  the  present  methods  of 
forecasting?  (41)  What  is  the  attitude  of  newspapers  and  periodicals  towards 
weather  forecasts?  (42)  What  is  meant  by  a  long-range  prediction?  (43) 
To  what  extent  can  the  station  normals  be  used  in  the  making  of 
long-range  predictions?  (44)  To  what  extent  can  departure  from  normal  be 
used  as  a  basis  for  long-range  predictions?  (45)  What  are  weather  cycles? 
(46)  To  what  extent  can  weather  cycles  be  used  in  forecasting  ?  (47)  Describe 
in  detail  the  tendency  of  a  weather  type  to  continue.  (48)  To  what  is  this 
tendency  due?  (49)  What  can  be  accomplished  along  the  line  of  scientific 
weather  prediction?  (50)  What  are  the  various  things  which  serve  as  a  basis 
for  long-range  weather  forecasts  ?  (51)  Describe  the  typical  long-range  weather 
forecast.  (52)  To  what  extent  can  predictions  from  the  readings  of  instruments 
and  the  appearance  of  the  sky  be  made?  (53)  Describe  weather  proverbs.  (54) 
Into  what  five  classes  may  the  weather  proverbs  be  divided  ?  (55)  What  basis 
have  weather  proverbs  ? 

TOPICS   FOR   INVESTIGATION 

(1)  The  Bowie  method  of  locating  the  center  of  a  low. 

(2)  The  rules  for  locating  a  low  24  hours  ahead. 

(3)  V-shaped  depressions. 

(4)  Secondary  lows  and  their  influence  on  the  weather. 

(5)  Local  influences  in  forecasting. 

(6)  The  area  for  which  to  predict  a  cold  wave. 

(7)  The  accuracy  attained  in  weather  forecasting. 

(8)  The  terms  used  in  expressing  weather  predictions  and  the  system  of 
verification  in  other  countries. 

(9)  The  methods  used  by  the  weather  bureaus  of  other  countries  in  making 
and  distributing  predictions. 

(10)  Centers  of  action. 

(11)  Long-range  predictions. 

(12)  Weather  cycles. 

(13)  Weather  proverbs. 

PRACTICAL   EXERCISES 

(1)  Make  several  exact  weather  predictions  and  also  in  the  form  issued  by  the 
officials  of  the  U.  S.  weather  Bureau,  when  a  low  is  dominant,  when  a  high  is 
dominant,  and  when  secondary  isobaric  forms  are  present.     In  each  case  verify 
the  forecast  according  to  the  regular  rules.      (For  this  work  a  file  of  weather 
maps  will  be  very  useful,  although  predicting  for  the  coming  day  is  always  much 
more  interesting.) 

(2)  In  several  cases  locate  the  center  of  a  low  24  hours  ahead,  using  the  Bowie 
method. 

(3)  From  a  file  of  weather  maps  select  those  which  illustrate  well  the  rules  for 
modifying  the  estimated  location  of  the  center  of  a  low. 

(4)  From  a  file  of  weather  maps  select  those  which  illustrate  well  forecasting 
when  the  secondary  isobaric  forms  are  of  importance. 

(5)  From  a  file  of  weather  maps  select  those  which  illustrate  well  the  predic- 
tion of  daijgerous  or  damage-causing  occurrences. 

(6)  From  a  long  series  of  observations  (those  at  a  regular  Weather  Bureau 
station  would  be  necessary)  compile  statistics  and  determine  if  certain  weather 
proverbs  or  weather  rules  or  things  used  as  the  basis  of  long-range  predictions 
have  any  foundation  or  value.     Determine  in  each  case  the  preponderence  of 
probability. 


WEATHER  PREDICTIONS  423 

REFERENCES 

The  following  are  the  books  and  pamphlets  which,  as  a  whole  or  in  part,  deal 

with  weather  and  weather  forecasting  in  general : 
ABBE,  CLEVELAND,    Preparatory    Studies   for    Deductive   Methods    in    Storm 

and  Weather  Predictions,  165  pp.,  Washington,    1890.     (Annual   Report 

of  the  Chief  Signal  Officer  for  1889 ;  Appendix  15.) 

ABBE,  CLEVELAND,  The  Aims  and  Methods  of  Meteorological  Work,  Balti- 
more, 1899.     (Special  publication  of  the  Maryland  Weather  Service.) 
ABERCROMBY,   RALPH,    Principles  of  Forecasting  by  Means  of  Weather  Charts, 

122  pp.,  London,  1885. 

ABERCROMBY,  RALPH,  Weather,  London,  1887. 
BEBBER,  W.  J.  VAN,  Handbuch  der  ausubenden   Witterungskunde,  8°,  2  parts, 

Stuttgart,  1885-6. 

BEBBER,  W.  J.  VAN,  Beurteilung  des  Welters  auf  mehrere   Tage  voraus,  Stutt- 
gart, 1896. 

BEBBER,  W.  J.  VAN,  Die  Wettervorhersage,  2d.  ed.,  219  pp.,  Stuttgart,  1898. 
BIGELOW,  FRANK  H.,  Storms,  Storm  Tracks,  and  Weather  Forecasting.     U.  S. 

Weather  Bureau,  Bulletin  No.  20  (W.  B.  No.  114). 
CHAMBERS,  GEORGE  F.,  The  Story  of  the  Weather,  London,  1897. 
DALLET,  G.,  La  prevision  du  temps. 

FREYBE,  OTTO,  Praktische  Wetterkunde,  173  pp.,  Berlin,  1906. 
GRANGER,  FRANCIS  S.,  Weather  Forecasting,  121  pp.,  Nottingham,  1909. 
GUILBERT,   GABRIEL,    Nouvelle  methode  de  prevision  du  temps,  343  pp.,  Paris, 

1909. 

KLEIN,  H.,  Wettervorhersage  fur  jedermann,  Stuttgart. 
KUHLENBAUMER,    TH.,    Unser    Wetter   und   seine    Vorherbestimmung,    164   pp., 

Minister,  1909. 
MOORE,  WILLIS  L.,  Weather  Forecasting.      U.  S.  Weather  Bureau,  Bulletin 

No.  25  (W.  B.  No.  191). 
MOORE,   WILLIS   L.,   "  Forecasting   the  Weather  and   Storms,"    The   National 

Geographic  Magazine,  June,  1905,  Vol.  XVI,  No.  6. 
PERNTER,  J.  M.,  Wetterprognose  in  Osterreich,  61  pp.,  Wien,  1907. 
PERNTER,  J.   M.,   "  Methods  of  Forecasting  the  Weather,"  Monthly  Weather 

Review,  December,  1903. 
SCOTT,   ROBERT  H.,    Weather   Charts   and  Storm    Warnings,  229  pp.,   London, 

1887. 
SCOTT,  ROBERT   H.,   Notes   on   Meteorology   and    Weather   Forecasting,   40   pp., 

London,  1909. 

SHAW,  W.  N.,  Forecasting  Weather,  8°,  xxvii  +  380  pp.,  London,  1911. 
Station  Regulations  of  the  U.  S.  Weather  Bureau,  Washington,  1905. 
WARD,  ROBERT  DE  C.,  Practical  Exercises  in  Elementary  Meteorology,  Ginn&  Co., 

1899. 
For  the  prediction  of  particular  and  damage-causing  occurrences  such  as  storm 

winds,  cold  waves,  frost,  floods,  etc.,  see : 
GARRIOTT,  EDWARD  B.,  Cold  Waves  and  Frosts  in  the  United  States.     U.  S. 

Weather  Bureau,  Bulletin  P  (W.  B.  No.  355). 
GARRIOTT,  EDWARD  B.,  Storms  of   the  Great  Lakes.      U.  S.  Weather  Bureau, 

Bulletin  K  (W.  B.  No.  288). 

In  connection  with  long-range  weather  predictions,  consult : 
GARRIOTT,  EDWARD  B.,  Long-range  Weather  Forecasts.     U.  S.  Weather  Bureau, 

BuUetin  No.  35  (W.  B.  No.  322). 


424  METEOROLOGY 

WALZ,  F.  J.,  "Fake  Weather  Forecasts,"  Popular  Science  Monthly,  pp.  503-513, 
Vol.  67,  1905. 

For   collections   of   weather   proverbs,  weather   rules,   weather   folklore,   and 

weather  prognostics,  see : 

CHAMBERS,  GEORGE  F.,  The  Story  of  the  Weather,  London,  1897. 
DUN  WOODY,  H.  H.  C.,  Weather  Proverbs.     Signal    Service   Notes,   No.    IX, 

Washington,  1883. 
GARRIOTT,  EDWARD  B.,  Weather  Folklore.     U.  S.  Weather  Bureau,  Bulletin  No. 

33  (W.  B.  No.  294). 

HORNER,  D.  W.,  Observing  and  Forecasting  the  Weather,  46  pp.,  London,  1907. 
INWARDS,  RICHARD,  Weather  Lore,  233  pp.,  London,  1898. 
MICHELSON,  W.  A.,  Wetterregeln,  17  pp.,  Braunschweig,  1906. 
SWAINSON,  CHARLES,  A  Handbook  of  Weather  Folklore,  275  pp,  1873. 


PART   II 

PART  I  consists  of  eight  chapters  which  treat  in  succession  the  atmos- 
phere, the  heating  and  cooling  of  the  atmosphere,  temperature,  pressure 
and  wind,  moisture  in  all  its  forms,  the  various  storms,  weather  bureaus 
and  their  work,  and  weather  predictions.  These  chapters  are  pre- 
sented in  full,  and  the  various  topics  are  treated  at  length.  This  mate- 
rial is  treated  in  full  in  practically  all  textbooks  on  meteorology,  and 
makes  a  complete  treatise  in  itself. 

Part  II  consists  of  five  chapters  which  are  treated  in  full  in  some  books 
and  passed  over  with  a  few  words  or  pages  in  others.  If  meteorology 
is  taken  in  the  largest  sense,  as  the  science  of  all  atmospheric  phenomena, 
and  should  include  all  the  work  of  weather  bureaus,  then  these  chapters 
should  be  included  in  a  treatise  on  the  subject.  These  chapters  treat 
climate,  floods  and  river  stages,  atmospheric  electricity,  atmospheric 
optics,  and  atmospheric  acoustics.  It  was  at  first  intended  to  consider 
these  five  chapters  as  an  appendix  and  present  simply  the  syllabus  of 
each  chapter,  and  the  references  to  the  literature.  If  this  book  is  used 
as  a  textbook,  the  student  has  had  sufficient  material  presented  to  him 
in  a  predigested  form,  and  it  would  be  best  for  him  to  work  up  these 
various  topics  for  himself.  The  general  reader,  however,  would  prob- 
ably prefer  to  know  what  would  be  covered  in  these  chapters  without 
working  up  the  subject  from  the  literature.  For  that  reason,  most  of 
the  topics  are  sketched  in  outline,  but  no  attempt  is  made  at  the  full- 
ness or  completeness  of  Part  I. 


425 


CHAPTER   IX 

CLIMATE 

DISTINCTION  BETWEEN  WEATHER  AND  CLIMATE,  403 
CLIMATIC  DATA  AND  CHARTS,  404 
THE  FACTORS  WHICH  DETERMINE  CLIMATE,  405 
THE  CLIMATIC  SUBDIVISIONS  OF  THE  WORLD 

Subdivision  on  the  basis  of  latitude,  406. 
Subdivision  on  the  basis  of  temperature,  407. 
Subdivision  on  the  basis  of  the  general  wind  system,  408. 
Subdivision  on  the  basis  of  surface  topography,  409.  "  « 

Other  climatic  subdivisions  of  the  world,  410,  411. 

THE  CLIMATE  OF  THE  UNITED  STATES 

Introduction,  412. 
Subdivision,  413. 
Detailed  treatment  of  certain  subdivisions,  414. 

THE  CLIMATE  OF  OTHER  COUNTRIES  AND  PLACES,  415 
THE  CONSTANCY  OF  CLIMATE,  416 
THE  SNOW  LINE,  417 

DISTINCTION  BETWEEN  WEATHER  AND  CLIMATE 

403.  Weather  is  the  condition  of  the  atmosphere  at  any  particular 
time  and  place,  and  is  best  described  by  stating  the  numerical  values  of 
Weather  *ne  va™us  meteorological  elements.  Climate  is  generalized 
and  climate  weather,  and  has  to  do  with  a  larger  area  and  longer  time. 
Climate  is  also  variously  defined  as  the  totality  of  the  weather 
or  the  customary  course  of  the  weather.  Weather  changes  from  moment 
to  moment,  but  climate  remains  the  same.  Weather  has  to  do  with 
the  particular  values  of  the  meteorological  elements,  while  climate  is 
concerned  more  with  the  normal  values. 

426 


CLIMATE  427 

Climatology  is  the  science  of  climate  and  includes,  not  only  a  descrip- 
tion of  the  climate  and  a  statement  of  the  causes  of  its  climatology 
characteristics,  but  also  its  effect  on  animal  and  vegetable  defined- 
life  and  its  relation  to  the  occupations  and  activities  of  man. 

CLIMATIC  DATA  AND  CHARTS 

404.   The  material  necessary  for  a  description  of  the  climate  of  a 
locality  may  be  presented  as  tables  of  statistics  or  by  means  of  graphs 
and  charts.     It  should  include  the  normal  hourly,   daily, 
monthly,  and  yearly  values  of  all  the  meteorological  elements  of  Jjjj  e 
and  all  the  tables  of  data  which  could  be  worked  up  in  connec-  which 
tion  with  the  various  elements.     It  should  also  include  tables  ^en. 
of  data  concerning  the  composition  of  the  atmosphere,  the 
amount  of  evaporation,  the  temperature  of  the  ground,  solar  radiation, 
thunderstorms,  fogs,  the  electrical  condition  of  the  atmosphere,  the 
haziness  of  the  sky,  and  the  like.     It  might  also  include  tables  of  data 
in  connection  with  the  dates  of  harvest,  the  freezing  over  of  rivers,  the 
arrival  and  departure  of  birds,  and  the  like. 

Hann,  in  his  Lehrbuch  der  Klimatologie,  mentions  thirty-six  sum- 
maries of  data  which  should  always  be  included  in  a  climatological 
study,  and  Abbe,  in  his  Aims  and  Methods  in  Meteorological  Work 
adds  six  more  to  the  list.  They  are  the  following : 

(1)  The  monthly  and  annual  mean  temperature  of  the  air. 

(2)  The  extent  of  the  mean  diurnal  range  of  temperature  for  each  month. 

(3)  The  mean  temperature  at  two  specific  hours,  namely,  the  early  morning 
and  midafternoon. 

(4)  The  extreme  limits,  or  total  secular  range,  of  the  mean  temperatures  of  the 
individual  months. 

(5)  The  mean  of  the  monthly  and  annual  extreme  temperatures,  and  the  result- 
ing non-periodic  range. 

(6)  The  absolute  highest  and  lowest  temperatures  that  occur  within  a  long 
interval  of  time. 

(7)  The  mean  variability  of  the  temperature  as  expressed  by  the  differences 
of  consecutive  daily  means. 

(8)  Mean  limit,  or  date,  of  frosts  in  spring  and  fall,  and  the  number  of  con- 
secutive days  free  from  frosts. 

(9)  The  elements  of  solar  radiation  as  measured  by  optical,  chemical,  and 
thermal  effects. 

(10)  The  elements  of  terrestrial  radiation  as  measured  by  radiation  ther- 
mometers. 


428  METEOROLOGY 

(11)  The  temperature  of  the  ground  at  the  surface,  and  to  a  depth  of  one  or 
two  yards. 

(12)  The  monthly  means  of  the  absolute  quantity  of  moisture  in  the  atmos- 
phere. 

(13)  The  monthly  means  of  the  relative  humidity  of  the  air. 

(14)  The  total  precipitation,  as  rain,  snow,  hail,  dew,  and  frost,  by  monthly 
and  annual  sums. 

(15)  The  maximum  precipitation  per  day  and  per  hour. 

(16)  The  number  of  days  having  0.01  inch  or  more  precipitation,  including 
dew  or  frost. 

(17)  The  percentage  of  rainy  days  in  each  month  or  the  probability  of  a  rainy 
day. 

(18)  The  number  of  days  of  snow,  with  the  depth  and  duration  of  the  snow 
covering. 

(19)  The  dates  of  first  and  last  snowfall. 

(20)  Similar  data  for  the  dates  of  hail. 

(21)  Similar  data  for  the  dates  of  thunderstorms. 

(22)  The  amount  of  cloudy  sky,  expressed  in  decimals  of  the  whole  celestial 
hemisphere. 

(23)  The  percentage  of  cloudiness  by  monthly  means,  for  three  or  more 
specific  hours  of  observation. 

(24)  The  thickness  of  the  cloud  layer,  or  the  amount  of  strong  sunshine  as 
shown  by  Campbell's  sunshine  recorder. 

(25)  The  number  of  foggy  days,  or  the  total  number  of  hours  of  fog. 

(26)  The  number  of  nights  with  dew ;  also  the  quantity  of  dew. 

(27)  The  monthly  means  or  total  of  wind  velocity,  or  estimated  wind  force. 

(28)  The  frequency  of  winds  from  the  eight  principal  points  of  the  compass, 
and  the  frequency  of  calms. 

(29)  The  frequency  of  winds  for  each  hour  of  observation  and  the  diurnal 
changes  in  the  winds. 

(30)  The  meteorological  peculiarities  of  each  wind  direction,  or  the  respective 
wind  roses  for  temperature,  moisture,  cloudiness,  and  rainfall. 

(31)  The  mean  annual  barometric  pressure. 

(32)  The  total  evaporation,  daily,  and  monthly,  or  some  equivalent  factor, 
such  as  the  depression  of  the  dewpoint  combined  with  the  velocity  of  the  wind. 

(33)  Variations  in  the  gases  contained  in  the  atmosphere,  provided  they  are 
suspected  to  be  of  importance. 

(34)  Impurities  in  the  atmosphere,  such  as  the  number  of  dust  particles,  and 
especially  the  number  of  spores  or  germs  of  organic  life. 

(35)  The  porportions  of  ozone,  the  peroxide  of  hydrogen,  and  nitric  acid. 

(36)  The  electrical  condition  of  the  atmosphere,  if  there  is  any  method  of 
obtaining  it. 

To  these  Abbe  adds  the  following : 


CLIMATE  429 

(37)  The  sensation  experienced  by  the  observer,  such  as  mild,  balmy,  invigorat- 
ing, depressing,  and  other  terms  used  to  express  the  effect  of  the  weather  upon 
mankind. 

(38)  The  number  of  storm  centers  that  pass  over  a  given  locality,  or  the 
storm  frequency,  monthly  and  annual. 

(39)  Frequency  of  severe  local  storms. 

(40)  The  duration  of  twilight. 

(41)  The  blueness  or  haziness  of  the  sky. 

(42)  The  number  and  extent  of  the  sudden  change  from  warm  to  cold,  or 
moist  to  dry  weather,  and  vice  versa. 

Even  the  above  list  is  by  no  means  complete,  as  many  more  summaries 
could  be  added.  Most  of  this  material  can  be  presented  by  means  of 
graphs  and  charts,  as  well  as  by  tables  of  statistics. 

THE  FACTORS  WHICH  DETERMINE  CLIMATE 

405.   The  climate  of  different  parts  of  the  world  is  very  different,  and 
the  chief  factor  which  determines    the  climate  is  the    latitude  of  the 
locality,  for  with  the   latitude  varies  the  amount  of  insola-       .        . 
tion  received  from  the  sun.     In  fact,  the  word  climate  comes  the  chief 
from  the  Greek  and  means  inclination.     Primarily,  then,  it  climatic 
was  the  varying  inclination  of  the  sun's  rays  at  different 
latitudes  which  was  considered.     If  latitude  were  the  only    factor  in 
determining  climate,  all  places  with  the  same  latitude  would  have  the 
same  climate.     The  climate  which  would  exist  if  it  depended  on  latitude 
alone  is  often   called  the  solar  climate.     There  are,  however,  several 
other  climatic  factors  and  the  climate  is  not  the  same  at  all  places  hav- 
ing the  same  latitude.     The  solar  climate  as  modified  by 
these  other  factors  is   often    called  the  physical   climate,  four  other 
These  modifying  factors  are  :  the  relative  distribution  of  land  climatic 
and  water,  the  altitude  above  sea  level,  mountain  ranges,  and 
the  topography  of  the  locality.     Under  this  last  is  included  the  nature 
of  the  soil,  and  whether  the  country  is  vegetation-covered  or  forested. 

It  is  sometimes  stated  that  the  type  of  storm,  the  amount  of  rainfall, 
the  direction  of  the  prevailing  winds,  etc.,  are  the  factors  which  deter- 
mine climate.     This  would  seem  to  be  a  mistake,  as  it  is  these   Things 
things  which  constitute  climate.     The  climate  is  different  which 
in  two  localities,  and  this  means  that  these  things  are  differ-  ^eTons^d- 
ent   in   the   two   localities.     The   climatic   factors   are  the  ered  climatic 
factors  which  determine  why  these  differences  exist,  and  the 
one  major  factor  and  the  four  minor  factors  have  just  been  stated. 


430  METEOROLOGY 

THE  CLIMATIC  SUBDIVISIONS  OF  THE  WORLD 

406.  Subdivision  on  the  basis  of  latitude. —  The  division  of  the  world 
into  geographical  or  climatic  zones  on  the  basis  of  latitude  goes  -back  to 
the  time  of  the  Greek  philosophers.  The  division  of  the 
WOI>ld  into  the  five  zones  used  at  present  is  generally  ascribed 
the  present  to  Parmenides,  who  flourished  about  450  B.C.  These  five 
fnto^ones11  zones  are  the  torrid  zone,  the  two  temperate  zones,  and  the 
two  frigid  zones.  The  torrid  zone  is  divided  into  two  equal 
parts  by  the  equator,  and  is  bounded  on  the  north  and  south  by  the 
tropics  of  Cancer  and  Capricorn.  The  width  of  the  zone  is  thus  47°, 
and  the  sun  reaches  the  zenith  on  at  least  one  day  during  the 
temperate,'  year  at  every  place  within  this  zone.  The  two  frigid  zones 
and  frigid  ije  wholly  within  the  Arctic  and  Antarctic  circles  and  sur- 
round the  two  poles.  The  sun  never  rises  at  least  one  day 
in  the  year  at  all  places  within  these  zones.  The  two  temperate  zones 
lie  between  the  frigid  zones  and  the  torrid  zone,  and  each  is  43°  wide. 
The  torrid  zone  covers  40  per  cent,  the  two  temperate  zones  52  per  cent, 
and  the  two  frigid  zones  8  per  cent  of  the  earth's  surface.  It  will  thus 
be  seen  that  they  are  of  very  unequal  size.  The  names  of  the  zones  are 
unfortunate,  as  they  would  seem  to  suggest  a  temperature  basis  for  the 
subdivision,  while  in  reality  the  subdivision  is  on  the  basis  of  latitude 
only.  The  torrid  zone  is  more  appropriately  called  the  tropical  zone, 
and  the  frigid  zones  the  polar  zones.  This  system  of  Parmenides  which 
has  come  down  to  us  was  used  by  Aristotle  (about  384  B.C.). 

Various  other  methods  of  dividing  the  world  into  zones  were  proposed 

by  different  Greek  philosophers.     Eudoxus  of  Cnidus,  who  lived  about 

366  B.C.,  divided  the  quadrant  of  the  earth  into  15  parts,  of 

terns^!?^-  wnic^  4  were  assigned  to  the  torrid  zone,  5  to  the  temperate, 

division  on     and  6  to  the  frigid.     The  tropics  and  polar  circles  were  thus 

latitude!8  °f  fixed  at  24°  and  54°  of  latitude  respectively.  Claudius 
Ptolemy,  the  great  astronomer  and  geographer,  who  flourished 
at  Alexandria  in  about  150  A.D.,  proposed  a  different  system.  Near  the 
equator,  the  width  of  a  zone  was  determined  by  a  difference  of  15  minutes 
in  the  length  of  the  longest  day.  In  higher  latitudes,  differences  of  half 
an  hour,  an  hour,  and  finally  a  month,  were  used.  Until  within  the 
last  century  or  two  all  systems  of  subdivision  were  based  on  latitude 
only.  This  means  that  latitude  was  practically  the  only  climatic  factor 
which  was  recognized,  although  a  few  mentioned  the  importance  of 
other  things. 


CLIMATE  431 

407.  Subdivision   on   the   basis   of   temperature.  —  Temperature   is 
the  most  important  of  the  meteorological  elements  in  its  influence  on 
plant  and  animal   life  and   the  occupations  and  habits  of 

life  of  man.     Isothermal  lines  do  not  follow  parallels  of 
latitude,  and  it  has  thus  been  proposed  to  use  them  instead  follow 
of  parallels  of  latitude  for  bounding  the  various  zones.  latitude8  ° 

According  to  Supan,  the  equatorial  or  torrid  zone  should 
be  limited  on  either  side  by  the  normal  annual  isotherm  of  68°  F.     This 
would  cross  the  United  States  from  the  southern  part  of  California  to 
the  northern  part  of  Florida.     The  intermediate  or  temper-   supan's 
ate  zone  has  for  its  poleward  limit  the  isotherms  of  50°  F.   subdivision, 
for  the  warmest  month.     This  would  cross  North  America  from  Alaska 
to  a  little  north  of  Newfoundland.     In  this  system  of  subdivision  there 
would  thus  be  five  zones  as  before,  only  they  would  be  bounded  by  iso- 
thermal lines  instead  of  parallels  of  latitude. 

Koppen  has  proposed  a  system  of  subdivision,  based  upon  tempera- 
ture, into  nine  belts  or  zones,  a  central  zone  and  four  on  each  side  of  it. 
Two  of  these  are  further  subdivided  into  three  parts.     These  Koppen's 
belts  are  as  follows  :  system  of 

(1)  Tropical  belt :   All  months  hot,  that  is,  with  a  normal  subdivision- 
temperature  of  over  68°  F.  during  all  months.     It  would  extend  roughly 
from  20°  N.  to  16°  S.  latitude. 

(2)  Subtropical  belts :  4  to  11  months  hot,  that  is,  over  68°  F. ;  1  to 
8  months  temperate  (50°  F.  to  68°  F.). 

(3)  Temperate  belts :  4  to  12  months  between  50°  F.  and  68°  F. 

(4)  Cold  belts :   1  to  4  months  temperate ;  the  rest  cold,  that  is  with 
a  normal  monthly  temperature  below  50°  F. 

(5)  Polar  belts :   all  months  below  50°  F.     The  two  temperate  belts 
are  subdivided  into  three  parts  with  these  characteristics  :   (a)  constant 
temperature  during  the  year,  (6)  hot   summers,  (c)  moderate  summers 
and  cold  winters. 

408.  Subdivision  on  the  basis  of  the  general  wind  system.  —  The 
importance  of  the  general  wind  system  in  determining  the  geographical 
distribution  of  the  various  meteorological  elements  has  been 
brought  out  in  previous  chapters.     It  was  shown  that  it  was  zone  sub_ 
the  chief  factor  in  determining  the  distribution  of  precipita-  division 
tion,   and  the  absolute  and  relative  humidity  were  both  the  generai 
closely  correlated  with  it.      It  would    thus  seem  that  the  wind 
world  might  be  subdivided  into  climatic  zones  on  the  basis  sys 

of  the  permanent    wind  system.      When  subdivided    on    this    basis, 


432  METEOROLOGY 

nine  zones  are  usually  recognized,  a  central  zone  and  four  on  each  side 
of  it.  The  central  zone  is  called  the  subequatorial  zone,  and  here  would 
be  experienced  the  calms  of  the  doldrums  and  trade  winds  blowing  in 
opposite  directions  at  different  times  of  year.  Next  to  this  central  zone, 
on  either  side,  would  be  the  two  trade  wind  zones.  Next  to  these  would 
come  the  two  subtropical  belts  or  zones.  Here  would  be  found  the  calms 
of  the  horse  latitudes  and  winds  blowing  from  opposite  directions  dur- 
ing different  parts  of  the  year.  Beyond  these  zones  would  come  the 
zones  of  the  prevailing  westerlies,  but  as  they  cover  such  a  very  large 
part  of  the  earth's  surface,  it  has  seemed  best  to  divide  each  of  these 
zones  into  two  by  the  polar  circles.  There  would  thus  be  nine  zones, 
corresponding  to  the  terrestrial  wind  system. 

409.  Subdivision  on  the  basis  of  surface  topography.  —  The  climate 
of  different  localities  in  the  same  zone  is  by  no  means  the  same.     This 

is  true  no  matter  if  latitude,  temperature,  or  the  general 

A  zone  may 

be  divided  wind  system  has  been  made  the  basis  of  subdivision  into 
mt°  S1usin  zones.  It  is  thus  necessary  to  subdivide  the  zones  on  the 
surface  basis  of  the  characteristics  of  the  surface.  Six  subdivisions 
topography  are  usuaiiy  made  :  ocean,  west  coast,  east  coast,  plain,  plateau, 
and  mountain.  The  climate  on  the  east  coast  of  an  ocean  is 
usually  somewhat  different  from  that  on  the  west  coast.  For  this 
reason,  two  coast  or  littoral  climates  must  be  recognized.  It  is  also  not 
sufficient  to  say  a  climate  is  continental  as  distinguished  from  marine 
or  littoral.  The  low-lying  areas,  the  plateaus,  and  the  mountain  regions 
must  be  treated  separately. 

Thus,  if  the  world  is  first  divided  into  zones  on  the  basis  of  latitude, 
or  temperature,  or  the  general  wind  system,  and  then  if  these  zones  are 
subdivided  on  the  basis  of  nature  of  the  surface,  the  resulting  subdivi- 
sion will  be  small  enough  that  a  general  description  of  the  climate  of 
each  one  can  be  given,  and  what  is  said  about  the  climate  of  one  locality 
will  hold  for  all  other  places  in  the  same  subdivision. 

410.  Other   climatic   subdivisions   of   the   world. — There   are   four 
other  climatic  subdivisions  of  the  world,  each  with  a  different  basis, 
which  deserve  a  brief  consideration.     These  are  the  subdivisions  of 
Supan,  Koppen,  Ravenstein,  and  Herbertson. 

Supan  arbitrarily  divides  the  world  into  35  so-called  climatic  provinces. 
The  attempt  is  here  made  to  group  together  those  near-by  places  which 
Supan's  have  the  same  surface  topography  and  where  the  meteoro- 
ciimatic™  logical  elements  have  the  same  characteristics.  The  number 
provinces.  of  these  provinces,  namely  35,  and  their  boundaries  are 


CLIMATE  433 

purely  arbitrary.     Provinces  No.  25,  26,  27,  and  28  include  the  United 
States,  and  their  characteristics  are  as  follows  : 

No.  25,  Californian  province.  Here  it  is  relatively  cool,  especially  in 
summer,  and  there  is  a  marked  subtropical  rainy  season. 

No.  26,  North  American  mountain  and  plateau  province.  Here  there 
are  great  daily  and  yearly  ranges  in  the  values  of  the  meteorological 
elements,  and  it  is  also  dry. 

No.  27,  Atlantic  province.  Here  the  chief  characteristics  are  contrast 
in  temperature  between  north  and  south  in  winter,  extreme  climate  even 
on  the  coast,  a  plentiful,  evenly  distributed  rainfall,  rapid  changes. 

No.  28,  West  Indian  province.  Here  there  is  an  equable  temperature 
and  rain  at  all  seasons,  but  with  a  well-marked  summer  maximum. 

Koppen's  climatic  subdivision  of  the  world  has  a  botanical   basis. 
Five  kinds  of  plants  are  recognized :   (megotherms)  those  which  need  a 
continuously    high    temperature    and    abundant    moisture;  ^oppen's 
(xerophytes)  those  which  like  high  temperature  and  dryness ;  botanical 
(mesotherms)   those  which  require  moderate  temperatures  syst< 
and  a  moderate  amount  of  moisture ;    (mikrotherms)  those  which  need 
lower  temperatures ;    (hekistotherms)  those  which  will  live  in  low  tem- 
peratures.    The  five  main  divisions  are  further  subdivided  until  the 
whole  number  reaches  twenty-four. 

According  to  Ravenstein,  the  world  is  subdivided  into  sixteen  cli- 
matic types,  and  the  basis  of  the  classification  is  tempera-  Raven_ 
ture  and  relative  humidity.  stein's 

According  to  Herbertson,  the  world  is  subdivided  into  syst< 
six  natural  geographical  regions,  and  these  are  subdivided  until  the 
whole  number  of  recognized  climates  reaches  fifteen.     His  Herbert- 
basis  of  classification  is  a  combination  of  temperature,  rain-  son's 
fall,   topography,   and  vegetation.  system. 

411.   It  will  thus  be  seen  that  there  are  many  methods  of  subdividing 
the  world  into  smaller  areas  for  the  purpose  of  discussing  the  climate, 
and  many  different  bases  have  been  used  for  the  various  The  purpose 
classifications.     The  purpose  in  each  case  has  been  to  form  of  sub- 
a  sufficient  number  of  climatic  provinces  so  that  the  climate     lvlslon> 
at  different  localities  in  the  same  province  would  be  essentially  the 
same.     In  general,  the  larger  the  number  of  provinces,  the  more  nearly 
alike  will  the  climate  be  at  different  places  in  the  same  province. 


434  METEOROLOGY 


THE  CLIMATE  OF  THE  UNITED  STATES 

412.  Introduction.  —  The  United  States  is  a  country  of  such  vast 
extent  and  with  such  a  diversified  surface  that  practically  no  statements 
The  climate    can  ^e  ma^e  about  the  climate  of  the  country  as  a  whole, 
in  different     The  statements  which  would  be  true  for  one  part  of  the 
country  is6     country  would  be  entirely  incorrect  for  another.     In  one 
very  dif-        part  of  the  country,  New  England  for  example,  the  weather 

is  dominated  by  an  almost  unbroken  procession  of  passing 
highs  and  lows.  As  a  result,  great  irregular  changes  in  the  values  of  the 
meteorological  elements  follow  each  other  in  quick  succession.  The 
precipitation  is  all  storm-caused  and  quite  evenly  distributed  throughout 
the  year.  The  climate  is  thus  extreme,  very  variable,  and  with  copious, 
evenly  distributed  precipitation.  In  another  part  of  the  country, 
California  for  example,  there  is  a  well-marked  rainy  season.  The  pre- 
cipitation here  is  caused  mostly  by  the  general  wind  system  of  the 
world.  The  irregular  changes  in  the  meteorological  elements  are  few, 
so  that  during  any  one  season,  one  day  is  much  like  another.  The  cli- 
mate is  thus  very  uniform  with  a  well-marked  wet  and  dry  season. 
Subdivision  The  first  essential,  then,  in  discussing  the  climate  of  the 
essential.  United  States  is  to  subdivide  the  country  into  smaller  areas 
so  that  the  climate  of  all  places  in  the  same  district  will  be  practically 
the  same. 

413.  Subdivision.  —  The  United  States  might   be  subdivided  into 
smaller  areas  for  the  purpose  of  discussing  the  climate,  using  any  one  of 
The  three       ^ne  systems  of  subdivision  and  classification  which  have  just 
systems  of      been  treated.     Since,  however,  the  U.  S.  Weather  Bureau 
usedTyThe    nas  c°Hected  practically  all  of  the  climatological  data,  it 
Weather        would  be  better  to  adopt  the  systems  of  subdivision  which 

have  been  followed  there.  Formerly  the  country  was  divided 
into  twenty-one  climatic  divisions  (see  section  357),  and  the  climatological 
data  and  statistics  were  summarized  for  each  of  these  districts  separately. 
At  present,  these  districts  are  still  used  to  a  slight  extent  in  summarizing 
data.  When  the  form  of  the  Monthy  Weather  Review  was  changed 
in  July,  1909,  the  country  was  subdivided  into  twelve  climatological 
districts  (see  section  357),  and  the  observations  are  now  summarized  for 
these  districts  as  a  whole.  These  districts  are  also  adhered  to  as  far  as 
practicable  in  matters  of  administration.  In  1910  was  commenced  the 
publication  of  the  summaries  of  the  climatological  data  of  the  United 
States,  by  sections,  and  for  this  purpose  the  country  was  divided  into 


CLIMATE  435 

106  districts.  Thus  in  discussing  the  climate  of  the  United  States,  it 
would  be  best  to  consider  the  country  divided  into  21, 12,  or  106  districts, 
following  the  subdivisions  of  the  U.  S.  Weather  Bureau. 

414.  Detailed  treatment  of  certain  subdivisions.  —  The  scope  of 
this  book  prevents  the  complete  discussion  of  the  climate  of  even 
one  place  or  district.  The  reader  who  is  interested  in  the  climate  of 
any  particular  locality  must  be  referred  to  the  literature  of  the  subject 
for  information. 

The  general  question,  however,  could  well  be  raised  as  to  what  would 
constitute  a  full  and  complete  discussion  of  the  climate  of  a  place. 
Such  a  complete  treatise  might  conveniently  consist  of  the 
following  nine  parts :  (I)  A  map  of  the  locality  and  surround-  full,  com- 
ing regions  is  usually  presented  first.  This  map  should  con-  P.lete  trea~ 
tain  the  usual  geographical  features  and  the  elevations.  A  climate  of  a 
brief  description  of  the  cities,  mountains,  etc.,  usually  place ,would 
accompanies  the  map.  (II)  Next  a  full  description  of  the 
surface  topography  might  be  given.  This  would  include  the  char- 
acteristics of  the  rivers,  the  nature  of  the  soil,  whether  forested  or  not, 
and  the  like.  (Ill)  Next  the  climatological  data  might  be  presented 
as  tables  of  statistics  or  as  charts  and  graphs.  The  source  of  the  data, 
the  length  of  the  records,  and  their  probable  accuracy  might  also  be 
discussed.  (IV)  Next  would  come  a  discussion  of  the  data,  perhaps 
considering  first  the  meteorological  elements  in  order  (temperature, 
pressure,  wind,  moisture,  cloud,  precipitation),  and  then  the  other 
meteorological  and  phenological  occurrences,  such  as  frost,  fog,  evapora- 
tion, time  of  harvest,  migration  of  birds,  etc.  (V)  Next  might  come  a 
discussion  of  the  types  of  storms,  their  prevalence  and  severity.  (VI) 
Next  the  influence  of  the  climate  on  certain  diseases  and  the  general 
healthfulness  of  the  climate  might  be  discussed.  (VII)  Next  the  influ- 
ence of  the  climate  on  agriculture  and  vegetation  might  be  discussed. 
Here  would  be  considered  such  questions  as  the  kind  of  crops  which  could 
be  best  grown,  the  possibility  of  fruit  growing,  the  kind  of  forest  trees 
which  would  be  most  common,  etc.  (VIII)  Next  the  general  influence 
of  the  climate  on  the  industries  and  habits  of  life  of  the  people  might  be 
discussed.  (IX)  A  bibliography  might  be  added.  These  nine  subdi- 
visions will  serve  to  indicate  what  one  might  expect  to  find  in  a  full, 
complete  discussion  of  the  climate  of  a  locality. 


436  METEOROLOGY 


THE  CLIMATE  OF  OTHER  COUNTRIES  AND  PLACES 

415.  It  is  entirely  impossible  to  present,  in  a  limited  space,  the  com- 
plete discussion  of  the  climate  of  even  one  country  or  place.  This  will 
not  be  attempted,  but  the  chief  characteristics  of  the  three  great  zones, 
the  torrid,  temperate,  and  frigid,  should  perhaps  be  stated.  The  zones 
are,  however,  of  such  large  extent  that  there  are  but  few  characteristics 
which  are  common  to  an  entire  zone. 

The  climate  of  the  torrid  zone  is  characterized  by  great  unifprmity, 

small  irregular  changes  in  the  meteorological  elements,  high  tempera- 

bar        ^ure>  and  a  small  yearly  variation  in  temperature.     Nowhere 

teristics  of      else  in  the  world  is  the  weather  so  nearly  the  same  day  after 

the  torrid       jay.     'pjjjg  means  a  verv  uniform  climate,  and  the  reason 

zone* 

for  it  is  the  type  of  storms.  Tropical  cyclones  and  thunder- 
showers  are  the  only  storms.  Tropical  cyclones  are  few  in  number,  occur 
at  certain  times  of  year,  and  cover  a  very  small  area.  Thundershowers 
are  very  prevalent,  occurring  at  many  places  almost  daily.  Large,  irregu- 
lar changes  in  the  meteorological  elements  following  each  other  in  quick 
succession  are  thus  almost  unknown.  The  noonday  sun  always  stands 
high  in  the  sky  and  the  change  in  temperature  during  the  year  is  small. 
Monsoons  are  well  marked  in  many  countries  in  the  torrid  zone,  and  the 
rainfall  occurs  either  during  the  rainy  season,  caused  by  a  monsoon,  or 
almost  daily,  usually  with  a  thundershower  in  the  afternoon,  when  con- 
vection is  most  powerful.  The  seasons  thus  depend  more  on  the  general 
wind  system  and  the  rainfall  than  on  changes  in  temperature. 

The  temperate  zone  is  characterized  by  a  very  variable  climate  and 
large  changes  in  temperature  between  summer  and  winter.  The  tem- 
The  temper-  peratures  are,  of  course,  lower  than  in  the  torrid  zone.  The 
ate  zone.  temperate  zone  is  constantly  being  traversed  by  passing 
highs  and  lows,  and,  as  a  result,  the  changes  in  the  meteorological  ele- 
ments are  abrupt  and  large.  This  makes  the  climate  very  variable. 
The  rainfall  results  both  from  the  general  wind  system  and  the  passing 
storms. 

The  frigid  zone  is  characterized  by  greater  uniformity  and  much  lower 
temperature  than  the  temperate  zone.  The  changes  in  the  meteoro- 
The  frigid  logical  elements,  particularly  temperature,  between  summer 
zone.  ancj  winter,  are  large.  The  daily  changes  at  times  are  very 

small.  Large,  irregular  changes  are  also  present. 


CLIMATE  437 

THE  CONSTANCY  OF  CLIMATE 

416.  The  question  of  the  constancy  of  climate  must  be  discussed  for 
three  different  time  intervals.  First,  has  the  climate  remained  constant 
during  the  recent  past,  say  the  last  hundred  years  ?  Secondly,  has  the 
climate  remained  constant  during  historic  times,  say  the  last  7000  years  ? 
Thirdly,  has  the  climate  remained  constant  during  recent  geologic  ages, 
say  the  last  10,000,000  years? 

There  are  many  stations  where  meteorological  observations  have 
been  made  for  more  than  a  hundred  years.     In  fact,  a  few  records  cover 
more  than  three  hundred  years.     Based  upon  these  observa- 
tions, the  statement  can  confidently  be  made  that  the  cli-  has  re- 


mate  is  essentially  the  same  now  as  it  was  many  years,  or  mained  un- 
even a  hundred  years  ago.     This  is  largely  contrary  to  popu-  during  the 
lar  belief.     It  means  that,  taking  one  year  with  another,  the  last  hun~ 
snowfall  is  just  as  large  now  as  then.     It  means  that  sleigh- 
ing lasts  just  as  long  now  as  then.     It  means  that  the  winters  are  no 
milder  now  than  then.     It  means  that  our  summers  are  no  hotter  now 
than  then.     The  constant  statements  by  the  older  people,  that  the 
climate  is  different  now  than  it  used  to  be  when  they  were  much  younger, 
are  due  to  the  tendency  to  magnify  and  remember  the  unusual  while 
the  ordinary  is  forgotten.     Thus,  in  time,  it  is  only  the  unusual  snowfall 
or  the  extremely  low  temperatures  that  are  well  remembered,  and  un- 
consciously the  abnormal  has  thus  been  substituted  for  the  normal. 
These  statements  are  also  due  to  the  fact  that  the  attitude  towards  life, 
the  amount  of  energy,  the  daily  occupations,  and  perhaps  the  place  of 
residence  of  the  older  people  are  very  different  now  than  when  they  were 
much  younger. 

In  discussing  possible  changes  in  climate  during  the  last  7000  years, 
inference  must  be  drawn  from  such  recorded  facts  as  the  dates  of  har- 
vest, the  kind  and  amount  of  crops  raised,  the  kind  of  cloth- 
ing  worn  by  the  people,  the  habits  of  life  of  the  people,  the  has  re- 
existence  of  certain  wild  animals  and  forest  trees,  the  size  of  mail^ed  es~ 
rivers,  the  height  of  lakes  and   inclosed  seas,  etc.     From  same  during 
evidence  of  this  kind,  the  conclusion  has  been  drawn  that  h.istoric 
there  have  been  no  marked  changes  in  climate  during  his- 
toric times.     It  has  been  often  thought  that  certain  climatic  cycles  have 
been  detected.     The  11-year  and  35-year  cycles  have  been  well  investi- 
gated, while  cycles  of  much  longer  duration  have  also  been  suspected. 
The  11  -year  cycle  corresponds  to  the  sun  spot  cycle  and  is  very  poorly 


438  METEOROLOGY 

marked.  Bruckner's  35-year  cycle  is  on  the  contrary  fairly  well  marked 
in  Europe,  both  in  temperature  and  precipitation.  None  of  these  cycles, 
with  the  possible  exception  of  the  35-year  cycle,  are  at  all  regular  or  well 
marked.  (See  section  397.) 

There  can  be  no  doubt  th&t  the  climate  has  changed  greatly  during 
recent  geologic  ages.  Almost  tropical  vegetation  has  existed  in  Green- 
Great  land,  and  glaciation  has  extended  many  times  far  towards 

changes  the  equator.  Various  explanations  of  these  changes  have 
togkdages  keen  advanced.  Among  them  are  a  change  in  the  location 
and  the  pos-  of  the  earth's  axis,  a  change  in  the  eccentricity  of  the  earth's 
sibie  causes.  OJfo^  the  precession  of  the  equinoxes  which  brings  the  long 
cold  winter  to  the  northern  (land)  hemisphere  every  25,000  years,  a 
change  in  the  energy  emitted  by  the  sun,  a  change  in  the  composition  of 
the  earth's  atmosphere,  a  change  in  the  elevation  of  the  place,  a  change 
in  the  distribution  of  land  and  water,  and  thus  the  ocean  currents. 

THE  SNOW  LINE 

417.  The  temperature  in  general  grows  less  with  increasing  altitude, 
and  thus  there  are  regions  on  high  mountains  even  in  the  torrid  zone 
Definition  of  near  ^e  equator  where  the  snow  which  falls  during  one 
the  snow  winter  has  not  sufficient  time  to  melt  entirely  during  the 
line*  following  summer  before  the  advent  of  the  snows  of  the  next 

winter.  These  are  regions  of  perpetual  snow,  and  the  lower  boundary  of 
these  regions  is  called  the  snow  line. 

On  the  equatorial  Andes,  the  height  of  the  snow  line  is  about  5000 
meters,  roughly  three  miles.  With  increasing  south  latitude,  the  height 
its  height  in  °^  ^ne  snow  ^me  increases  somewhat  due  to  scantier  precipi- 
South  tation,  and  reaches  a  maximum  height  of  about  6000  meters 

America.  in  ^0  soui^  latitude.  The  height  of  the  snow  line  then  de- 
creases rapidly  with  increasing  latitude.  Its  height  is  about  3500 
meters  in  latitude  32°  ;  1600  meters  in  latitude  42°  ;  800  meters  in  lati- 
tude 50° ;  400  meters  in  latitude  55° ;  and  according  to  the  observa- 
tions made  by  certain  Antarctic  expeditions,  it  reaches  sea  level  in  from 
67°  to  70°  south  latitude. 

In  the  northern  hemisphere,  the  height  of  the  snow  line  is  somewhat 
its  height  in  Sreater  than  in  South  America.  In  the  Alps  in  Europe  (lati- 
other  parts  tude  about  46°)  it  is  about  2900  meters.  In  Norway  (lati- 
of  the  world.  tude  about  61o)  it  ig  about  1500  meters  In  the  Himalaya 

Mountains  the  height  is  about  5000  meters.     In  North  America,  at 


CLIMATE  439 

latitude  19°  N.,  in  Mexico,  the  height  of  the  snow  line  is  about  4600 
meters.  On  Mt.  Shasta  the  height  is  2400  meters;  in  the  Cascade 
Mountains,  at  the  northern  boundary  of  the  United  States,  it  is  2000 
meters  ;  on  Vancouver  Island  it  is  1700  meters  ;  on  Mt.  St.  Elias  (lat. 
60°  N.)  it  is  800  meters.  Even  in  polar  regions  it  does  not  everywhere 
absolutely  reach  sea  level. 

The  height  of  the  snow  line  does  not  depend  on  latitude  alone,  and 
thus  is  often  somewhat  different  on  two  mountains  in  the  same  latitude. 
The  other  factors  which  influence  the  height  of  the  line  are  The  factors 


the  amount  of  snow  during  the  winter,  the  change  in  tern-  ei 


perature  between  winter  and  summer,  the  steepness  of  the  height. 
mountains,  the  exposure  of  its  slopes,  and  the  general  wind  system. 

TOPICS   FOR   INVESTIGATION 

(1)  What  a  full  list  of  climatic  data  would  consist  of. 

(2)  The  factors  which  determine  climate. 

(3)  Some  system  of  climatic  subdivision  and  its  basis. 

(4)  The  contrast  between  marine  and  littoral  climate  in  the  same  latitude. 

(5)  The  climatic  characteristics  of  the  continents. 

(6)  Climatic  changes  during  historic  times. 

(7)  Geological  changes  in  climate. 

(8)  The  change  in  the  height  of  the  snow  line  between  winter  and  summer. 

(9)  The  temperature  of  the  snow  line. 

(10)  Effect  of  climate  on  the  mental  characteristics  of  the  inhabitants. 

PRACTICAL   EXERCISES 

(1)  Prepare  tables  of  climatic  data. 

(2)  Express  these  tables  graphically. 

(3)  Summarize  the  observations  of  some  station  with  a  long  record  to  show 
the  constancy  of  climate. 

(4)  Treat  fully  the  climate  of  some  place. 

REFERENCES 

The  following  books  and  pamphlets  deal  with  climate  or  climatology  in  gen- 

eral: 

BEBBER,  W.  J.  VAN.,  Hygienische  Meteorologie,  Stuttgart,  1895. 
BONACINA,  L.  C.  W.,  Climatic  Control,  viii  +  167  pp.,  London,  1911. 
CULLIMORE,  D.  H.,  The  Book  of  Climates  for  all  Lands,  London,  1890. 
HANN,  JULIUS,  Handbuch  der  Klimatologie,  3  vols.,    2d  ed.,  Stuttgart,  1897; 

3d  ed.  of  Vol.  I,  1908;   of  Vol.  II,  1910;   of  Vol.  Ill,  1911. 
HERBERTSON,  A.  J.  and  F.  D.,  Man  and  his  Work,  London,  1899. 
KOPPEN,  Klimakunde,  2d  ed.,  Leipsig,  1906. 
MEYER,  HUGO,  Anleitung   zur   Bearbeitung   meteorologischer  Beobachtungen  fur 

die  Klimatologie,  8°,  viii  +  187  pp.,  Berlin,  1891. 


440  METEOROLOGY 

MOORE,  WILLIS  L.,  Climate.     Bulletin  No.  34  of  U.  S.  Weather  Bureau  (W.  B. 
publication  No.  311). 

RATZEL,  Anthropogeographie,  2d  ed.,  Stuttgart,  1899. 

STOCKMAN,  WILLIAM  B.,  Invariability  of  our  Winter  Climate,  (W.  B.  publica- 
tion No.  312). 

SUPAN,  A.,  Grundzuge  der  physischen  Erkunde,  4th  ed.,  Leipzig,  1908. 

WARD,  R.  DEC.,  Climate,  G.  P.  Putnam's  Sons,  New  York,  1908. 

WARD,  R.  DEC.,  Harm's  Handbook  of  Climatology  (translation  of  Vol.  I),  The 
Macmillan  Co.,  1903. 

WOEIKOF,  A.,  Die  Klimate  der  Erde,  Jena,  1887. 

(Of  the  books  just  mentioned,  the  two  by  Ward  are  by  far  the  most  complete 

and  readable  in  English.         Taken  together  they  cover  the  subject  of  climate 

in  a  most  complete  and  interesting  way.     The  three-volume  work  by  Hann  in 

German  is  by  far  the  most  complete  and  useful  treatise  on  the  subject.     It  is 

a  veritable  mine  of  information.) 

For  a  description  of  the  climate  of  the  United  States  as  a  whole  and  of  various 

places  in  the  country,  see  : 
BLODGET,  Louis,  Climatology  of  the  United  States,  xvi  -f-  536  pp.,  Philadelphia 

1857. 

FASSIG,  OLIVER  L.,  The  Climate  and  Weather  of  Baltimore,  Baltimore,  1907. 
GREELY,  A.  W.,  Report  on  the  Climate  of  Colorado  and  Utah,  Washington,  1891 
HAZEN,  HENRY  A.,  The  Climate  of  Chicago.     Bulletin  No.  10  of  U.  S.  Weather 

Bureau. 
HENRY,  ALFRED  J.,  Climatology  of  the  United  States,  1012  pp.,  Washington,  1906. 

Weather  Bureau,  Bulletin  Q.     (The  standard  descriptive  and  statistical 

work  on  this  subject.) 
M'ADIE,  ALEX.  G.,  and  WILLSON,  GEORGE  H.,  The  Climate  of  San  Francisco 

Bulletin  No.  28  of  the  U.  S.  Weather  Bureau,  (W.  B.  publication  No.  211). 
M'ADIE,  ALEX.  G.,  Climatology  of  California  Bulletin  L  of  U.  S.  Weather  Bu- 
reau, (W.  B.  publication  No.  292). 
SMOCK,  JOHN  C.,  Climate  of  New  Jersey,  Trenton,  1888.     (Part  of  the  final  report 

of  the  state  geologist.) 
Summary  of  the  Climatological  Data  for  the  United  States,  by  Sections  (106 

are  to  be  issued,  covering  the  whole  country). 
WALDO,  FRANK,  Elementary  Meteorology  (Chapter  13). 

For  a  description  of  the  climate  of  various  countries  and  place,  outside  of  the 
United  States,  see : 

ABBE,  CLEVELAND,  JR.,  The  Climate  of  Alaska,  Washington,  1906.     (Extract 

from  Professional  Paper  No.  45,  U.  S.  Geological  Survey.) 
ABBOT,  HENRY  L.,  Climatology  of  the  Isthmus  of  Panama  (W.  B.  publication 

No.  201). 
ALEXANDER,  WILLIAM  H.,  Climatology  of  Porto  Rico,  Monthly  Weather  Review, 

July,  1906. 

ALGUE,  JOSE,  The  Climate  of  the  Philippines,  103  pp.,  Manila,  1904. 
BEHRE,  OTTO,  Das  Klima  von  Berlin,  8°,  158  pp.,  Berlin,  1908. 
BLANFORD,  Climates  and  Weather  of  India,  8°,  382  pp.,  London,  1889. 
DAVIS,  WALTER  G.,  Climate  of  the  Argentine  Republic,  vi  +  154  pp.,  Buenos 

Aires,  1902. 

ELIOT,  SIR  JOHN,  Climatological  Atlas  of  India,  Edinburgh,  1906. 
KNOX,  ALEXANDER,  The  Climate  of  the  Continent  of  Africa,  8°,  xii  +  552  pp., 

Cambridge,  1911. 


CLIMATE  441 

MOORE,  JOHN  W.,  Meteorology,  2d  ed.  London,  1894.     (Climate  of  the  British 

Islands,  Chapters  XXV  and  XXVI.) 

NAKAMURA,  K.,  The  Climate  of  Japan,  109  pp.,  Tokio,  1893. 
PHILLIPS,  W.  F.  R.,  Climate  of  Cuba,  Bulletin  22  of  U.  S.  Weather  Bureau, 

(W.  B.  publication  No.  163). 

QUETELET,  A.,  Sur  le  climat  de  la  Belgique,  4°,  2  vols.,  Bruxelles,  1849. 
Rizzo,  G.  R.,  II  Clima  di  Torino. 
ROSTER,  GIORGIO,  Climatologia  delV  Italia  nelle  sue  attineze  con  Vigiene  e  con 

agricoltura,  8°,  xxix  +  1040  pp.,  Torino,  1909. 

For  a  treatment  of  the  variations  in  climate,  see  : 

BUCKNER,  EDWARD,  Klimaschwankungen  seit  1700,  viii  +  324  pp.,  Wien, 
1890. 

ECKARDT,  WILHELM  R.,  Das  Klimaproblem  der  geologischen  Vergangenheil  und 
historischen  Gegenwart,  8°,  vi  +  183  pp.,  Braunschweig,  1909. 

Die  Veranderungen  des  Klimas  seit  dem  Maximum  der  letzten  Eiszeit  (Pub. 
by  llth  International  Geological  Congress),  4°,  Iviii  +  459  pp.,  Stock- 
holm, 1910. 

For  climatic  charts  see  the  references  in  connection  with  the  charts  of  the  meteor- 
ological elements  at  the  end  of  the  various  chapters. 


CHAPTER  X 
FLOODS  AND  RIVER  STAGES 

DEFINITION  OF  THE  TERMS  USED  IN  CONNECTION  WITH  RIVERS,  418. 
THE  MEASUREMENTS  MADE  IN  CONNECTION  WITH  RIVERS 

Velocity  of  flow,  419. 
River  stage,  420. 
Cross  section  of  a  river,  421. 
River  discharge  422. 

RIVER  DATA,  423 

THE  DIFFERENT  KINDS  OF  FLOODS,  424 

THE  CHARACTERISTICS  OF  INDIVIDUAL  RIVERS,  425 

THE  PREDICTION  OF  RIVER  STAGES  AND  FLOODS,  426,  427 

SUDDEN  RISES  OF  OCEANS  AND  LAKES,  428 

DEFINITION  OF  THE  TERMS  USED  IN  CONNECTION  WITH  RIVERS 

418.  There  are  several  terms  used  in  connection  with  rivers  and 
floods  which  require  at  the  outset  exact  definition  and  brief  consideration. 

Drainage  area.  —  By  the  drainage  area  of  a  river  is  meant  the  tract 
of  country  from  which  the  water  drains  into  the  river.  This  is  some- 
Drainage  times  called  the  catchment  basin  or  the  watershed.  The 
area-  area  drained  by  some  of  the  rivers  of  the  world  is  enormous, 

that  by  the  Amazon  probably  being  the  largest.  The  drainage  area  of 
the  Mississippi  River  and  its  tributaries  probably  stands  next  in  size. 
The  Missouri  River  drains  about  527,150  square  miles ;  the  Ohio  River, 
about  201,700 ;  the  Arkansas  River,  about  186,300 ;  the  Red  River,  about 
90,000;  so  that  the  Mississippi  River  and  its  tributaries  drain  about 
1,240,000  square  miles.  Even  the  smallest  brook  with  a  name  usually 
has  a  drainage  area  of  a  good  many  square  miles.  A  complete  descrip- 
tion of  the  drainage  area  of  a  river  would  include  an  account  of  its 
topography,  meteorology,  and  climate. 

Inland  drainage  area.  —  If  a  tract  of  country  is  landlocked,  that  is, 
surrounded  on  all  sides  by  land  of  greater  elevation,  the  rivers  will  flow 

442 


FLOODS  AND  RIVER  STAGES  443 

to  the  lowest  point  and  form  a  lake  or  sea  without  an  outlet  to  the 
ocean.     Such  an  area  is  called  an  inland  drainage  area,  and  the  evapora- 
tion must  here  equal  the  precipitation.     The  sea  or  lake  is  Inland 
often  below  sea  level  and  is  usually  salt.     The  reason  is  drainage 
because  the  rivers  always  carry  down  a  small  amount  of  salt 
and  other  minerals,  and  these  are  left  behind  when  evaporation  takes 
place.     The  Caspian  Sea,  with  an  area  of  180,000  square  miles,  is  the 
largest  sea  of  this  kind  in  the  world.     Great  Salt  Lake,  Utah,  contains 
17  per  cent  of  salt  and  other  minerals,  and  is  the  best  known  inland 
lake  in  the  United  States.      In  Australia,  52  per  cent  of  the  whole 
country  consists  of  inland  drainage  areas;  in  Africa,  31  per  cent;  in 
Europe  and  Asia,  28  per  cent;  in  South  America,  7.2  per  cent;  in 
North  America,  3.2  per  cent. 

Run-off.  —  By  run-off  is  meant  the  percentage  of  the  precipitation 
which  eventually  drains  into  a  river,  and  this  varies  all  the  way  from  a 
few  per  cent  to  nearly  90  per  cent  in  some  extreme  cases.  The 
run-off  depends  chiefly  upon  the  characteristics  of  the  drainage 
area,  but  also  upon  the  amount  and  rapidity  of  the  rainfall.  Ground  is 
ordinarily  classified  as  permeable  or  impermeable.  If  rock  or  a  stratum 
of  material  impervious  to  water  is  near  the  surface,  the  ground  is  im- 
permeable, and  the  run-off,  in  this  case,  will  be  large  and  occur  very 
shortly  after  the  precipitation.  If  the  ground  is  permeable,  the  run-off 
is  much  smaller  and  much  more  gradual  and  regular.  Most  watersheds 
have  also  a  decided  seasonal  change  in  characteristics.  At  one  time  of 
year  vegetation  may  be  luxuriant.  At  another  time  of  year  the  ground 
may  be  frozen  hard  or  covered  with  a  layer  of  snow  and  ice.  All  these 
seasonal  changes  make  a  tremendous  difference  in  the  run-off.  The 
run-off  also  depends,  to  a  large  extent,  on  the  amount  and  rapidity  of  the 
rainfall.  It  increases  both  with  the  amount  of  the  rainfall  and  the 
rapidity  with  which  it  falls.  One  of  the  hardest  problems  in  connection 
with  rivers  is  to  try  to  estimate  the  run-off  when,  over  a  drainage  area  in 
a  certain  condition,  a  certain  amount  of  rain  falls  in  a  certain  time 
interval.  The  average  run-off  for  all  watersheds  in  the  world  is  between 
20  and  30  per  cent. 

Thalweg.  —  The  term  thalweg  is  sometimes  used  to  designate  the 
valley  bottom  through  which  a  river  runs.  Thalweg. 

Regimen.  —  The  term  regimen  is  used  to  designate  the 
characteristics  of  a  river.      Its  normal  height,  its  greatest 
and  least  height,  its  normal  discharge  of  water,  its  normal  velocity  of 
flow,  its  cross  section  —  all  these  things  go  to  make  up  its  regimen. 


444  METEOROLOGY 

River  stage.  —  By  river  stage  is  meant  the  height  of  the  surface  of  a 

river  above  some  arbitrarily  chosen  zero  point.     The  zero  point  may 

be  mean  sea  level,  or  the  lowest  point  reached  by  the  river 

or  the  normal  height  of  the  river,  or  any  arbitrarily  chosen 

point.     Thus,  if  a  river  stage  is  stated  as  36  feet,  it  simply  means  that 

the  surface  of  the  river  is  36  feet  above  the  definite  zero  point   from 

which  all  heights  are  reckoned. 

Flood  line.  —  The  flood  line  is  some  definite  river  stage  so  chosen 

because  a  greater  height  than  this  can  be  considered  a  flood. 

Thus,  if  the  flood  line  is  40  feet,  it  means  that  a  river 

stage  above  40  feet  would  result  in  an  overflow  and  a  damage-causing 

flood. 

River  slope.  —  By  river  slope  is  meant  the  change  in  elevation  of  the 

river  surface  with  distance.     It  is  usually  expressed  as  so  many  inches 

per  mile.     In  the  case  of  great  rivers,  it  is  never  more  than 

a  few  inches  per  mile.     In  rapidly  flowing  streams  it  is  much 

more  and  may  amount  to  several  feet. 

Wetted  perimeter.  — ^  The  length  of  a  line  from  one  side  of  a  river 
Wetted  to  the  other  measured  along  the  bottom  is  called  the  wetted 
perimeter  perimeter. 

Mean  hydraulic  depth.  —  The  area  of  the  cross  section  of  a  river 
-,  .  divided  by  the  wetted  perimeter  is  called  the  mean  hy- 

Meanhy-  J 

draulic  depth,  draulic  depth. 

THE  MEASUREMENTS  MADE  IN  CONNECTION  WITH  RIVERS 

419.  Velocity  of  flow.  —  The  velocity  of  flow  of  a  river  is  determined 
ordinarily  by  means  of  a  current  meter,  which  consists  essentially  of  a 
A  current  propeller  wheel  which  revolves  faster  the  greater  the  velocity 
meter.  of  ^ne  current.  The  Price  current  meter  as  made  by  W.  and 

L.  E.  Gurley  of  Troy,  N.Y.,  is  pictured  in  Fig.  146.  This  instrument 
is  used  by  the  U.  S.  Coast  and  Geodetic  Survey  and  by  many  hydraulic 
engineers  in  different  parts  of  the  country.  It  consists  essentially  of 
five  conical  buckets  so  arranged  that  they  turn  easily  with  the  slightest 
current.  They  are  provided  with  a  rudder  consisting  of  four  light 
metal  wings  or  vanes  in  order  to  keep  the  wheel  in  line  with  the  current. 
A  heavy  weight  (about  sixty  pounds)  with  a  wooden  rudder  is  attached 
for  deep-water  work  or  where  the  current  is  particularly  swift.  The 
instrument  is  so  constructed  that  electrical  contact  is  made  after  every 
revolution  of  the  wheel,  and  the  number  of  revolutions  is  counted  auto- 


FLOODS  AND  RIVER  STAGES 


445 


matically  by  an  electric  register.     A  reduction  table  is  furnished  with 
the  instrument  for  finding  the  velocity  which  corresponds  to  a  given 


FIG.   146.  —  The  Price  Current  Meter. 


number  of  revolutions  per  second.     It  would  be  a  little  more  accurate 
to  determine  the  reduction  table  for  each  instrument  separately.     This 


446  METEOROLOGY 

is  done  by  dragging  it  at  a  known  rate  of  speed  through  still  water. 
Sometimes,  instead  of  an  electric  register  for  counting  the  number  of 
revolutions,  the  wheel  is  so  constructed  that  a  hammer  strikes  against  a 
diaphragm  after  every  ten  revolutions,  and  the  sound  is  conveyed  to  the 
ear  of  the  observer.  These  are  called  acoustic  current  meters. 

In  making  a  complete  determination  of  the  velocity  of  flow  of  a  river, 
observations  must  be  made  at  different  depths  and  in  different  parts  of 
a  river.  This  practically  amounts  to  determining  the  velocity  of  flow 
at  all  points  in  the  cross  section. 

If  only  the  surface  velocity  is  desired,  it  can  be  determined  roughly  by 
watching  some  floating  object  and  determining  the  time  required  for  it  to 
be  carried  a  known  distance. 

The  velocity  of  flow  can  also  be  determined  by  computation  from 

the  slope  of  the  river  and  the  mean  hydraulic  depth.      In  order  to 

determine  the  slope,  the  difference  in  level  of  the  river  at 

methods  of     Pomts   several  miles  apart  must  be  accurately  determined 

determin-      by    surveying  methods.      Only  approximate   results    can, 

onflow?"*7     however,  be  obtained,  as  the  constants  in  the  formula  for 

computing  velocity  from   slope  and  mean  hydraulic  depth 

are  too  uncertain  and  depend  upon  too  many  things. 

420.  River  stage.  —  The  river  stage  or  the  height  of  the  river  surface 
above  some  arbitrarily  chosen  point  is  determined  by  means  of  a  river 
The  river  gauge.  This  consists  ordinarily  of  a  heavy  plank,  8  or  10 
gauge.  inches  wide  and  a  couple  of  inches  thick,  and  of  sufficient 

length  to  cover  the  greatest  possible  fluctuations  in  the  height  of  the 
surface  of  the  river.  The  gauge  is  ordinarily  divided  into  feet,  possibly 
inches  or  tenths  of  a  foot,  and  the  foot  marks  are  usually  numbered. 
The  gauge  is  placed  vertically  and  securely  fastened  to  a  bridge  pier,  the 
end  of  a  wharf,  or  the  like.  The  height  of  the  surface  of  the  river  can 
thus  be  readily  read  off.  Sometimes  the  gauge  is  not  placed  vertically, 
but  is  inclined  to  follow  the  river  bank.  It  should  always  be  graduated 
to  show  vertical  heights,  however. 

Every  river  gauge  should  be  provided  with  a  permanent  bench  mark 
near  by  on  shore.  A  bench  mark  is  simply  a  very  stable,  permanent 
The  bench  point  whose  elevation  above  sea  level  is  not  supposed  to 
mark.  change.  A  copper  bolt  in  the  stone  foundation  of  a  building, 

the  water  table  of  a  firmly  placed  building,  the  surface  of  some  large  stone 
in  a  building,  all  serve  well  as  bench  marks.  By  surveying  methods,  the 
height  of  the  bench  mark  above  mean  sea  level  should  be  determined, 
and  also  the  difference  in  level  between  the  bench  mark  and  the  zero  of 


FLOODS  AND  RIVER  STAGES  447 

the  river  gauge.  If  a  river  gauge  is  then  repaired  or  carried  away  by  a 
flood,  the  new  one  can  be  placed  with  its  zero  mark  at  exactly  the  same 
level  as  before.  The  zero  of  a  river  gauge  may  be  placed  anywhere  on 
the  gauge,  but  it  is  customary  to  put  it  so  low  that  the  lowest  water  will 
never  reach  it.  Negative  values  of  river  stages  are  thus  avoided. 

The  following  descriptions  of  the  river  gauges  at  Albany,  N.Y.,  and 
New  Orleans,  La.,  taken  from  W.  B.  publication  No.  227,  will  serve  as 
illustrations. 

Albany,  New  York 

"  Albany,  N.Y.,  is  on  the  Hudson  River,  150  miles  from  its  mouth. 

"  The  gauge  is  a  self -registering  tide  gauge,  patterned  after  those  used 
by  the  United  States  Coast  and  Geodetic  Survey  in  former  years.  It  is 
the  property  of  the  United  States  Engineer  Corps,  and  is  located  on  the 
east  side  of  the  State  Street  Bridge. 

"  The  bench  mark  was  established  in  1896  by  the  United  States  Engi- 
neer Corps,  is  on  the  southeast  corner  of  the  east  basement  window  on 
the  south  or  State  Street  front  of  the  United  States  Government  build- 
ing near  Dean  Street.  It  is  18  feet  above  the  zero  of  the  gauge  and 
18.2  feet  above  mean  sea  level. 

"  The  highest  water  was  21.4  feet  on  February  9, 1857.  It  was  due  to 
back  water.  On  October  4  and  5,  1869,  the  water  reached  a  stage  of 
18.5  feet,  the  highest  stage  due  to  rainfall  alone.  The  lowest  water  was 
-1.2  feet  on  Sept.  30,  1867." 

New  Orleans,  Louisiana 

"  New  Orleans,  La.,  is  on  the  Mississippi  River,  108  miles  above  the 
Gulf.  The  river  is  2400  feet  wide.  The  drainage  area  above  the  sta- 
tion is  1T235,500  square  miles. 

"  The  river  gauge  is  the  property  of  the  city  and  is  situated  at  the  foot 
of  Canal  Street  among  a  cluster  of  piles  in  rear  of  ferry  wharf.  It  is 
made  of  cypress,  and  is  painted  white  with  markings  in  black. 

"  Bench  mark  at  corner  of  Common  and  Delta  Streets,  on  iron  cornice, 
6  inches  above  sidewalk  at  E.  Conery's  store,  is  16.5  feet  above  zero 
of  gauge,  and  14  feet  above  mean  sea  level.  Curbstone  under  third 
window  of  customhouse  from  Decatur  Street,  and  on  Customhouse 
Street,  is  11  feet  above  zero  of  gauge,  and  8.5  feet  above  mean  sea  level. 

"  Graduation  is  from  zero  to  17  feet  above.  Highest  water  was  19.5 
feet  on  May  13,  1897  ;  lowest,  -0.2  feet  on  December  27,  1872.  Dan- 
ger line  is  at  16  feet." 


448        .  METEOROLOGY 

421.  Cross  section  of  a  river.  —  The  determination  of  the  cross  section 
of  a  river  belongs  to  hydrographic  surveying.     It  is  necessary  to  deter- 
The  cross       mine  by  sounding  the  depth  of  the  water  every  few  feet  all 
section  of  a    the  way  across  the  river.     These  observations  can  then  be 

plotted  to  scale  and  a  cross  section  of  the  river  determined. 

422.  River  discharge.  —  River  discharge  may  be  found  in  two  ways: 
by  means  of  a  weir  or  dam,  and  by  computation  from  the  cross  section 

and  the  velocity  of  flow. 

we*r  me^hod  is  by  far  the  most  accurate,  but  can  be 


termining       applied  only  to  small  streams.     It  consists  in  forcing  the 

charge!*        water  to  flow  over  a  weir  or  dam.     By  measuring  the  width 

of  the  stream  and  the  depth  of  the  water  flowing  over  the 

dam,  fairly  exact  values  of  the  discharge  can  be  found  by  computation. 

If  the  cross  section  of  a  river  is  known  and  the  average  velocity  of 

flow  has  been  determined,  the  discharge  can  be  computed.     The  product 

of  the  area  of  the  cross  section  in  square  feet  by  the  velocity  in  feet  per 

second  gives  the  discharge  in  cubic  feet  per  second. 


RIVER  DATA 

423.  A  complete  description  of  a  river  and  its  characteristics  would 
include  material  and  data  concerning  both  the  watershed  and  the  river 
What  a  itself.  In  connection  with  the  watershed  or  drainage  area, 
complete  a  full  treatment  would  include  an  account  of  its  topography, 
ofTriver*1  meteorology,  and  climate.  The  most  important  items  are 
would  a  map  of  the  watershed  showing  the  elevations  and  the  char- 

acter of  the  surface,  and  normal  values  and  data  concerning 
the  precipitation,  snowfall,  temperature,  evaporation,  and  run-off. 
All  other  facts  in  connection  with  the  topography,  meteorology,  or 
climate  would  be  of  interest  and  value,  but  of  secondary  importance. 

A  treatment  of  the  river  itself  would  include  a  detailed  description  of 
the  course  of  the  river  and  data  in  connection  with  the  flow  of  water. 
The  cross  section  of  the  river  at  various  points,  its  length,  the  height  of 
its  banks,  and  the  area  which  would  be  covered  by  a  rise  of  a  given  amount 
should  all  be  known.  In  connection  with  the  flow  of  water,  normal 
values  and  data  for  the  river  stages,  the  velocity  of  flow  at  different 
stages,  the  river  discharge  at  various  stages,  the  causes  of  floods,  and  the 
velocity  of  progression  of  a  flood  wave  should  all  be  known. 

Complete  data  for  a  river  and  its  drainage  area  are  probably  available 
for  very  few  rivers,  but  the  important  items  are  known  for  most  rivers. 


FLOODS  AND  RIVER  STAGES  449 


THE  DIFFERENT  KINDS  OF  FLOODS 

424.  Floods  in  rivers  may  be  caused  in  a  variety  of  ways.  (1)  Floods 
may  be  caused  by  the  breaking  of  a  dam,  by  the  breaking  of  levees,  or 
by  a  sudden  change  of  course  by  a  river.  (2)  Floods  may  . 

be  caused  by  the  temporary  blocking  of  a  river  by  an  ava-  kinds  of 
lanche,  landslide,  or  glacier.  This  choking  of  the  river  channel  fl.oods  in 
would  cause  a  temporary  lake  back  of  the  obstruction,  and 
the  giving  way  of  this  barrier  might  cause  disastrous  floods  along  the 
lower  course  of  the  river.  (3)  Floods  can  be  caused  by  the  luxuriant 
growth  of  vegetation  which  may  choke  the  river  channel  and  thus 
cause  a  rise  of  water.  (4)  Floods  may  be  caused  by  the  formation  of 
ice  dams  when  the  ice  breaks  up  in  the  spring.  These  cause  floods  both 
above  and  later  below  the  dam.  (5)  Floods  are  often  caused  by  rather 
sudden  melting  of  the  snow  and  ice  over  a  considerable  portion  of  a 
watershed.  (6)  Floods  due  to  excessive  precipitation  over  the  water- 
shed are  the  most  common  of  all  floods.  Of  these  various  kinds  of  floods, 
those  caused  in  the  first  two  ways  are  of  unusual  occurrence,  and  are 
far  from  common.  Those  caused  by  vegetation  and  ice  dams  are  fairly 
common  in  certain  rivers.  Those  caused  by  the  melting  of  snow  and 
excessive  precipitation  are  the  commonest  of  all  floods. 

The  best-known  flood  due  to  the  breaking  of  a  dam  is  the  one  which 
occurred  at  Johnstown,  Pa.,  June  1,  1889.  A  reservoir  about  3  miles 
long  and  1  mile  wide  and  perhaps  100  feet  deep  was  held  by  a  iuustrations 
dam  1000  feet  wide.  The  sudden  breaking  of  this  dam  pre-  of  the  va- 
cipitated  the  water  upon  Johnstown,  18  miles  below,  and  nous  kinds* 
caused  the  loss  of  nearly  3000  lives. 

The  breaking  of  levees  along  the  lower  Mississippi  River,  particularly 
when  the  river  is  in  flood,  is  of  fairly  common  occurrence  and  often  floods 
large  areas. 

The  sudden  changing  of  the  course  of  a  river  usually  occurs  in  moun- 
tainous regions  where  the  river  is  small  and  swift,  or  at  the  delta  where 
a  very  large  river  flows  into  the  ocean.  In  the  first  instance,  very  little 
damage  is  usually  done.  Disastrous  results  ordinarily  follow  when  a 
large  river  changes  its  course.  It  is  said  that  the  Hwangho  river  in 
China  is  particularly  prone  to  change  its  channel,  and  that  the  place 
where  it  empties  into  the  ocean  has  varied  as  much  as  300  miles  during 
the  last  4000  years.  These  great  changes  have  caused  the  loss  of  many 
millions  of  lives. 

The  temporary  blocking  of  a  river  by  an  avalanche  or  landslide  or  by 
2a 


450  METEOROLOGY 

the  forward  movement  of  a  glacier  occurs  only  in  mountainous  regions 
near  the  source  of  a  river.  This  blocking  of  the  channel  may  cause, 
however,  a  fairly  large  lake ;  and  if  the  barrier  breaks  suddenly,  disas- 
trous floods  are  sure  to  occur  along  the  course  of  the  river. 

Floods  due  to  vegetation  are  said  to  be  common  along  the  Upper  Nile 
and  in  the  Parana  River  in  South  America. 

Ice  dam  floods  are  common  in  all  rivers  which  freeze  over  to  a  con- 
siderable depth  in  winter.  When  the  ice  breaks  up  in  the  spring,  it  is 
apt  to  form  a  dam  at  a  narrow  part  of  the  channel  or  where  it  is  ob- 
structed by  a  bridge.  Such  a  dam  causes  floods  above  it,  and,  if  it 
breaks  suddenly,  floods  below  it  are  likely  to  occur. 

Floods  due  to  the  sudden  melting  of  the  snow  and  ice  on  the  watershed 
or  to  excessive  precipitation  over  the  watershed  are  common  in  all  parts 
of  the  world.  These  two  causes  are  also  likely  to  occur  together  during 
the  late  winter  and  early  spring,  in  which  case  the  rise  of  the  rivers  will 
be  particularly  large. 

Of  these  six  kinds  of  floods,  the  last  two  are  the  only  ones  which  can  be 
predicted. 

THE  CHARACTERISTICS  OF  INDIVIDUAL  RIVERS 

425.  Limited  space  does  not  permit  the  detailed  treatment  of  the 
characteristics  of  even  one  particular  river.  The  reader  must  be  referred 
to  the  literature  on  the  subject  for  the  characteristics  of  any  river  in 
which  he  may  be  especially  interested.  What  would  be  included  in 
such  a  complete  treatise  has  already  been  stated  in  connection  with 
river  data. 

A  few  facts  about  the  more  important  rivers  of  the  world  may  be  of 

interest.     In  the  Nile  the  lowest  water  occurs  ordinarily  in  June  and 

the  highest  in  September  or  October.     At  Cairo,  Egypt,  the 

about  indi-     rise  is  from  15  to  30  feet,  and  the  country  is  inundated 

annually  to  a  considerable  distance  on  each   side  of   the 

river.     The  fertility  of  Lower  Egypt  is  due  to  the  water 

gained  in  this  way,  and  to  the  alluvium  brought  down  by  the  river. 

The  rise  in  the  Nile  is  due  to  large  rainfall  in  Abyssinia  of  the  monsoon 

type. 

In  the  Yangtse-Kiang,  the  Amur,  and  the  Hwangho  rivers,  the  rise 
occurs  during  the  late  summer.  This  is  due  to  monsoon  rains  over  the 
interior  of  Asia  and  shows  to  what  extent  the  monsoon  penetrates  the 
continent.  Floods  during  the  summer  are  particularly  damage-causing 


FLOODS  AND  RIVER  STAGES  451 

as  they  occur  during  the  time  of  growing  crops.  A  summer  rise  due  to 
monsoon  rains  is  also  true  of  the  Congo,  the  Ganges,  and  the  Brahma- 
putra. 

The  Amazon  River  changes  but  little  during  the  year,  as  the  rainfall 
is  more  uniform,  and  when  the  tributaries  on  one  side  are  in  flood,  those 
on  the  other  side  are  usually  not. 

In  the  Mississippi  and  Ohio  rivers,  and  in  the  Rhine,  Seine,  and  Elbe 
in  Europe,  the  floods  occur  during  the  winter  and  spring.  They  are 
caused  by  large  rainfall  over  the  watershed  while  the  ground  is  frozen,  or 
by  the  sudden  melting  of  large  quantities  of  snow.  Very  often  the  two 
causes  operate  together.  The  rivers  of  New  England  and  near-by  states 
also  have  floods  during  the  winter  and  spring  for  the  same  reasons.  In 
these  rivers  ice  dams  often  form  when  the  ice  breaks  up  in  the  spring. 

THE  PREDICTION  OF  RIVER  STAGES  AND  FLOODS 

426.   The  prediction  of  river  stages  and  the  height  and  time  of  occur- 
rence of  a  flood  crest  may  be  made  in  two  different  ways.     One  is  on 
the  basis  of  what  has  occurred  on  the  watershed,  and  the 
other  -is  on  the  basis  of  river  stages  which  have  been  ob-  methods  of 
served  on  gauges  higher  up  the  river  and  on  its  various  Predicting 

,    .,  .  river  stages. 

tributaries. 

If  the  first  method  is  followed,  the  condition  of  the  watershed  must 
be  known ;  that  is,  its  extent,  whether  the  ground  is  frozen  or  not,  the 
amount  of  snow  which  may  rest  on  it,  etc.     A  measured  int^^st 
amount  of  rain  at  a  known  temperature  has  fallen  on  the  method  oc- 
whole  or  a  part  of  the  watershed  in  a  certain  time.     The  S£lJJS£lon 
problem  is  to  determine  the  resulting  rise  in  the  river  due  to  shed  are 
the  run-off.     It  might  seem  that  the  problem  could  be  solved  used< 
theoretically;    that  is,  that  it  could  be  determined  what  the  run-off 
would  be  for  the  watershed  in  a  known  condition  due  to  the  rainfall 
in  question.     As  a  matter  of  fact,  the  problem  is  too  complex  to  be 
treated  with  any  accuracy  in  this  way.     If,  however,  records  for  a  con- 
siderable time  are  available,  it  is  possible  to  determine  from  past  occur- 
rences about  what  the  resulting  rise  in  a  river  will  be  for  certain  happen- 
ings on  the  watershed.     But  even  when  done  in  this  way,  the  results 
are  so  uncertain  that  this  method  is  never  used  by  the  U.  S.  Weather- 
Bureau  in  forecasting. 

The  second  method  of  predicting  floods  and  river  stages  is  by  means 
of  gauges  placed  higher  up  the  river  and  on  its  various  tributaries. 


452  METEOROLOGY 

When  a  flood  occurs  the  water  rises,  attains  its  greatest  height,  and 
then  falls.  A  flood  may  thus  be  considered  as  a  wave  which  progresses 

down  a  river  at  a  certain  speed.  By  means  of  the  river  gauges 
omTmethod  Placed  higher  up  the  river  and  on  its  tributaries  the  location 
gauge  read-  and  height  of  the  different  flood  waves  may  be  determined, 
used"'  Based  on  past  experience,  the  height  of  the  flood  wave  at 

the  station  in  question  and  its  time  of  occurrence  can  be 
predicted.  The  whole  prediction  of  floods  rests  upon  a  series  of  rules  or 
tables  derived  by  studying  critically  the  records  of  previous  floods. 
This  method  can,  of  course,  be  applied  only  to  the  lower  part  of  the 
course  of  a  river  and  not  to  its  sources. 

427.  The  prediction  of  river  stages  and  floods  is  part  of  the  regular 
work  of  the  U.  S.  Weather  Bureau,  and  belongs  to  the  river  and  flood 
The  work  of  secti°n-     It  was  formerly  under  one  man,  but  the  predictions 
the  Weather  are  now  made  by  many  near  the  various  rivers.     Daily  river 

gauge  readings  are  made  at  8  A.M.  at  many  stations  located 
on  the  various  rivers.  These  observations  are  telegraphed  to  32  centers, 
and  the  preparation  of  forecasts  and  warnings  is,  in  most  cases,  intrusted 
to  the  officials  of  the  Weather  Bureau  at  these  centers,  under  the  super- 
vision of  the  forecast  official  at  Washington.  The  regulations  concern- 
ing the  issuing  of  forecasts  have  already  been  stated  in  section  391. 

SUDDEN  RISES  OF  OCEANS  AND  LAKES 

428.  Sudden  rises  of  the  ocean  are  caused  either  by  earthquakes  or 
tropical   cyclones.     There   are  several   notable   examples  where  tidal 
Sudden  rise    waves  more  than  50  feet  high  have  been  caused  by  earth- 
of  the  ocean.  quakes  and  great  loss  of  life  has  resulted.     The  rise  of  water 
caused  by  the  Galveston  cyclone  (see  section  271)  is  a  good  example  of 
the  second  kind. 

A  sudden  increase  in  the  height  of  the  surface  of  a  lake  at  a  certain 
point  is  called  a  seiche^  and  these  are  common  in  the  Great  Lakes  of  the 
United  States  and  in  some  lakes  in  Switzerland,  and  other 
parts  of  the  world.  They  are  probably  caused  by  thunder- 
showers  or  a  sudden  change  in  wind  direction  or  velocity.  They  some- 
times amount  to  several  feet.  It  is  said  that  the  surface  of  the  water  on 
the  south  side  of  Lake  Erie  is  always  several  feet  higher  when  the  north 
wind  blows  than  when  the  south  wind  blows.  Thus  a  sudden  change  in 
wind  direction  might  readily  cause  a  seiche.  After  the  surface  of  a  lake 
had  once  been  changed  from  a  level  surface  by  the  action  of  the  wind,  if 


FLOODS  AND  RIVER  STAGES  453 

the  wind  should  die  down  to  a  calm,  the  surface  would  then  oscillate 
about  a  nodal  line  near  the  center  of  the  lake  until  finally  brought  to 
rest  by  friction.  The  continuance  of  seiches  after  the  cessation  of  wind 
can  thus  be  explained. 

TOPICS   FOR   INVESTIGATION 

(1)  The  run-off  under  various  conditions. 

(2)  Inland  drainage  basins. 

(3)  The  various  kinds  of  current  meters. 

(4)  The  complete  description  of  some  river. 

(5)  The  seiche. 

PRACTICAL  EXERCISES 

(1)  Investigate  critically  some  small  brook,  determining  the  characteristics 
of  the  watershed  and  the  discharge  and  behavior  of  the  brook  under  all  condi- 
tions. 

(2)  Determine  the  cross  section  of  some  river  and  the  velocity  at  all  points 
in  it. 

REFERENCES 

FRANKENFIELD,  H.  C.,  The  Floods  of  the  Spring  of  1903  in  the  Mississippi 
Watershed.  Bulletin  M  ;  W.  B.  publication  No.  303. 

HENRY,  ALFRED  J.,  Wind  Velocity  and  the  Fluctuations  of  Water  Level  on 
Lake  Erie.  Bulletin  J  ;  W.  B.  publication  No.  262. 

MOORE,  WILLIS  L.,  The  Influence  of  Forests  on  Climate  and  on  Floods. 

MORRILL,  PARK,  Floods  of  the  Mississippi  River.  Bulletin  E ;  W.  B.  publica- 
tion No.  143. 

RUSSEL'L,  THOMAS,  Meteorology,  New  York,  1895.  (Chapters  IX  and  X  cover 
rivers  and  floods  and  river  stage  predictions.) 

Daily  River  Stages  at  River  Gauge  Stations  on  the  principal  Rivers  of  the 
United  States  (6  parts  have  been  issued.  The  Publication  was  com- 
menced by  the  Signal  Service  and  continued  by  the  Weather  Bureau.) 

Monthly  Weather  Review.  (The  condition  of  the  rivers  and  the  occurrence  of 
floods  are  here  summarized  monthly.  The  hydrographs  of  the  seven  prin- 
cipal rivers  are  also  given.) 


CHAPTER  XI 
ATMOSPHERIC  ELECTRICITY 

INTRODUCTION  —  HISTORY,  429 

THE  ELECTRICITY  OF  THE  EARTH,  AIR,  CLOUDS,  AND  RAINDROPS 

The  measurement  of  potential  differences  due  to  the  earth's  electric  field,  430. 

The  electric  field  of  the  earth,  431. 

The  conductivity  of  the  atmosphere,  432. 

The  source  of  the  charged  ions  and  the  earth's  negative  charge,  433,  434. 

The  source  of  the  electricity  of  clouds  and  raindrops,  435. 

Air  currents  and  earth  currents,  436,  437. 

THE  NATURE  AND  KINDS  OF  LIGHTNING 

The  cause  and  kinds  of  lightning,  438. 

Zigzag  lightning,  439. 

Other  kinds  of  lightning,  440. 

DANGER  FROM  LIGHTNING  AND  PROTECTION  FROM  LIGHTNING 

Loss  of  life  and  property  due  to  lightning,  441. 
Protection  from  lightning,  442. 

OTHER  MANIFESTATIONS  OF  ATMOSPHERIC  ELECTRICITY,  443. 

INTRODUCTION  —  HISTORY 

429.  Lightning  is  an  electric  spark  on  a  tremendous  scale.  Vague, 
indefinite  opinions  that  this  might  be  the  case  were  expressed  by  various 
Early  opin-  scientists  from  1600  on,  but  the  first  definite  assertion  was 
ions  and  ex-  made  by  J.  H.  Winkler  in  Leipzig,  in  1746,  and  he  attempted 
lts*  to  prove  it  by  analogy.  Benjamin  Franklin  proposed  experi- 
mental proofs  in  1749,  and  in  1752  sent  a  communication  to  the  Royal 
Society  in  London,  recommending  the  use  of  rods  with  points  as  light- 
ning conductors.  D'Alibard,  at  Marly  sur  Ville,  near  Paris,  through 
translating  Franklin's  communication,  received  the  incentive  to  carry 
out  a  series  of  experiments.  An  iron  rod  some  forty  feet  long  and  pro- 
vided with  a  point  was  attached  to  an  insulated  support.  When  a 
thundershower  approached,  sparks  an  inch  or  more  in  length  could  be 
drawn  from  the  rod.  Franklin's  famous  kite  experiment  was  performed  a 
little  later.  In  a  letter  dated  October  19,  1752,  he  describes  it  as  follows: 

454 


ATMOSPHERIC  ELECTRICITY  455 

"  Make  a  small  cross  of  light  sticks  of  cedar,  the  arms  so  long  as  to 
reach  to  the  four  corners  of  a  large,  thin  silk  handkerchief  when  extended. 
Tie  the  corners  of  the  handkerchief  to  the  extremities  of  the  fTanj/^nya 
cross,  so  you  have  the  body  of  a  kite  which,  being  properly  kite  ex- 
accommodated  with  a  tail,  loop,  and  string,  will  rise  in  the  penment- 
air  like  those  made  of  paper,  but  being  made  of  silk  is  better  fitted  to 
bear  the  wet  and  wind  of  a  thunder  gust  without  tearing.  To  the  top 
of  the  upright  stick  of  the  cross  is  to  be  fixed  a  very  sharp-pointed  wire, 
rising  a  foot  or  more  above  the  wood.  To  the  end  of  the  twine  next  the 
hand  is  to  be  tied  a  silk  ribbon,  and  where  the  silk  and  twine  join  a  key 
may  be  fastened.  This  kite  is  to  be  raised  when  a  thunder  gust  appears 
to  be  coming  on,  and  the  person  who  holds  the  string  must  stand  within 
a  door  or  window,  or  under  some  cover,  so  that  the  silk  ribbon  may  not 
be  wet  ;  and  care  must  be  taken  that  the  twine  does  not  touch  the  frame 
of  the  door  or  window.  As  soon  as  the  thunder  clouds  come  over  the 
kite,  the  pointed  wire  will  draw  the  electric  fire  from  them,  and  the  kite, 
with  all  the  twine,  will  be  electrified,  and  stand  out  every  way  and  be 
attracted  by  an  approaching  finger.  And  when  the  rain  has  wet  the 
kite  and  twine,  you  will  find  the  electric  fire  stream  out  plentifully  from 
the  key  on  the  approach  of  your  knuckle." 

De  Ramas,  in  France,  a  few  years  later  (1757)  was  able  to  get  sparks 
ten  to  twelve  feet  long  by  means  of  a  kite.     A  few  days  after  D'Ali- 
bard's  experiment,  Le  Monnier  was  able,  by  means  of  better  The  basis 
constructed  and  insulated  apparatus,  to  prove  that  there  was  of  the  old 
a  difference  of  potential  between  a  point  at  a  given  height  in  concePtion- 
the  air  and  the  earth  at  all  times,  even  when  the  sky  was  clear.     From 
this  time  on,  long  series  of  observations  were  made  by  many  investigators 
in  different  parts  of  the  world.     The  digest  of  all  this  experimental  and 
observational  material  led  to  the  formation  of  what  we  may  now  call 
the  old  conceptions  concerning  atmospheric  electricity.     This  old  con- 
ception has  been  considerably  modified  during  the  last  twenty  years, 
but  still  deserves  careful  consideration. 

The  earth  itself  is  a  conductor,  and  is  surrounded  by  a  non-conducting 
medium,  the  atmosphere.     The  earth  is  highly  charged  with 
negative  electricity  and  in  the  non-conducting  atmosphere  charged 
around  it,  there  is  thus  a  field  of  force.     There  would  thus  be  ««rthsur  - 

rounded  by 

a  difference  of  potential  between  a  point  at  a  given  height  in  a  non-con- 


the  atmosphere  and  the  earth,  and  this  potential  difference 
would  be  greater,  the  greater  the  elevation  of  the  point  in 
question.  Since  the  earth  has  a  charge  of  negative  electricity,  lines  of 


456  METEOROLOGY 

force  must  start  from  the  earth  and  extend  outward.  The  great  ques- 
tion was,  where  these  lines  of  force  ended,  in  other  words,  where  the 
corresponding  charges  of  positive  electricity  were  located.  Some  said 
in  the  clouds,  others  in  the  outer  regions  of  the  atmosphere,  or  on  the 
heavenly  bodies,  or  in  the  remote  depths  of  space.  The  origin  of  the 
earth's  negative  charge  was  never  satisfactorily  explained,  it  being 
generally  supposed  that  it  had  had  its  charge  from  the  beginning. 

Floating  in  the  atmosphere  and  carried  from  one  point  to  another,  are 
innumerable  dust  particles  and  minute  water  drops.  These  are  con- 
The  charged  ductors  and  charged  with  either  negative  or  positive  elec- 
particies.  tricity.  Various  ways  in  which  these  particles  may  have 
become  charged  have  been  suggested.  (1)  They  may  have  become 
electrified  by  friction.  Carried  rapidly  by  the  wind,  these  particles 
might  strike  against  material  objects  or  each  other.  Ice  crystals  and 
snowflakes  in  the  upper  air  may  strike  against  each  other.  (2)  At  the 
moment  of  evaporation,  a  particle  may  have  become  charged  nega- 
tively by  induction.  (3)  If  a  cloud  forms,  the  lower  side  would  have  a 
positive  charge  induced  on  it,  and  the  upper  side  would  have  a  negative 
charge.  If  such  a  cloud  should  be  suddenly  broken  through  horizontally 
by  the  wind,  the  two  portions  would  be  charged  and  with  opposite  kinds 
of  electricity.  (4)  Every  particle  floating  in  the  atmosphere  would  have 
positive  electricity  on  its  lower  side  and  negative  electricity  on  its  upper 
side  due  to  induction  by  the  charged  earth.  Now  ultra-violet  light 
readily  discharges  negative  electricity.  Thus  the  negative  electricity 
on  these  particles  might  be  discharged  and  scattered,  which  would  leave 
the  particle  charged  with  positive  electricity.  There  are  thus  at  least 
four  different  ways  in  which  these  little  conducting  particles  might  be- 
come charged  with  negative  or  positive  electricity.  When  a  cloud 
forms,  these  particles  become  nuclei  of  condensation,  and  thus  the  cloud 
particles  become  highly  electrified.  When  these  cloud  particles  further 
unite  to  form  raindrops,  the  quantity  of  electricity  steadily  increases, 
until  finally  there  is  a  lightning  flash  between  two  clouds  charged  dif- 
ferently or  between  a  cloud  and  the  earth. 

This  whole  conception,  then,  briefly  summarized  is  as  follows.     The 

earth  is  a  nearly  spherical  conductor  charged  with  negative  electricity 

and  surrounded  by  a  non-conducting  medium.     It  will  be 

surrounded  by  equipotential  surfaces  and  there  will  be  lines 

of  force  going  out  from  it.     In  the  non-conducting  surrounding  medium 

there  are  conducting  particles  which  become  positively  or  negatively 

electrified.     These  serve  as  nuclei  of  condensation  when  a  cloud  forms, 


ATMOSPHERIC  ELECTRICITY  457 

and  thus  cloud  particles  and  raindrops  collect  electricity  until  a  light- 
ning flash  occurs. 

The  first  difficulty  with  this  explanation  was  encountered  when  it  was 
found  that  the  atmosphere  was  not  a  non-conductor,  but  a  poorly  con- 
ducting medium.     It  was  found  that  a  charged  body  in  the 
atmosphere  was  slowly  discharged.     This  was  at  first  laid  cuities  with 
to  poor  insulation  of  the  body,  or  the  presence  in  the  atmos-  the  ?ld  con" 
phere  of  dust  and  water  particles.     It  was  soon  found  that 
this  discharging  of  a  charged  body  coulcl  not  be  accounted  for  in  this 
way,  as  the  rate  of  discharge  was  slower  when  the  moisture  was  high  or 
even  when  it  was  foggy.     Since  the  atmosphere  conducted  the  elec- 
tricity more  like  an  electrolyte  than  a  metallic  conductor  it  was  soon 
assumed  that  there  were  present  in  the  atmosphere,  numerous  small 
particles  or  portions  of  molecules,  called  gas  ions,  which  were  charged 
some  positively,  some  negatively.     The  actual  existence  of  these  ions 
has   since   been   experimentally   demonstrated.     The  presence   of  the 
charged  ions  at  once  raised  new  questions.     What  was  the  origin  of  these 
ions,  and  how  had  they  become  charged,  some  positively,  some  nega- 
tively ?     How  did  the  earth  retain  its  negative  charge ;   in  other  words, 
why  was  not  the  earth  discharged?     It  was  also  soon  found  that  these 
ions,  particularly  the  negatively  charged  ones,  also  served  as  Read just_ 
nuclei  of  condensation.     Another  source  of  the  electrifica-  mentneces-] 
tion  of   the  cloud    particles  and   raindrops  had  thus  been  sary* 
found.     A  readjustment  of  the  old  conception  was  thus  necessary,  and 
this  will  be  shortly  given. 


THE  ELECTRICITY  OF  THE  EARTH,  AIR,  CLOUDS,  AND  RAINDROPS 

430.    The  measurement  of  potential  differences  due  to  the  earth's 
electric  field.  —  Since  the  earth  is  charged  with  negative  electricity,  it 
must  be  surrounded  by  an  electric  field  of  force  and  a  poten-  Two  pieces 
tial  difference  must  exist  between  any  point  in  the  atmos-  of  apparatus 
phere  and  the   charged  earth.     In  order  to  measure  the  B    Jssary- 
potential  differences  between  the  earth  and  a  given  point  in  the  atmos- 
phere, an  electrometer  and  a  "  collector  "  are  necessary.     In  the  early 
determinations,  a  simple  crude  electroscope  was  used  as  the  electrom- 
eter.    It  consisted  of  a  glass  globe  or  case,  containing  a  metal  rod  to 
which  was  attached  two  pith  balls  or  two  light  straws,  or  The  eiec- 
two  leaves  of  thin  gold  foil.     By  the  divergence  of  these  light  frometer. 
objects  the  charge  and  thus  the  potential  difference  could  be  judged. 


458 


METEOROLOGY 


In  Fig.  147,  two  simple  electroscopes  which  may  serve  as  electrometers 
are  shown.  One  is  a  gold  leaf  electroscope  which  has  been  made  more 
sensitive  and  accurate  and  has  been  provided  with  a  scale.  The  other 
is  Braun's  electrometer.  Here  a  light  aluminum  pointer  moves  over  a 
scale  and  indicates  the  potential  difference.  For  more  precise  measure- 
ments, some  form  of  quadrant  electrometer  must  be  used.  For  the 
description,  theory,  and  use  of  these  well-known  pieces  of  electrical 
apparatus,  the  reader  must  be  referred  to  text-books  on  physics. 


FIG.   147.  —  Two  Simple  Electroscopes. 
(Froni  GOCKEL'S  Die  Luftelektrizitat.) 

The  so-called  collector  is  placed  at  the  point  in  the  atmosphere  for 
which  the  potential  difference  as  regards  the  earth  is  to  be  determined. 
The  In  the  early  experiments,  it  consisted  of  an  insulated  point 

collector.  or  pOints  connected  by  means  of  a  wire  with  the  electrometer. 
It  would  take  up  the  potential  of  the  point  where  it  was  located,  and  the 
difference  of  potential  between  this  point  and  the  earth  would  thus  be 
indicated  by  the  electrometer.  Later,  the  flame  of  a  lamp  placed  on  an 
insulated  support  or  some  slowly  burning  substance  was  used.  Still 
later,  the  water-dropping  collector  was  devised.  This  consists  simply  of 
a  vessel  of  water  on  an  insulated  support,  from  which  the  water  is  allowed 


ATMOSPHERIC  ELECTRICITY 


459 


to  fall  drop  by  drop.  This  last  collector  has  probably  been  the  most 
widely  used  of  any.  In  still  more  recent  experiments,  a  small  plate  or 
rod  covered  with  a  radioactive  substance  has  been  used. 

The  method  of  determining  the  potential  difference  is  thus  to  connect 
one  part  of  the  electrometer  with  the  earth,  and  the  other 
part  with  the  collector  which  is  placed  at  the  point,  and  the  ofhdetermind 
electrometer  reading  indicates  the  potential  difference.  ingpoten- 

431.    The   electric  field  of  the   earth.  —  Since  the  earth 
is  a  conductor  highly  charged  with  negative  electricity,  it 
must  be  surrounded  by  an  electric  field  of  force,  extending  out  indefi- 
nitely through  the  atmosphere,  and  an  equipotential  surface  could  be 
drawn  through  any  point  in  this  field  of  force.     By  an  equi- 
potential surface  is  meant   a  surface  containing  all  points  tial  surfaces 
which  have  the  same  potential  difference  as  regards  the  and  thcir 
earth.     If  the  earth  were  a  perfectly  smooth  conductor,  the 
equipotential  surfaces  would  be  parallel  to  the  earth's  surface,  that  is, 
concentric    with    the    earth.      As    a 
matter    of   fact,  the  surface   of   the 
earth  is  far  from  smooth  and  level, 
and  the  irregularities  greatly  distort 
the  equipotential  surface.     Numerous 
investigations    have    been    made    to 
determine  the  effect  of  a  hill,  moun- 
tain, tree,  building,   or  the  like,  on 
the   equipotential   surfaces.      It   has 
been    found  that  the  general  effect 

of  projections  is  to  warp  the  equipotential  surfaces  upward  and  cause 
them  to  be  closer  together.  This  is  represented  roughly  in  Fig.  148. 
It  will  be  seen  from  this  that  the  change  in  potential  with  elevation 
would  be  very  small  beside  a  building  or  hill.  On  the  other  hand,  above 
a  tree  or  hill  it  would  be  particularly  large.  This  must  be  held  in  mind 
in  choosing  a  point  for  which  to  determine  the  potential  difference  as 
regards  the  earth.  The  most  typical  and  usual  values  would  The  geo_ 
be  found  by  choosing  a  point  over  a  level  plain.  graphical, 

The  change  in  potential  with  elevation  amounts  ordinarily  n^ff'^ 
to  about  100  volts  per  yard,  and  a  point  in  the  atmosphere  irregular  va- 
is  positive  as  compared  with  the  earth.     This  change  in  ^  poten_ 
potential  with  elevation  is   by  no  means  a  constant.     It  tial  differ- 
grows  rapidly  less  with  altitude.     It  is  very  different   in 
different  parts  of  the  world.     It  has  a  periodic  daily  and  annual  varia- 


FIG.  148.  —  The  Equipotential  Surfaces 
over  an  Irregular  Surface. 

(From  HANN'S  Lehrbuch  der  Meteorologie.) 


460  METEOROLOGY 

tion  and  very  large  irregular  fluctuations,  which  are  closely  correlated 
with  the  meteorological  elements  and  storms.  Near  the  earth's 
surface,  as  just  stated,  the  change  in  potential  amounts  to  about 
100  volts  for  an  ascent  of  one  yard.  At  a  height  of  two  or  three  miles, 
the  change  per  yard  has  decreased  to  nearly  one  half  its  value  at  the 
earth's  surface  and  there  is  some  evidence  that,  at  the  height  of  five 
miles  or  more,  a  change  with  elevation  practically  ceases  to  exist.  This 
proves  that  the  earth  is  not  the  only  charged  conductor,  but  that  there 
are  charges  of  electricity  in  the  atmosphere  itself.  If  the  earth  alone 
were  charged,  the  change  in  potential  with  elevation  would  not  cease  to 
exist.  The  values  found  at  various  places  on  the  earth's  surface  are 
very  different.  This  may  be,  in  a  large  part,  due  to  irregularities  in 
the  surface,  but  the  larger  values  seem  to  be  found  in  middle  latitudes. 
Smaller  values  seem  to  be  found  for  cold  or  dry  places.  The  daily 
variation  is  very  complicated,  and  seems  to  show  two  maxima  and  two 
minima  very  similar  to  the  daily  variation  in  barometric  pressure.  The 
maxima  occur  in  the  middle  of  the  morning  and  in  the  early  evening. 
The  minima  occur  in  the  early  afternoon  and  before  sunrise.  The  graph 
which  represents  the  daily  variation  is  very  different  for  different  places 
and  sometimes  has .  only  one  maximum  and  minimum.  The  annual 
variation  shows  a  maximum  in  winter  and  a  minimum  in  summer. 
The  values  of  potential  difference  grow  less  with  higher  temperatures. 
Under  long-continued  bright  sunshine,  the  values  are  usually  less.  The 
values  also  grow  less  with  increasing  cloudiness.  With  increasing 
dampness  and  during  foggy  weather,  the  values  are  usually  larger.  The 
effect  of  wind  and  pressure  is  extremely  small  and  has  never  been  defi- 
nitely determined.  During  a  snowstorm  or  thundershower  tremendous 
irregular  fluctuations  occur.  The  positive  potential  difference  often 
becomes  negative  and  may  attain  values  as  high  as  10,000  volts  per  yard. 
If  under  normal  fair  weather  conditions,  the  change  in  potential  per 
yard  is  taken  as  100  volts,  and  if  it  is  furthermore  assumed  that  this  state 
The  charge  °^  things  is  the  same  over  the  whole  earth,  the  negative 
of  the  potential  to  which  the  earth  must  be  charged  can  be  com- 

puted.    The  value  would  be  about  600,000,000  volts. 
432.    The  conductivity  of  the  atmosphere.  —  Until  recent  times,  the 
atmosphere  was  always  considered  a  non-conductor.     The 
SeCc<race°-     °^  conception  was  that  the  atmosphere  was  a  non-conduct- 
tion  that  the    ing  gaseous  medium  surrounding  a  highly  charged  conductor, 
TsTcon^6     the  earth<     Coulomb,  in  1785,  was  the  first  to  note  that  an 
ductor.  insulated  charged  body  exposed  to  the  free  atmosphere  lost 


ATMOSPHERIC  ELECTRICITY  461 

its  charge,  and  he  stated  definitely  that  this  was  not  due  to  poor  insula- 
tion of  the  supports  of  the  charged  body,  but  that  the  charged  body 
gave  up  its  electricity,  in  part  at  least,  to  the  air  itself.  This  was  at 
once  ascribed  to  the  conducting  particles,  dust  particles,  and  minute 
water  drops  which  were  present  in  the  atmosphere.  More  than  a  hun- 
dred years  passed  without  any  noticeable  progress  or  change  in  opinion. 
In  1887  Linss  commenced  the  quantitative  investigation  and  measure- 
ment of  the  conductivity  of  the  atmosphere.  From  that  time  on,  an 
immense  amount  of  research  work  has  been  done  on  the  conductivity 
of  the  atmosphere ;  and  among  the  numerous  investigators,  the  names 
of  Elster  and  Geitel,  Wilson,  Ebert,  and  Gerdien  are  particularly  worthy 
of  mention.  It  was  soon  found  that  the  atmosphere  was  not  a  non- 
conductor, but  a  poor  conductor,  and  that  it  conducted  like  an  electro- 
lyte, rather  than  like  a  metallic  conductor.  The  presence  of  positively 
or  negatively  charged  gas  ions  as  the  cause  of  the  conductivity  was  thus 
suspected  in  the  atmosphere.  The  actual  presence  of  the  . 
ions  was  later  experimentally  demonstrated,  and  it  is  now 
possible,  by  means  of  rather  elaborate  experimentation,  to  determine 
the  number  present,  their  velocity  of  motion,  and  the  charges  which 
they  carry.  High  conductivity  thus  indicates  a  large  number  of  these 
charged  ions  present  in  the  atmosphere,  and  low  conductivity  a  small 
number.  It  has  also  been  found  that  there  are  two  sizes  of  ions, 
and  these  may  be  designated  as  large  and  small  ions.  The  small  ions 
are  of  about  the  same  size  as  molecules,  while  the  large 
ions  are  very  much  larger.  The  large  ions  correspond  very  Jindsof ions 
likely  to  the  dust  and  moisture  particles  and  are  thus  made  and  their 
up  of  perhaps  millions  of  molecules.  The  number  of  large 
ions  was  found  by  Langevin  in  1905  to  be  about  fifty  times 
as  large  as  the  number  of  small  ions,  but  the  small  ions  were  about  3000 
times  as  rapid  in  their  movements.  This  means  that  in  the  conductivity 
of  the  atmosphere  the  small  ions  play  by  far  the  larger  part,  while  in 
serving  as  nuclei  for  condensation,  the  large  ions  are  by  far  the  most 
important. 

The  number  of  ions  present  in  the  atmosphere,  that  is,  its  conductivity, 
is  by  no  means  constant.     It  is  very  different  in  different  places,  changes 
markedly  with  elevation,  has  a  well  marked  periodic  daily  The  varia_ 
and  annual  variation,  and  large  irregular  fluctuations  which  tions  in  the 
are  closely  correlated  with  the  meteorological  elements  and  c 
storms.     The  conductivity  at  various  places  on  the  earth's  surface  is 
very  different  and  seems  to  be  due  to  local  influences  such  as  the  presence 


462  METEOROLOGY 

of  mountains,  the  presence  of  radioactive  material  in  the  soil,  etc.  The 
conductivity  near  the  earth's  surface  and  in  caves  is  large.  It  then 
decreases  somewhat  with  altitude,  but  grows  very  large  again  at  the 
height  of  a  few  miles.  The  daily  variation  is  very  different  at  different 
places,  but  ordinarily  there  are  minima  at  sunrise  and  sunset,  and 
maxima  in  the  early  afternoon  and  about  midnight.  The  annual  varia- 
tion shows  higher  values  in  summer,  and  lower  values  in  winter.  The 
conductivity  of  the  atmosphere  is  particularly  small  during  hazy,  foggy 
weather,  when  the  moisture  is  large,  when  the  sky  is  cloud-covered,  or 
when  precipitation  is  falling.  This  is  directly  contradictory  to  the  old 
view  that  it  was  the  presence  of  dust  and  moisture  particles  in  the 
air  which  gave  it  its  conductivity.  The  conductivity  is  particularly 
large  when  it  is  very  clear  and  dry.  The  influence  of  temperature, 
pressure,  and  wind  has  never  been  definitely  determined  when  the  influ- 
ence of  these  has  been  separated  from  the  influence  of  the  other  things, 
which  usually  change  in  value  at  the  same  time. 

The  change  in  potential  with  elevation  and  the  conductivity  of  the 
atmosphere  usually  vary  in  value  at  the  same  time  but  in  opposite 
directions.  That  is,  when  the  conductivity  increases,  the  change  in 
potential  with  elevation  decreases,  and  vice  versa.  The  reason  for  this 
will  be  seen  later. 

433.  The  source  of  the  charged  ions  and  the  earth's  negative  charge. 
—  There  are  several  possible  sources  of  the  ions  which  are  found  in  the 
atmosphere  and  which  import  to  it  its  conductivity.  The 
sources  of  most  important  cause  of  ionization  is  without  doubt  the 
the  charged  presence  of  radioactive  material  or  the  decomposition  prod- 
ucts and  emanations  of  such  material.  These  substances 
are  found  in  minute  quantities  in  the  soil,  in  the  rocks  of  the  earth's 
crust,  in  the  water  from  springs,  in  the  air  in  caves  and  below  the  earth's 
surface,  and  in  the  lower  layers  of  the  atmosphere  itself.  These  sub- 
stances possess  the  power  of  forming  ions,  and,  in  fact,  one  of  the  best 
methods  of  determining  the  amount  of  radioactive  material  present  and 
its  strength  is  by  means  of  the  number  of  ions  which  it  produces  in  the 
air.  Experimental  determinations  of  the  number  of  ions  present  in  the 
air  in  caves,  or  closed  cellars,  or  in  the  air  taken  from  below  the  surface 
of  the  ground  shows  that  it  always  contains  a  particularly  large  number 
of  ions,  and  this  is  exactly  what  would  be  expected.  Another  cause  of 
ions  is  ether  waves  of  very  short  wave  length,  that  is,  ultra-violet  light. 
This  cause  of  ionization  would  probably  be  most  effective  in  the  upper 
layers  of  the  atmosphere  where  most  of  these  rays  are  absorbed.  An- 


ATMOSPHERIC  ELECTRICITY  463 

other  source  of  ions  is  the  breaking  up  of  air  molecules  into  positive  and 
negative  ions  either  spontaneously  or  possibly  due  to  impact  or  friction. 
Again,  charged  dust  particles  and  electrons  pushed  away  from  the  sun 
by  the  pressure  of  light  waves  may  make  their  way  into  the  earth's 
atmosphere  and  cause  the  formation  of  ions.  It  has  also  been 
found  that  when  raindrops  are  broken  up  into  smaller  ones,  both 
positive  and  negative  ions  are  given  off.  The  number  of  negative 
ions  is  usually  much  larger  than  the  number  of  positive,  so  that  the 
broken  raindrop  is  left  positively  charged.  If  these  are  the  causes 
of  the  ions  in  the  atmosphere,  it  will  be  seen  at  once  that  the  number 
of  these  ions  and  thus  the  conductivity  of  the  atmosphere  ought 
to  be  very  different  in  different  localities.  Furthermore  one  would 
expect  a  change  with  altitude  and  both  periodic  and  irregular 
fluctuations. 

434.  Since  the  atmosphere  which  surrounds  the  negatively  charged 
earth  is  not  a  non-conductor,  but  a  poor  conductor,  due  to  the  ions 
present  in  it,  the  maintenance  of  the  negative  charge  of  the 
earth  must  be  explained.  As  long  as  the  atmosphere  was  JjJ^ti^*" 
considered  a  non-conductor,  it  could  readily  be  supposed  of  the  nega- 
that  the  earth  had  received  its  negative  charge  in  the  begin-  oJth^eaftii 
ning,  and  it  would  always  retain  it.  Since  the  atmosphere  is 
a  poor  conductor  due  to  the  ions  in  it,  the  earth  would  attract  the  positive 
ions  and  repel  the  negative  ones  and  soon  lose  its  charge.  The  contin- 
uance of  the  negative  charge  must  thus  be  explained.  One  explanation 
is  that  it  is  due  to  precipitation.  It  has  been  determined  by  experiments 
that  raindrops  and  snowflakes  are  frequently  negatively  charged.  This 
would  mean  that  precipitation  would  bring  to  the  earth  negative  elec- 
tricity, and  thus  the  negative  charge  of  the  earth  might  be  maintained. 
Recent  experiments,  however,  seem  to  show  that  precipitation  brings  to 
the  earth  positive  electricity  more  often  and  in  as  great  quantity  as 
negative  electricity.  The  larger  raindrops  seem  to  be  charged  posi- 
tively and  the  smaller  ones  negatively.  Another  explanation  is  that  the 
earth's  negative  charge  is  due  to  the  ionization  of  the  air  in  the  cracks 
and  crevices  of  the  earth's  surface  itself.  The  radioactive  material 
ionizes  the  air  in  the  cracks  and  crevices  below  the  surface  of  the  earth. 
This  air  eventually  makes  its  way  out,  particularly  when  the  baro- 
metric pressure  is  lessening.  The  negative  ions  have  much  greater 
rapidity  of  motion  than  the  positive  ions.  As  a  result,  as  this  air  makes 
its  way  out,  the  negative  ions  strike  the  sides  of  the  vents  in  greater 
number,  and  thus  charge  the  earth  negatively  while  the  positive  ions 


464  METEOROLOGY 

escape  in  greater  numbers.     Both  of  the  causes  may  be  operative  even 
at  the  same  time. 

435.    The   source  of  the  electricity  of  clouds  and  raindrops.  —  The 

nine  methods  of  cloud  formation  have  been  fully  discussed  in  a  previous 

chapter.     In  eight  of  these  methods,  condensation  is  caused 

methods  of     by  the  lowering  of  the  temperature.     The  air  grows  colder, 

cloud  forma-  reaches  the  dew  point,  and  becomes  saturated  with  moisture. 

If  the  cooling  continues,  the  air  must  become  supersaturated 

or  condensation  must  take  place.     In  the  one  method  moisture  is  added 

to  the  air  which  is  already  saturated,  and  thus  either  condensation  or 

supersaturation  must  result.     When  condensation  takes  place,  the  dust 

and  other  particles  serve  as  nuclei  of  condensation. 

It  has  been  shown  experimentally  that  if  particles  are  absent,  the 
ions  can  serve  as  nuclei  of  condensation,  and  it  has  been  furthermore 
found  that  the  negatively  charged  ions  will  promote  conden- 
sation  when  supersaturation  has  been  carried  to  a  much  less 
become  extent  than  is  necessary  if  the  positive  ions  must  be  used  as 
nuclei.  The  negative  ions  are  also  much  more  rapid  in  their 
movements  than  positive  ions,  so  that  they  will  join  them- 
selves to  small  cloud  particles  in  greater  numbers  than  the  positive  ions. 
As  a  result,  cloud  particles  are  usually  charged  with  negative  electricity. 
These  cloud  particles  join  together  to  form  raindrops  and  snowflakes, 
and  then,  under  the  action  of  gravity,  commence  their  fall  to  the  earth's 
surface.  As  a  result,  an  excess  of  positive  ions  has  been  left  behind,  and 
it  is  a  fact  of  observation  that  the  cirrus,  the  highest  cloud,  is  usually 
positively  electrified.  As  the  negatively  charged  raindrops  or  snow- 
flakes  grow  in  size,  they  become  more  and  more  highly  charged  with 
electricity,  and  as  they  approach  the  earth,  the  positive  difference  of 
potential  between  a  point  in  the  atmosphere  and  the  earth  must  lessen. 
In  fact,  it  often  becomes  zero,  changes  to  a  negative  difference  of  poten- 
tial, and  may  attain  values  so  large  that  a  lightning  flash  is  the  result. 
The  raindrops  and  snowflakes  thus  bring  down  large  quantities  of 
negative  electricity  to  the  earth's  surface. 

On  the  other  hand,  the  raindrops,  particularly  the  large  ones,  must 
be  frequently  broken  up  by  the  wind.  When  this  occurs, 
How  rain  more  negative  than  positive  ions  are  released,  so  that 
become"17  ^  drop  becomes  positively  electrified.  This  may  be 
positively  repeated  many  times.  Thus  there  may  be  positively 
charged.  charged  raindrops  as  well  as  negatively  charged  ones,  and 
both  kinds  of  electricity  may  be  brought  to  the  earth  by  precipitation. 


ATMOSPHERIC  ELECTRICITY  465 

436.  Air  currents.  —  The  atmosphere  contains  these  positively  and 
negatively  charged  ions  in  large  numbers  and  is  a  conductor  for  this 
reason.     The  atmosphere  is  also  an  electric  field  of  force, 

since  the  earth  always  has  a  negative  charge,  and  clouds  fortheexist- 
are  charged  sometimes  positively,  sometimes  negatively.  ence  of  *"" 
As  a  result,  the  charged  ions  must  move  along  the  lines  of 
force  at  right  angles  to  the  equipotential  surfaces.  The  negatively 
charged  bodies  will  attract  the  positive  ions  and  repel  the  negatively 
charged  ions,  and  a  positively  charged  body  will  do  the  opposite.  Fur- 
thermore, these  charged  ions  will  be  carried  about  by  the  wind  and 
carried  up  by  rising  air  currents.  As  a  result,  there  ought  to  be  electric 
currents  in  the  atmosphere,  usually  vertical  ones,  but  sometimes  in 
any  direction,  particularly  in  the  upper  atmosphere  and  among  the 
clouds.  The  existence  of  such  currents  has  been  experimentally 
verified. 

Since  precipitation  must  bring  down  more  electricity  to  some  parts  of 
the  earth  than  others,  there  ought  to  be  earth  currents  to  neutralize 
these  discrepancies.     These,  again,  have  been  found  experi-  Earth 
mentally ;   and,  in  fact,  it  is  their  presence  in  long  telegraph  cummfcj- 
and  telephone  lines  which  at  times  causes  great  inconvenience. 

437.  The  view  concerning  the  electricity  of  the  atmosphere,  earth, 
clouds,  and  raindrops  has  thus  in  very  recent  times  changed  from  what 
might  be  called  a  statical  conception  to  a  dynamical  concep-  The  old  and 
tion.     The    statical    conception    was    that    the   earth    had  modern  con- 
received  its  negative  charge  once  for  all,  and  since  it  was  atmospheric 
surrounded  by  a  non-conducting  atmosphere,  it  would  always  electricity 
retain  it.     In  the  present  conception  also,  the  earth  has  a  con 
negative  charge,  but  this  must  be  constantly  replenished,  and  two  possible 
and  probable  sources  have  been  discussed.     The  atmosphere  is  a  poor 
conductor  by  reason  of  the  charged  ions  which  exist  in  it.     The  ions 
are  constantly  coming  into  existence  in  several  different  ways.     Radio- 
active material  and  ultra-violet  light  are  probably  the  two  chief  causes. 
The  ions  go  out  of  existence  by  joining  themselves  to  the  earth  and 
other  masses,  and  by  serving  as  nuclei  of  condensation  for  the  moisture. 
As  condensation  takes  place,  the  cloud  particles  gain  charges  of  elec- 
tricity, and  as  these  fall  later  to  the  earth  as  raindrops  or  snowflakes, 
great  changes  in  potential  are  caused  which  may  result  in  lightning. 
These  ions,  by  their  movements,  also  give  rise  to  air  currents  and  earth 
currents.     The    picture  is  thus  not  one  of  static  equilibrium,  but  of 
constant  motion  and  interchange  which  has  reached  a  steady  stage. 

2n 


466  •  METEOROLOGY 


THE  NATURE  AND  KINDS  OF  LIGHTNING 

438.  The  cause  and  kinds  of  lightning.  —  The  origin  and  nature  of  the 
electrification  of  the  cloud  particles  and  raindrops  have  already  been 
The  cause      fully  discussed.     As  the  raindrops  or  snowflakes  begin  their 
of  a  light-       fall  to  the  earth's  surface  under  the  action  of  gravity,  they 
ning  flash.      steadily  increase  in  size  until  the  bottom  of  the  cloud  is 
reached.     This  is  due  to  impact  with  other  cloud  particles  or  condensa- 
tion on  their  cold  surface.     As  they  increase  in  size,  they  just  as  steadily 
become  more  and  more  highly  charged.     If  the  cloud  is  a  thick  one,  or  if 
condensation  is  particularly  copious,  or  if  the  raindrops  are  especially 
large,  the  charge  may  become  very  large.     Thus  it  is  that  thick  clouds 
with  copious  rainfall    and    large  raindrops  are  especially  likely  to  be 
attended  by  lightning.     As  these  highly  charged  raindrops  come  towards 
the  earth  under  the  action  of  gravity,  great  changes  take  place  in  the 
earth's  electric  field  of  force.     Ordinarily,  the  positive  potential  differ- 
ence between  a  point  in  the  atmosphere  and  the  earth  grows  less,  becomes 
zero,  and  then  negative,  and  may  reach  enormous  values.     Eventually 
the  potential  difference  becomes  sufficient  to  cause  an  electric  spark  to 
pass  between  the  cloud  and  the  earth,  and  a  lightning  flash  is  the  result. 
It  has  been  found  experimentally  that  in  about  60  per  cent  of  the  cases 
a  lightning  flash  conveys  negative  electricity  to  the  earth,  and  in  the 
remaining  40  per  cent,  positive.     Different  parts  of  the  same  cloud  or 
different  clouds  may  be  charged  with  different  kinds  of  electricity. 
If  these  are  brought  nearer  together,  a  lightning  flash  may  result.     Light- 
ning occurs  between  two  clouds  probably  much  more  frequently  than 
between  a  cloud  and  the  earth. 

There  are  several  different  kinds  of  lightning.  The  commonest  form 
by  far  is  the  lightning  flash  which  occurs  between  two  clouds,  or  between 
The  kinds  of  a  cloud  and  the  earth.  This  is  usually  classified  as  zigzag 
lightning.  lightning.  Four  other  kinds  are  usually  recognized,  viz. 
sheet  lightning,  heat  lightning,  ball  lightning,  and  beaded  lightning. 

439.  Zigzag  lightning.  —  The  ordinary  lightning  flash  which  occurs 
between  two  clouds,  or  a  cloud  and  the  earth,  is  usually  considered  zig- 

.   .        zag   in    appearance.     Sinuous   would   probably    better   de- 
Description.  "  J 

scribe  its  appearance,  as  it  resembles  a  river  course  more  than 

a  series  of  straight  lines  joining  each  other  at  an  angle.     The  exact 

appearance  can  be  much  better  studied  by  photography  than 

hig°a°Ugh?"  ky  observing  directly,  and  many  good  lightning  photographs 

ning  flash,      are  now  in  existence.     They  can  best  be  secured  by  point- 


FIG.  149.  —  A  Lightning  Flash. 


ATMOSPHERIC  ELECTRICITY  467 

ing  the  camera  at  night  towards  an  approaching  thundershower  and 
leaving  the  shutter  open  several  minutes  in  the  hope  that  a  favorably 
placed  lightning  flash  may  occur  during  the  interval.  The  same  plate 
can  be  exposed  as  long  as  twenty  minutes  if  the  night  is  dark,  and  there 
are  no  artificial  lights  near  to  fog  the  plate.  A  lightning  flash  from  an 
"  untouched  "  negative  is  reproduced  in  Fig.  149,  and  the  sinuous  or 
zigzag  appearance  is  easily  seen.  There  are  several  causes  which  con- 
tribute to  giving  the  lightning  flash  this  appearance.  It  was 
formerly  supposed  that  the  lightning  flash  always  followed 
the  line  of  least  resistance,  that  is,  that  the  electric  discharge  zag  or 
sought  out  the  pockets  of  air  which,  on  account  of  the  presence 
of  dust  or  moisture,  or  some  other  cause,  had  the  greatest 
conductivity.  This  is  probably  to  some  extent  true,  but  the  sinuous 
appearance  is  due  largely  to  other  things.  If  a  photograph  of  a  light- 
ning flash  is  studied  carefully,  it  will  be  seen  that  wherever  there  is  a 
particularly  sharp  bend,  a  branch  extends  off  to  one  side.  When  a 
lightning  flash  occurs,  the  eye  is  somewhat  blinded  by  the  glare,  so  that 
only  the  trunk  flash  is  seen,  and  this,  furthermore,  so  claims  the  atten- 
tion that  the  side  branches  are  overlooked.  It  is  this  branching  struc- 
ture of  lightning,  however,  which  is  largely  responsible  for  its  sinuous 
appearance.  Again,  a  long  lightning  flash  is  not  seen  through  a  uniform, 
homogeneous  medium.  There  are  various  layers  in  the  atmosphere  of 
different  temperature  and  different  density,  and  these  are  being  moved 
about  and  mixed  by  the  wind.  As  a  result,  refraction  is  very  different 
in  some  directions  than  in  others,  and  the  broken,  sinuous  appearance  is 
the  result.  This  is  exactly  the  same  as  the  appearance  of  telegraph 
wires  seen  through  the  windowpane  of  a  moving  train.  Again  the  light- 
ning flash  might  have  a  slightly  spiral  form.  This  seen  in  projection 
against  a  dark  background  would  have  a  sinuous  appearance.  Prob- 
ably all  these  causes  work  together  to  give  the  lightning  flash  the  appear- 
ance which  it  has. 

The  length  of  a  lightning  flash  between  a  cloud  and  the  earth  is  not 
usually  more  than  three  quarters  of  a  mile  or  a  mile  at  most.     Between 
two  clouds,  however,  lightning  flashes  as  long  as  twenty  miles 
have  been  observed.     The    color    of  a    lightning  flash    is 
usually  white,  although  red,  yellow,  blue,  and  violet  tinges   spectrum  of 
have  at  times  been  observed.     The  spectrum  of  lightning  is  flash.*11"18 
a  bright  line  spectrum,  showing  generally  the  lines  of  nitrogen, 
although  the  lines  of  oxygen,  hydrogen,  and  some  of  the  other  constituents 
of  the  atmosphere  are  sometimes  glimpsed. 


468  METEOROLOGY 

Formerly  it  was  thought  that  a  lightning  flash  was  simply  an  electric 
spark  which  passed  from  the  cloud  to  the  earth,  or  possibly  from  the 

earth  to  the  cloud.  It  is  now  known  that  the  discharge  is 
anosciiiat-  oscillatory  like  the  discharge  of  a  Leyden  jar.  That  is, 
ing  dis-  instead  of  a  single  spark  or  discharge  passing  from  the  cloud 

to  the  earth  or  from  the  earth  to  the  cloud,  the  discharge  goes 
backward  and  forth  many  times,  growing  constantly  weaker.  This 
oscillation  occurs  many  times,  perhaps  in  some  cases  scores  of  times,  in 
an  extremely  short  time  interval,  a  time  interval  certainly  less  than  a 
thousandth  of  a  second.  This  has  been  determined  by  photographing 
a  lightning  flash  with  a  rapidly  revolving  camera.  It  has  thus  been 
possible  to  separate  and  photograph  as  separate  the  successive  oscilla- 
tions. Such  a  photograph  shows  the  /ightning  flash,  not  as  a  single 
sinuous  line,  but  as  a  series  of  sinuous  lines  side  by  side  and  nearly 
parallel.  There  is  some  evidence  that  the  beginning  of  a  flash  does  not 
consist  of  oscillations,  but  of  a  series  of  impulses  going,  say,  from  the 
cloud  towards  the  earth,  and  each  penetrating  a  greater  distance,  until 
finally  the  earth's  surface  is  reached.  That  a  lightning  flash  is  practi- 
cally instantaneous  can  be  shown  in  a  variety  of  ways.  A  fast  moving 

object  like  a  railway  train  is  seen  by  the  light  of  a  lightning 

flask  as  stationary.  Trees  swayed  by  the  wind  are  also 
practically  as  stationary,  and  a  photograph  of  them  shows  no  blurring 
taneous  ^ue  ^°  movmS-  A  simple  experiment  can  also  be  tried. 

Cover  a  bicycle  wheel  with  pieces  of  colored  paper,  and  when 
a  thundershower  occurs  at  night,  spin  the  wheel  rapidly.  Each  light- 
ning flash  will  reveal  the  wheel  as  stationary,  and  the  colored  pieces  of 
paper  will  be  seen  distinctly  and  without  blurring  or  running  together. 
A  variously  colored  spinning  top  will  also  serve  as  well  as  the  bicycle 
wheel.  It  is  a  well-known  fact  of  experience,  however,  that  a  lightning 
flash  seems  many  times  to  last  at  least  nearly  a  second.  This  is  due 
partly  to  the  observer.  A  lightning  flash  is  a  rather  unexpected,  start- 
ling, dazzling  occurrence.  The  various  details  which  have  impressed 
themselves  upon  the  eye  are  slow  in  coming  to  the  conscious  attention  of 
the  observer,  and  the  duration  is  thus  estimated  as  much  longer  than  it 
really  is.  But  photography  has  again  revealed  the  cause  of  this  long 
Multiple  duration  in  many  cases.  After  an  oscillatory  discharge 
flashes.  lasting  say  a  thousandth  of  a  second  has  taken  place  over  a 
certain  path,  there  is  a  decided  tendency  for  another  discharge  to  take 
place  over  the  same  path  a  few  tenths  of  a  second  later,  and  even  a  third 
or  fourth  discharge  may  follow,  so  that  the  duration  of  the  whole  occur- 


ATMOSPHERIC  ELECTRICITY  469 

rence  may  be  nearly  a  second,  but  we  have  to  do  here  with  a  multiple, 
not  a  single,  flash.  The  reason  why  a  second  discharge  follows  over  the 
same  track  is  probably  because  the  resistance  has  been  lessened  by  the 
first  discharge.  This  can  often  be  photographed  without  a  moving 
camera.  The  path  will  drift  enough  with  the  wind  between  the  first 
and  second  discharge,  to  allow  them  to  be  photographed  separately. 
This  was  the  case  in  the  photograph  which  is  reproduced  as  Fig.  149. 

A  lightning  flash  is  thus  an  oscillatory  discharge  lasting  but  an  ex- 
tremely short  time  and  followed  often  by  subsequent  discharges  a  few 
tenths  of  a  second  later  over  nearly  the  same  path.  Attempts  The  amount 
have  been  made  to  estimate  the  number  of  amperes  of  elec-  of  current- 
tricity  in  a  lightning  flash.  The  basis  for  these  estimations  is  the  induc- 
tion effects  of  the  discharge.  Twenty  thousand  amperes  is  the  figure 
usually  given  for  an  ordinary  lightning  flash.  Attempts  have  also  been 
made  to  reproduce  on  a  small  scale,  by  means  of  electrical  machines,  the 
characteristics  and  effects  of  lightning  and  with  fair  success. 

440.  Other  kinds  of  lightning.  —  Sheet  lightning,  heat  lightning,  ball 
lightning,  and  beaded  lightning  are  the  other  kinds  of  lightning  which 
deserve  a  passing  consideration. 

The  term  sheet  lightning  is  applied  to  the  sudden,  brief  lighting  up  of 
a  whole  cloud.  Sometimes  the  edges  of  the  cloud  appear  more  brilliantly 

illuminated  than  the  center.     At  times  it  appears  as  if  a  cur-  f 

,   .  •••   i    •  i    Sheet  Hiht- 

tam  were  suddenly  drawn  away,  disclosing  the  bright  cloud,  ning  and  its 

It  occurs  only  during  a  thundershower,  and  thunder  is  often  nature  and 
heard.     The  duration  is  fairly  long,  perhaps  a  second  or  two, 
and  the  illumination  often  occurs  in  pulses.     The  explanation  which 
most  naturally  suggests  itself  is  that  it  is  due  to  a  regular  lightning  flash 
which  is  hidden  from  view  by  the  cloud.     The  spectrum,  however, 
consists  of  bands  due  to  nitrogen  rather  than  lines.     It  may  thus  be  of 
the  nature  of  a  brush  discharge  rather  than  a  spark  discharge.     It  will 
be  remembered  that  when  the  poles  of  an  electrical  machine  are  pulled 
too  far  apart,  the  spark  discharge  changes  to  a  brush  discharge. 

Heat  lightning  is  the  term  applied  to  the  sudden  lighting  up  of  the 
atmosphere,  usually  when  it  is  hazy  and  misty  but  when  no  thunder- 
shower  is  visible.  It  is  generally  so  indefinite  that  it  is  hard 
to  localize  it  and  thunder  is  never  heard.  It  is  usually  ex- 
plained  as  the  reflection  from  the  hazy  air  of  the  lightning 
which  accompanies  a  thundershower  below  the  horizon  of  the  observer. 
It  may  also  be  a  brush  discharge  and  thus  of  the  same  nature  as  sheet 
lightning. 


470  METEOROLOGY 

Ball  lightning  is  the  least  known  and  the  hardest  to  explain  of  all, 
and  many  even  deny  its  existence.  It  sometimes  appears  as  a  ball  of  fire 
Ball  descending  from  the  clouds  but  more  usually  it  suddenly 

lightning.  makes  it  appearance  on  some  object  near  the  ground.  It 
appears  often  on  objects  inside  of  houses.  The  ball  of  fire  is  variously 
described  as  being  as  small  as  an  egg,  or  in  some  extreme  cases  even  as 
large  as  a  man's  head.  It  always  moves  slowly,  sometimes  in  a  zigzag 
line,  and  is  often  accompanied  by  a  hissing,  sputtering  sound.  Its 
path  is  sometimes  indicated  by  charring,  but  it  may  leave  no  trace.  It 
sometimes  silently  disappears,  sometimes  goes  into  the  ground,  some- 
times explodes  with  a  loud  report.  A  lightning  flash  of  the  usual  kind 
and  loud  thunder  usually  accompany  it.  As  has  just  been  stated,  there 
are  many  who  deny  the  existence  of  ball  lightning  and  would  explain  it  as 
imagination  or  an  optical  illusion.  There  are,  however,  rather  too  many 
carefully  described  authentic  cases  to  deny  its  existence.  Gockel  in 
his  book  Das  Gewitter  gives  twenty-four  good  examples,  and  if  all 
literature  were  searched,  hundreds  of  cases  would  be  found.  The  things 
observed  can  probably  be  explained  as  the  result  of  an  ordinary  lightning 
flash  together  with  induction  effects,  and  a  peculiar  mind  state  on  the 
part  of  the  observer  caused  perhaps  by  the  lightning. 

Beaded  lightning  is  of  very  rare  occurrence  and  is  the  term  applied 
to  a  lightning  flash  which  appears,  not  of  the  same  intensity  throughout, 
Beaded  but  like  a  series  of  luminous  beads  strung  together.  There 
lightning.  are  no^  enough  cases  on  record  to  make  an  attempt  at  an 
explanation  possible  or  worth  while.  It  might  be  an  ordinary  lightning 
flash  seen  through  a  layer  of  rippled  clouds. 

DANGER  FROM  LIGHTNING  AND  PROTECTION  FROM  LIGHTNING 

441.  Loss  of  life  and  property  due  to  lightning.  —  It  is  no  easy  matter 
to  get  accurate  statistics  for  a  whole  country  as  to  the  loss  of  life  and 
Sources  of  property  due  to  lightning.  Vital  statistics  and  the  accounts 
information.  wnich  appear  in  newspapers  must  be  relied  upon  for  in- 
formation concerning  the  loss  of  life.  The  records  of  insurance  com- 
panies and  newspaper  accounts  must  be  used  to  determine  the  property 
loss.  The  work  of  determining  the  loss  was  begun  by  the  Weather 
Bureau  in  the  United  States  in  1890  and  the  work  ended  with  the  year 
LOSS  of  life  1900.  During  this  last  year  a  particular  effort  was  made  to 
Untod  £e*  as  comPle^e  and  accurate  statistics  as  possible.  The 

states.          average  number  of  persons  killed  in  the  United  States  each 


ATMOSPHERIC  ELECTRICITY  471 

year  for  the  period  1891  to  1900  inclusive  was  377.  This  means 
that  on  the  average  six  persons  are  killed  each  year  out  of  every 
million  inhabitants.  During  the  single  year  1900,  when  the  particular 
effort  was  made  to  secure  complete  statistics,  713  were  killed.  This 
would  be  an  average  of  about  10  per  million.  Of  the  713  persons  killed 
during  1900,  291  were  killed  in  the  open,  158  in  houses,  57  under  trees, 
and  56  in  barns.  The  circumstances  attending  the  death  of  the  remain- 
ing 151  were  not  known.  Of  the  973  persons  who  were  more  or  less 
injured  by  lightning  during  the  same  year,  327  were  injured 
in  houses,  243  in  the  open,  57  in  barns,  and  29  under  trees.  The 
circumstances  attending  the  injury  of  the  remaining  317  were  not  known. 
The  number  killed  and  injured  in  the  open  is  a  rather  remarkable  showing. 
Most  of  those  killed  in  the  open  were  either  raised  above  their  surround- 
ings because  they  were  on  horseback  or  on  a  load  of  grain  or  on  a  wagon 
or  agricultural  implement  or  they  had  some  metallic  tool  in  their  hands. 
Statistics  as  to  the  number  killed  yearly  per  million  of  inhabitants  have 
been  prepared  by  various  investigators  for  various  countries  LOSS  of  life 
in  Europe.  For  England  and  Wales  the  number  killed  inEurope- 
annually  is  about  1  in  a  million ;  for  France  about  3  ;  for  Belgium  about 
2 ;  for  Sweden  about  3  ;  for  Prussia  about  6 ;  for  Hungary  about  16. 

All  of  the  figures  just  given  are  certainly  too  small  rather  than  too 
large.  There  is  but  a  slight  chance  that  the  same  case  would  be  recorded 
and  counted  twice,  while  a  great  many  cases,  particularly  in  sparsely 
populated  districts,  must  escape  notice  entirely.  The  figures  are  with- 
out doubt  sufficiently  accurate,  however,  to  give  a  fair  idea  as  to  the 
danger  from  lightning. 

The  property  loss  due  to  lightning  is  even  harder  to  determine  than 
the  loss  of  life.     During  1898  in  the  United   States  there  were  1866 
property  losses,  aggregating  $2,000,000.     Certain  kinds  of  The  ^nd  of 
buildings  are  struck  more  often  than  others.     A  summary  buUdings 
for  Schleswig-Holstein  for  the  years  1874  to  1883  shows  that  struck* 
each  year,  out  of  every  million  buildings,  163  ordinary  buildings  with 
hard  roofs,  386  ordinary  buildings  with  soft  roofs,  6277  churches,  8524 
windmills,  and  306  factories  were  struck  by  lightning.     The  character 
of  the  soil  also  makes  a  difference  with  the  number  of  lightning  strokes. 
The  order  of  frequency  of  lightning  strokes  on  the  various   The  effect  of 
soils  in  percentages,  deduced  from  380  reports  in  the  United   the  soil- 
States,  is  as  follows ;   loam,  26  per  cent ;   sand,  24  per   cent ;    clay,  19 
per  cent ;    prairie,  19  per  cent ;    scattering,  12  per  cent.     According  to 
Hellmann,  for  North  Germany,  if  the  liability  for  chalk  formation  is 


472  METEOROLOGY 

considered  1,  then  it  is  2  for  marl,  7  for  clay,  9  for  sand,  and  22  for  loam. 
If  lightning  strikes  sandy  soil,  a  glazed  tube  called  a  fulgurite,  some- 
times two  or  three  feet  in  length,  is  formed.  If  lightning  strikes  a  rock, 
the  surface  is  sometimes  glazed  in  spots.  There  is  also  a  great  difference 
in  the  kind  of  trees  that  are  struck  by  lightning.  Observations  made 
The  kinds  of  on  an  area  °^  aDOU^  45,000  acres  in  a  German  forest  showed 
trees  that  that  the  various  kinds  of  trees  were  struck  as  follows  :  oaks, 
ack*  159  times  ;  beeches,  21  times ;  pines,  20  times ;  firs,  59  times ; 
birches,  4  times  ;  larches,  7  times ;  ashes,  5  times.  And  this  occurred  in 
a  forest  which  was  composed  approximately  as  follows :  beech,  70  per 
cent ;  oak,  11  per  cent ;  pine,  13  per  cent ;  fir,  16  per  cent.  Using  these 
and  similar  observations  as  a  basis,  the  statement  can  be  made  that  the 
oak  is  particularly  liable  to  be  struck,  that  the  elm,  chestnut,  and  pine 
stand  next,  and  that  the  beech,  birch,  and  maple  are  almost  never  struck. 
When  a  tree  is  struck,  it  is  usually  the  trunk  which  is  injured,  not  the 
branches. 

.  The  question  is  often  asked  if  the  danger  from  lightning  is  increasing. 
It  is  a  question  which  it  is  very  hard  to  answer  from  the  point  of  view  of 
I  h  d  n  statistics.  The  actual  number  of  deaths  and  injuries  from 
ger  from  lightning  and  the  actual  number  of  property  losses  is,  of  course, 
lightning  increasing.  But  the  number  of  inhabitants  and  the  number 

increasing : 

of  buildings  is  also  increasing  very  rapidly.  Again,  the 
means  of  gathering  news  and  the  wide  publicity  given  to  each  death  or 
each  property  loss  have  become  much  greater.  It  is  not  to  be  wondered 
at,  then,  that  the  number  of  deaths  and  the  number  of  property  losses 
seem  very  much  larger  than  they  used  to  be.  Statistics  on  the  subject 
give  no  very  definite  answer,  but  it  would  seem  from  them  that  there 
has  been  no  marked  increase  in  the  danger  from  lightning. 

442.  Protection  from  lightning.  —  The  first  suggestion  to  protect 
buildings  by  means  of  lightning  rods  was  made  by  Benjamin  Franklin. 
History  of  ^  'ls  contained  in  a  communication  to  the  Royal  Society  in 
lightning  London,  and  dated  July  29,  1752.  The  first  lightning  rod 

was  probably  erected  by  Franklin  on  his  own  house  in  Sep- 
tember, 1752,  about  a  month  after  his  famous  kite  experiment,  and  con- 
sisted of  a  pointed  iron  rod  which  extended  about  nine  feet  above  the 
chimney  of  the  building  and  was  connected  by  means  of  a  thick  iron 
wire  with  a  well.  Lightning  rods  soon  became  quite  common  in  the 
States,  but  in  Europe  their  introduction  was  slow.  In  1760  a  lightning 
rod  was  placed  on  the  Eddystone  lighthouse  at  Plymouth,  and  this  was 
probably  the  first  one  in  Europe.  A  lightning  rod  on  the  tower  of  the 


ATMOSPHERIC  ELECTRICITY  473 

Jakobikirche  in  Hamburg,  in  1796,  was  probably  the  first  in  Germany. 
Up  to  thirty  years  ago,  the  lightning  rod  was  very  common  and  most 
large  buildings  were  protected  by  them.     During  the  last 
thirty  years,  however,  the  use  of  lightning  rods  has  fallen  off  rdds  now 
greatly.     This  is  not  only  a  fact  of  popular  observation,  but  Iess  com- 
it  is  born  out  by  the  statistics  of  fire  insurance  companies 
and  by  the  reports  of  the  companies  engaged  in  the  manufacture  of 
lightning  rods.     The  reasons  for  this  are  probably  the  changed  ideas  as 
to  the  nature  of  lightning  and  the  kind  of  protection  furnished  by  rods, 
and  also  the  fact  that  nearly  all  buildings  are  now  insured  and  that 
covers  loss  due  to  lightning  as  well  as  fires  started  in  other  ways.     It  is 
sometimes  answered  that  it  is  cheaper  to  insure  them  than  to  put  up 
lightning  rods. 

The  damage  caused  by  lightning  is  mechanical  as  well  as  thermal. 
If  a  tree  is  struck,  it  may  be  completely  shattered  or  the  bark  may  be 
torn  from  the  trunk  in  a  long  strip.     Figures  150  and  151  Mechanical 
illustrate  an  oak  and  a  black  walnut  which  were  struck  by  damage- 
lightning.     If  i  building  is  struck,  bricks  may  be  torn  from  the  wall  or 
chimney,  masonry  may  be  cracked,  doors  may  be  splintered,  and  objects 
broken  to  pieces.     A  church  spire  struck  by  lightning  is  shown  in  Fig. 
152.     All  these  are  the  mechanical  effects  of  lightning ;  and  it  is  some- 
times thought  that  the  sudden  expansion  of  inclosed  pockets  of  air  or  the 
sudden  evaporation  of  inclosed  moisture  is  responsible  for  a  large  part 
of  the  damage.    Woodwork  may  be  charred,  inflammable  or  easily  ignited 
material  may  be  set  on  fire,  gas  pipes  may  be  punctured  and  Thermal 
the  gas  ignited.    All  these  are  the  thermal  effects  of  lightning;  effects, 
and,  in  fact,  the  greatest  damage  comes  from  the  fires  which  are  started. 
Not  only  is  damage  caused  by  the  main  discharge,  but  currents  are 
induced  in  near-by  metal  objects  and  conductors,  and  these  are  often 
damage-causing.      Many  of  the  vagaries  and  unusual  things  done  by 
lightning  are  due  to  these  induction  effects. 

It  was  formerly  supposed  that  a  lightning  rod  afforded  protection  in 
two  ways.     The  pointed  rod  was  supposed  to  carry  the  accumulation 
of  electricity  to  the  ground,  so  that  a  disruptive  discharge  Are  light- 
would  never  occur ;  and  if  one  did  occur,  it  would  lead  it  to  the  ning  rods 
ground,  as  it  would  be  the  best  conductor.     It  is  now  known  worth  whil 
that  the  potential  difference  between  a  point  in  the  air  and  the  ground 
changes  so  rapidly  during  a  thundershower  that  a  lightning  rod  could  not 
possibly  equalize  the  potential  difference  and  prevent  a  discharge.     Fur- 
thermore, the  quantity  of  electricity  in  a  lightning  flash  is  so  enormous 


474 


METEOROLOGY 


FIG.  152.  — A  Church  Spire  Struck  by  Lightning. 
Spire  (198  feet,  brick  and  brownstone)  struck 
July  29,  1894. 

(From  Scientific  American,  F.  J.  MOULTON.) 


that  only  a  very  large,  well- 
constructed  and  grounded 
lightning  rod  could  carry  it 
all  safely  to  the  ground.  It 
is  now  also  known  that 
lightning  is  an  oscillatory 
discharge,  and  there  are 
many  induced  currents 
which  may  be  as  damage- 
causing  through  their  me- 
chanical and  thermal  effects 
as  the  main  flash.  For  ex- 
ample, lightning  has  per- 
haps struck  the  lightning 
rod  of  a  house  and  has  been 
conveyed  to  the  ground.  A 
little  induced  current  in  a 
gas  pipe  may  have  set  on 
fire  some  easily  ignited 
substance,  and  the  building 
is  destroyed  by  fire  in  spite 
of  the  fact  that  the  light- 
ning rod  did  its  full  duty. 
The  question  is  thus  a 
very  pertinent  one  as  to 
whether  it  is  worth  while 
putting  lightning  rods  on  a 
building.  In  a  city  where 
buildings  are  close  to- 
gether, where  iron  is  much 
used  in  construction,  where 
there  are  many  metallic 
gutters,  and  where  fire 
companies  are  efficient, 
it  is  very. likely  not  worth 
while  to  instal  lightning 
rods.  In  the  country, 
however,  where  buildings 
stand  alone  and  condi- 
tions are  very  different,  a 


ATMOSPHERIC   ELECTRICITY  475 

considerable    protection   would    probably   be    gained    from    lightning 
rods. 

The  question  is  often  asked  as  to  what  kind  of  lightning  rods  should 
be  used,  and  how  they  should  be  attached  to  the  building.  The  best 
answer  to  such  questions  is  contained  in  the 

RULES  TOR  THE  ERECTION  OF  LIGHTNING  CONDUCTORS,  AS  ISSUED  BY 
THE  LIGHTNING  ROD  CONFERENCE  IN  1882,  WITH  OBSERVATIONS 
THEREON  BY  THE  LIGHTNING  RESEARCH  COMMITTEE,  1905 

(NOTE.  —  Paragraphs  beginning  with  odd  numbers  refer  to  Lightning  Rod 
Rules,  1882;  those  with  even  numbers  to  Lightning  Research  Committee's 
observations,  1905.) 

(1)  Points.  —  The  point  of  the  upper  terminal  should  not  be  sharp,  not  sharper 
than  a  cone  of  which  the  height  is  equal  to  the  radius  of  its  base.     But  a  foot 
lower  down  a  copper  ring  should  be  screwed  and  soldered  on  to  the  upper  ter- 
minal, in  which  ring  should  be  fixed  three  or  four  sharp  copper  points,  each  about 
six  inches  long.     It  is  desirable  that  these  points  be  so  platinized,  gilded,  or 
nickel  plated  as  to  resist  oxidation. 

(2)  It  is  not  necessary  to  incur  the  expense  of  platinizing,  gilding,  or  electro- 
plating.    It  is  desirable  to  have  three  or  more  points  beside  the  upper  terminal, 
which  can  also  be  pointed ;  these  points  must  not  be  attached  by  screwing  alone, 
and  the  rod  should  be  solid  and  not  tubular. 

(3)  Upper  terminals.  —  The  number  of  conductors  or  points  to  be  specified 
will  depend  upon  the  size  of  the  building,  the  material  of  which  it  is  con- 
structed, and  the  comparative  height  of  the  several  parts.     No  general  rule 
can  be  given  for  this,  but  the  architect  must  be  guided  by  the  directions  given. 
He  must,  however,  bear  in  mind  that  even  ordinary  chimney  stacks,  when  ex- 
posed, should  be  protected  by  short  terminals  connected  to  the  nearest  rod, 
inasmuch  as  accidents  often  occur  owing  to  the  good  conducting  power  of  the 
heated  air  and  soot  in  the  chimney. 

(4)  This  is  dealt  with  below  in  suggestion  3. 

(5)  Insulations.  —  The  rod  is  not  to  be  kept  from  the  building  by  glass  or 
other  insulators,  but  attached  to  it  by  metal  fastenings. 

(6)  This  regulation  stands. 

(7)  Fixing.  —  Rods  should  preferentially  be  taken  down  the  side  of  the  build- 
ing which  is  not  exposed  to  rain.    They  should  be  held  firmly,  but  the  holdfast 
should  not  be  driven  in  so  tightly  as  to  pinch  the  rod  or  prevent  the  contraction 
and  expansion  produced  by  changes  of  temperature. 

(8)  In  most  cases  it  would  be  advantageous  to  support  the  rods  by  holdfasts 
(which  should  be  of  the  same  metal  as  the  conductor)  in  such  a  manner  as  to 
avoid  all  sharp  angles.     The  vertical  rods  should  be  carried  a  certain  dis- 
tance away  from  the  wall  to  prevent  dirt  accumulating  and  also  to  do  away 


476  METEOROLOGY 

with  the  necessity  of  their  being  run  around  projecting  masonry  or  brick- 
work. 

(9)  Factory  chimneys.  —  These  should  have  a  copper  band  around  the  top, 
and  stout,  sharp,  copper  points,  each  about  one  foot  long,  at  intervals  of  two  or 
three  feet  throughout  the  circumference,  and  the  rod  should  be  connected  with 
all  bands  and  metallic  masses  in  or  near  the  chimney.     Oxidation  of  the  points 
must  be  carefully  guarded  against. 

(10)  As  an  alternative,  the  rods  above  the  band  might,  with  advantage,  be 
curved  into  an  arch  provided  with  three  or  four  points.     It  is  preferable  that 
there  should  be  two  lightning  rods  from  the  band  carried  down  to  the  earth 
in  the  manner  previously  described.      Oxidation   of    the    points    does   not 
matter. 

(11)  Ornamental  ironwork.  —  All  vanes,  finials,  ridge  ironwork,  etc.,  should 
be  connected  with  the  conductor,  and  it  is  not  absolutely  necessary  to  use  any 
other  point  than  that  afforded  by  such  ornamental  ironwork,  provided  the  con- 
nection be  perfect  and  the  mass  of  ironwork  considerable.    As,  however,  there  is 
a  risk  of  derangement  through  repairs,  it  is  safer  to  have  an  independent  upper 
terminal. 

(12)  Such  ironwork  should  be  connected  as  indicated  below  in  suggestion  3. 
In  the  case  of  a  long  line  of  metal  ridging,  a  single  main  vertical  rod  is  not  suffi- 
cient, but  each  end  of  the  ridging  should  be  directly  connected  to  earth  by  a  rod. 
Where  the  ridge  is  non-metallic,  a  horizontal  conductor  (which  need  not  be  of 
large  sectional  area)  should  be  run  at  a  short  distance  above  the  ridge  and  be 
similarly  connected  to  earth. 

(13)  Material  for  rod.  —  Copper,  weighing  not  less  than  6  ounces  per  foot 
run,  and  the  conductivity  of  which  is  not  less  than  90  per  cent  of  that  of  pure 
copper,  either  in  the  form  of  tape  or  rope  of  stout  wires  —  no  individual  wire 
being  less  than  No.  12  B.  W.  G.     Iron  may  be  used,  but  should  not  weigh  less 
than  2*  pounds  per  foot  run. 

(14)  The  dimensions  given  still  hold  good  for  main  conductors.     Subsidiary 
conductors  for  connecting  metal  ridging,  etc.,  to  earth  may  with  advantage  be 
of  iron  and  of  a  smaller  gauge,  such  as  No.  4  S.  W.  G.  galvanized  iron.    The 
conductivity  of  the  copper  used  is  absolutely  unimportant,  except  that  high 
conductivity  increases  the  surges  and  side  flashes,  and  therefore  is  positively 
objectionable.     It  is  for  that  reason  that  iron  is  so  much  better. 

(15)  Joints.  —  Although  electricity  of  high  tension  will  jump  across  bad 
joints,  they  diminish  the  efficacy  of  the  conductor,  therefore  every  joint, 
besides  being  well  cleaned,  screwed,  scarfed,  or  riveted,  should  be  thoroughly 
soldered. 

(16)  Joints  should    be    held   together  mechanically  as  well  as  connected 
electrically,  and  should  be  protected  from  the  action  of  the  air,  especially  in 
cities. 

(17)  Protection.  —  Copper  rods  to  the  height  of  10  feet  above  ground  should 


ATMOSPHERIC  ELECTRICITY  477 

be  protected  from  injury  and  theft  by  being  inclosed  in  an  iron  pipe  reaching 
some  distance  into  the  ground. 

(18)  This  regulation  stands. 

(19)  Painting.  —  Iron  rods,  whether  galvanized  or  not,  should  be  painted ; 
copper  ones  may  be  painted  or  not,  according  to  the  architectural  requirements. 

(20)  This  regulation  stands. 

(21)  Curvature.  —  The  rod  should  not  be  bent  abruptly  round  sharp  corners. 
In  no  case  should  the  length  of  the  rod  between  two  points  be  more  than  half  as 
long  again  as  the  straight  line  joining  them.    Where  a  string  course  or  other 
projecting  stonework  will  admit  it,  the  rod  may  be  carried  straight  through, 
instead  of  around  the  protection.     In  such  a  case,  the  hold  should  be  large 
enough  to  allow  the  conductor  to  pass  freely,  .and  allow  for  expansion,  etc. 

(22)  The  straighter  the  run  the  better.    Although  in  some  cases  it  may 
be  necessary  to  take  the  rod  through  the  projection,  it  is  better  to  run  out- 
side, keeping  it  away  from  the  structure  by  means  of  holdfasts,  as  described 
above. 

(23)  Extensive  masses  of  metal.  —  As  far  as  practicable,  it  is  desired  that 
the  conductor  be  connected  to  extensive  masses  of  metal,  such  as  hot-water 
pipes,  etc.,  both  internal  and  external ;  but  it  should  be  kept  away  from  all  soft 
metal  pipes,  and  from  internal  gas  pipes  of  every  kind.     Church  bells  inside  well- 
protected  spires  need  not  be  connected. 

(24)  It  is  advisable  to  connect  church  bells  and  turret  clocks  with  the  con- 
ductors. 

(25)  Earth  connections.  —  It  is  essential  that  the  lower  extremity  of  the 
conductor  be  buried  in  permanently  damp  soil ;  hence  proximity  to  rain  water 
pipes  and  to  drains  is  desirable.     It  is  a  very  good  plan  to  make  the  conductor 
bifurcate  close  below  the  surface  of  the  ground,  and  adopt  two  of  the  following 
methods  for  securing  the  escape  of  the  lightning  into  the  earth.     A  strip  of  copper 
tape  may  be  led  from  the  bottom  of  the  rod  to  the  nearest  gas  or  water  main  — 
not  merely  to  a  lead  pipe  —  and  be  soldered  to  it ;  or  a  tape  may  be  soldered 
to  a  sheet  of  copper,  3  feet  by  3  feet  and  -fa  inch  thick,  buried  in  permanently 
wet  earth,  and  surrounded  by  cinders  or  coke ;  or  many  yards  of  the  tape  may 
be  laid  in  a  trench  filled  with  coke,  taking  care  that  the  surfaces  of  copper  are, 
as  in  the  previous  cases,  not  less  than  18  square  feet.     Where  iron  is  used  for  the 
rod,  a  galvanized-iron  plate  of  similar  dimensions  should  be  employed. 

(26)  The  use  of  cinders  or  coke  appears  to  be  questionable  owing  to  the 
chemical  or  electrolytic  effect  on  copper  or  iron.     Charcoal  or  pulverized  carbon 
(such  as  ends  of  arc  light  rods)  is  better.    A  tubular  earth  consisting  of  a  per- 
forated steel  spike  driven  tightly  into  moist  ground  and  lengthened  up  to  the 
surface,  the  conductor  reaching;  to  the  bottom  and  bein<z;  packed  with  granulated 
charcoal,  gives  as  much  effective  area  as  a  plate  of  larger  surface,  and  can  easily 
be  kept  moist  by  connecting  it  to  the  nearest  rain  water  pipe.    The  resistance  of 
a  tubular  earth  on  this  plan  should  be  very  low  and  practically  constant. 


478  METEOROLOGY 

(27)  Inspection.  —  Before  giving  his  final  certificate,  the  architect  should 
have  the  conductor  satisfactorily  examined  and  tested  by  a  qualified  person,  as 
injury  to  it  often  occurs  up  to  the  latest  period  of  the  work  from  accidental 
causes,  and  often  from  carelessness  of  workmen. 

(28)  Inspection  may  be  considered  under  two  heads : 

(A)  The  conductor  itself. 

(B)  The  earth  connection. 

(A)  Joints  in  a  series  of  conductors  should  be  as  few  as  possible.    As  a  rule, 
they  should  only  be  necessary  where  the  vertical  and  horizontal  conductors  are 
connected,  and  the  main  conductors  themselves  should  always  be  continuous 
and  without  artificial  joints.     Connections  between  the  vertical  and  horizontal 
conductors  should  always  be  in  places  readily  accessible  for  inspection.    Visible 
continuity  suffices  for  the  remainder  of  the  circuit.    The  electrical  testing  of  the 
whole  circuit  is  difficult  and  needless. 

(B)  The  electrical  testing  of  the  earth  can,  in  simple  cases,  be  readily  effected. 
In  complex  cases,  where  conductors  are  very  numerous,  tests  can  be  effected  by 
the  provision  of  test  clamps  of  a  suitable  design. 

(29)  Collieries.  —  Undoubted  evidence  exists  of  the  explosion  of  fire  damp  in 
collieries  through  sparks  from  atmospheric  electricity  being  led  into  the  mine 
by  the  wire  ropes  of  the  shaft  and  the  iron  rails  of  the  galleries.    Hence  the  head 
gear  of  all  shafts  should  be  protected  by  proper  lightning  conductors. 

Suggestions  of  the  Committee 

The  investigations  of  the  committee  warrant  them  in  putting  forward  the 
following  practical  suggestions: 

(1)  Two  main  lightning  rods,  one  on  each  side,  should  be  provided,  'extending 
from  the  top  of  each  tower  or  high  chimney  stack  by  the  most  direct  course  to 
earth. 

(2)  Horizontal  conductors  should  connect  all  the  vertical  rods  (a)  along  the 
ridge,  or  any  other  suitable  position  on  the  roof ;  (6)  at  or  near  the  ground. 

(3)  The  upper  horizontal  conductor  should  be  fitted  with  aigrettes  or  points, 
at  intervals  of  20  or  30  feet. 

(4)  Short  vertical  rods  should  be  erected  along  minor  pinnacles  and  connected 
with  the  upper  horizontal  conductor. 

(5)  All  roof  metals,  such  as  finials,  ridging,  rain  water  and  ventilating  pipes, 
metal  cowls,  lead  flashing,  gutters,  etc.,  should  be  connected  with  the  horizontal 
conductors. 

(6)  All  large  masses  of  metal  in  the  building  should  be  connected  either 
directly  or  by  means  of  the  lower  horizontal  conductor. 

(7)  Where  roofs  are  partially  or  wholly  metal  lined,  they  should  be  connected 
to  earth  by  means  of  vertical  rods  at  several  points. 

(8)  Gas  pipes  should  be  kept  as  far  away  as  possible  from  the  position  occu- 
pied by  lightning  conductors,  and  as  an  additional  protection,  the  service  mains, 


ATMOSPHERIC  ELECTRICITY  479 

to  the  gas  meter  should  be  metallically  connected  with  house  services  leading 
from  the  meter. 

(Signed)  JOHN  SLATER,  Chairman, 

E.  ROBERT  TESTING, 

OLIVER  LODGE, 

J.  GAVEY, 

W.  N.  SHAW, 

A.  R.  STENNING, 

ARTHUR  VERNON, 

KILLINGWORTH  HEDGES,  Honorary  Secretary, 

G.  NORTHOVER,  Secretary. 

These  rules  were  formulated  in  England,  and  the  most  active  member 
of  the  Research  Committee  was  without  doubt,  Sir  Oliver  Lodge,  F.R.S. 

There  are  a  few  illusions  and  prevalent  misapprehensions  with  regard 
to  lightning  which  should  receive  passing  mention.     It  is  not  true  that 
lightning  never  strikes  twice  in  the  same  place.     Nearly  A  few  popu_ 
every  person  knows  of  houses  or  barns  which  have  been  larmisap- 
struck  more  than  once.     It  is  also  foolish  to  believe  that  a  pre 
few  inches  of  glass  or  a  few  feet  of  air  or  a  quarter  of  an  inch  of  rubber 
will  serve  to  bar  the  progress  of  a  lightning  flash  which  has  forced  its 
way  through  perhaps  a  mile  of  air.     It  is  also  not  the  highest  point 
which  is  always  struck. 

OTHER  MANIFESTATIONS  OF  ATMOSPHERIC  ELECTRICITY 

443.  There  are  two  other  manifestations  of  atmospheric  electricity 
which  deserve  brief  consideration.  These  are  the  aurora  borealis  and 
St.  Elmo's  fire. 

The  aurora  borealis,  or  northern  lights,  as  seen  in  middle  latitudes, 
usually  consists  of  a  whitish  arc  of  light  or  quivering,  rapidly  moving 
beams.     Sometimes   a  faint   illumination  without   definite  Description 
form  is  seen,  and  again  it  takes  the  form  of  clouds  or  patches  of  the  va- 
of    light.     The  rays  or  beams  seem  sometimes  to  form  a  prances 
curtain  or  to  radiate  from  a  crown.     The  arc  may  also  take  of  the 
the  form  of  a  vertically  suspended  curtain  and  sometimes 
appears  folded  or  convoluted.     The  simple  arc  and  the  quivering  rays 
are,  however,  the  most  common  forms  in  middle  latitudes.     The  arc  is 
usually  seen  in  the  north,  but  at  times  it  may  pass  through  the  zenith 
or  even  be  seen  in  the  south.     The  quivering  beams  generally  come  out 
of  the  north,  and  usually  follow  the  direction  of  the  magnetic  north  and 


480  METEOROLOGY 

south  line.  The  color  of  the  light  is  usually  pale  white,  although  red- 
dish, yellowish,  and  greenish  tinges  have  often  been  noticed.  It  has 
sometimes  been  stated  that  there  is  a  peculiar  odor  which  has  been 
noticed,  but  this  has  never  been  determined  with  certainty.  An  auroral 
display  usually  commences  in  the  early  evening  and  lasts  a  few  hours, 
although  it  has  been  observed  to  last  several  days.  Auroras  do  not 
occur  in  all  parts  of  the  world  with  the  same  frequency.  The  belt  of 
maximum  frequency  in  the  northern  hemisphere  extends  from  about 
Belt  of  65  to  80°  north  latitude.  In  middle  latitudes  they  are  much 

maximum  less  frequent  and  also  in  the  immediate  vicinity  of  the  north 

pole.  This  belt  extends  farther  south  over  North  America 
than  over  Europe  and  Asia,  and  thus  seems  to  be  related  to  the  magnetic 
north  pole,  which  it  seems  to  surround  rather  than  the  geographical 
pole.  Auroras  are  of  very  rare  occurrence  in  the  torrid  zones.  There 
is  a  poorly  marked  daily  period.  An  auroral  display  generally  com- 
mences in  the  early  evening  and  lasts  a  few  hours,  although  auroras  have 
Daily  and  been  observed  at  any  time  of  the  day  or  night.  There  is  a 
annual  va-  well-marked  annual  periodicity,  the  maximum  number 

occurring  in  March  and  again  in  October.  The  minima  occur 
in  midwinter  and  midsummer.  There  are  also  several  longer  cycles 
in  connection  with  their  frequency  of  occurrence  which  are  well  marked. 
The  sun  spot  cycle  of  a  little  more  than  eleven  years  is  extremely  well 
marked  and  easily  noticeable.  In  fact,  the  auroras  are  closely  related 
with  all  solar  disturbances.  A  particularly  large  sun  spot  or  an  outburst 
of  solar  activity  which  causes  earth  currents  and  magnetic  disturbances 
is  almost  sure  to  be  attended  by  brilliant  auroral  displays  as  well.  The 
various  determinations  of  the  height  of  auroral  arches  or  streamers  are 
•  ht  very  discordant.  Values  of  from  10  to  15  up  to  nearly  1000 

miles  have  been  found.  The  average  height  seems  to  be 
a  hundred  miles  or  more,  and  the  height  is  much  less  in  higher  than  in 
middle  latitudes.  This  is,  without  doubt,  due  to  the  fact  that  the  auroral 
streamers  follow  the  lines  of  magnetic  force  which  actually  enter  the 
earth  in  the  region  of  the  magnetic  north  pole.  It  was  once  thought 
that  the  light  of  the  aurora  was  due  to  the  reflection  of  sunlight 
from  the  ice  crystals  of  the  upper  atmosphere  or  to  the  reflection  of 

sunlight  from  the  snow  and  ice  in  the  polar  regions.     It  is 

now  known  that  it  is  not  reflected  light  at  all,  because  it 
shows  no  trace  of  polarization  due  to  reflection,  and  the  spectrum  is  not 
that  of  sunlight.  More  than  fifty  bright  lines  or  bands  have  now  been 
mapped  in  the  spectrum  of  the  aurora,  and  many  have  thought  to  identify 


ATMOSPHERIC  ELECTRICITY  481 

them  with  the  lines  in  the  spectra  of  the  rarer  constituents  of  the  atmos- 
phere, such  as  argon,  neon,  and  krypton.  The  aurora  then  is  caused  by 
electrical  discharges  in  the  rare  upper  atmosphere,  and  is  of  Nature  of 
very  much  the  same  character  as  cathode  rays.  All  agree  the  aurora- 
that  the  rarefied  upper  atmosphere  is  in  an  ionized  condition,  and  is  thus 
a  fairly  good  conductor,  so  that  these  discharges  are  easily  possible. 
These  ions  may  be  negatively  charged  particles  pushed  away  from  the 
sun  by  the  pressure  of  light  waves,  or  they  may  have  originated  in  the 
earth's  atmosphere  in  ways  which  have  already  been  discussed.  Such 
long  since  disproved  theories  as  to  the  cause  of  the  aurora  as  that  it  is 
due  to  cosmic  dust  which  strikes  the  atmosphere  and  becomes  luminous, 
or  that  it  is  due  to  the  phosphorescence  of  snow  or  ice  or  dust,  need  only 
be  mentioned  in  passing.  A  complete  theory  of  the  aurora  which  will 
explain  all  its  varied  forms,  its  place  and  time  of  occurrence,  its  period- 
icity, and  the  many  facts  observed  in  connection  with  it  has  hardly  yet 
been  worked  out. 

St.  Elmo's  fire  consists  of  brushlike  tufts  of  light  which   sometimes 
appear  on  all  pointed  objects  or  objects  with  sharp  angles,  during  a 
thundershower  or  snowstorm.     It    has    been   occasionally  st.  Elmo's 
observed  at  low  levels,  but  it  is  of  commonest  occurrence  at  fire- 
high  elevations  on  mountains.     It  has  also  appeared  on  the  masts  of 
ships  at  sea.     A  hissing  sound  is  usually  heard,  and  sometimes  an  odor 
is  noticed.     It  is  particularly  visible  and  noticeable  just  before  a  light- 
ning flash,  when  the  potential  difference  is  very  large.     It  is  simply  a 
brush  discharge  of  electricity  due  to  the  large  change  in  potential  with 
height. 

TOPICS   FOR   INVESTIGATION 

(1)  Benjamin  Franklin  and  atmospheric  electricity. 

(2)  The  history  of  atmospheric  electricity  to  1760. 

(3)  Electrometers. 

(4)  Form  of  the  equipotential  surfaces  over  projections. 

(5)  The  variations  in  potential  difference. 

(6)  The  variations  in  conductivity. 

(7)  The  earth's  negative  charge  and  its  maintenance. 

(8)  Earth  currents. 

(9)  Photography  of  lightning. 

(10)  Ball  lightning. 

(11)  Heat  lightning. 

(12)  Effects  of  lightning  on  trees. 

(13)  The  kinds  of  trees  struck  by  lightning. 

(14)  Loss  of  life  due  to  lightning. 

(15)  Lightning  rods. 

(16)  The  aurora. 

(17)  St.  Elmo's  fire. 

2i 


482  METEOROLOGY 

PRACTICAL   EXERCISES 

(1)  Determine  the  potential  difference  between  a  point  in  the  atmosphere 
and  the  earth  under  various  conditions. 

(2)  Determine  whether  raindrops  are  positively  or  negatively  electrified. 

(3)  If  suitable  apparatus  is   available,   determine   the  conductivity  of  the 
atmosphere  under  different  conditions,  and  repeat  some  of  the  experiments  in 
connection  with  the  ions. 

(4)  Photograph  some  lightning  flashes. 

(5)  Determine  the  duration  of  lightning  flashes. 

REFERENCES 

For  an  admirable  presentation  of  the  modern  point  of  view  in  connection  with 

atmospheric  electricity,  see : 

GOCKEL,  ALBERT,  Die  Luftelectrizitdt,  8°,  vi  +  206  pp.,  Leipzig,  1908. 
MACHE,  H.,  UND  SCHWEIDLER,  E.  VON,  Die  atmospherische  Electrizitdt,  8°,  xi  + 

247  pp.,  Braunschweig,  1909. 

For  a  treatment  of  the  kinds,  nature,  and  effects  of  lightning,  see : 
FLAMMARION,  CAMILLE,  Thunder  and  Lightning,  translated  by  Walter  Mostyn, 

12°,  281  pp.,  Boston,  1906. 

GOCKEL,  ALBERT,  Das  Gewitter,  8°,  264  pp.,  Koln,  1905. 
For  a  treatment  of  lightning  rods  and  protection  from  lightning,  see : 
ANDERSON,  RICHARD,  Lightning    Conductors,  8°,  xv  +  470    pp.,  London,  1885. 
HEDGES,    KILLINGWORTH,    Modern    Lightning    Conductors,   4°,  vi  +  119    pp., 

London, 1905. 
LODGE,  SIR    OLIVER  J.,   Lightning    Conductors  and  Lightning   Guards,   12°,  xii 

+  544  pp.,  London,  1892. 
SPANG,  HENRY  W.,  A  Practical  Treatise  on  Lightning  Protection,  12°,  63  pp.,  New 

York,  1883. 

In  connection  with  the  aurora  borealis,  see  : 

ANGOT,  ALFRED,  The  Aurora  Borealis,  8°,  xii  +  264  pp.,  London,  1896. 
ARRHENIUS,  SVANTE  A.,  Lehrbuch  der  cosmischen  Physik,  Chapter  XVII,  Leip- 
zig, 1903. 

CAPRON,  J.  RAND,   Aurorce;    their  Characters  and  Spectra,  London,  1879. 
LEMSTRON,  Uaurore  boreale,  Paris,  1886. 
Chronological  list  of  Auroras  1870  to  1879.     Professional  Papers  of  the  Signal 

Service,  No.  3,  by  A.  W.  GREELY. 
Catalog  der  in  Norwegen   bis   June    1878  beobachteten  Nordlichter    (Sopnus 

TROMHOLT). 

Six  pamphlets   have  been  published  by  the  U.  S.  Weather  Bureau  on  atmos- 
pheric electricity  and  lightning : 

McAoiE,  ALEXANDER,  Protection    from  Lightning   (Circular  of    Information). 
McAoiE,  ALEXANDER,  Protection  from  Lightning  (Bulletin  No.  15,  1895). 
McAoiE  AND  HENRY,  Lightning  and  Electricity  of  the  Air  (Bulletin  No.  26; 

W.  B.  publication  No.  197). 
HENRY  AND  McAoiE,  Property  Loss  by  Lightning,  1898   (W.  B.  publication 

No.  199). 
HENRY,  ALFRED  J.,  Loss  of   Life  in  the   United  States  by  Lightning    (Bulletin 

No.  30;  W.  B.  publication  No.  256). 
HENRY,  ALFRED  J.,  Recent  Practice  in  the  Erection  of   Lightning  Conductors 

(Bulletin  No.  37;  W.  B.  publication  No.  349),  1906. 


CHAPTER  XII 

ATMOSPHERIC  OPTICS 

INTRODUCTION,  444 

THE  OPTICAL  PHENOMENA  DUE  TO  THE  GASES  OP  THE  ATMOSPHERE 

Refraction,  445. 
Twinkling,  446. 
Mirage  and  looming,  447. 

THE  OPTICAL  PHENOMENA  DUE  TO  THE  PARTICLES  SOMETIMES  PRESENT 
IN  THE  ATMOSPHERE 

Halos  and  related  phenomena,  448. 
Rainbow,  449. 
Cloud  shadows,  450. 

THE  OPTICAL  PHENOMENA  DUE  TO  THE  SMALL  PARTICLES  ALWAYS  PRES- 
ENT IN  THE  ATMOSPHERE 

The  blue  color  of  the  sky,  451. 
Sunrise  and  sunset  colors,  452. 
Twilight,  453. 

MISCELLANEOUS  OPTICAL  PHENOMENA 

Cloud  colors,  454. 

Transparency  of  the  atmosphere  —  haze,  455. 

INTRODUCTION 

444.   The  light  of  the  sun,  moon,  planets,  and  stars  must  pass  through 
the  earth's  atmosphere  before  reaching  the  eye  of  an  observer.     Under 
atmospheric  optics  are  considered  the  effects  of  the  atmos- 
phere on  this  light  and  the  optical  phenomena  to  which  its 
passage  through  the  atmosphere  gives  rise.     The  phenomena  under  at- 
are  varied  and  complex,  and  can  be  conveniently  grouped 
under  three  heads  :  (1)  those  phenomena  which  are  due  to  the 
gases  of  the  atmosphere  themselves ;  (2)  those  due  to  the  particles  some- 
times present  in  the  atmosphere ;   (3)   those  due  to  the  small  particles 
always  present  in  the  atmosphere.     The  first  group  includes  refraction, 
twinkling,  mirage,  and  looming.     The  second  group  includes  The  sub_ 
halos,  the  rainbow,  and  cloud  shadows.     The  third  group  divisions  of 
includes  the  blue  color  of  the  sky,  sunrise  and  sunset  colors,   t 
and  twilight.     At  the  end  of  the  chapter,  under  the  head  of  miscellaneous 

483 


484 


METEOROLOGY 


The  effect 
of  the 
earth's  at- 
mosphere. 


optical  phenomena,  cloud  colors  and  the  transparency  of  the  atmos- 
phere will  be  briefly  treated. 

THE  OPTICAL  PHENOMENA  DUE  TO  THE  GASES  OF  THE  ATMOSPHERE 

445.  Refraction.  —  It  is  a  well-known  law  of  optics  that  when  a  ray 
of  light  passes  from  a  medium  of  one  optical  density  into  that  of  an- 
The  defini-  other,  it  is  bent  from  its  course,  being  bent  toward  the  normal 
tion  of  re-  to  the  bounding  surface  when  passing  from  a  rarer  to  a  denser 
fraction.  medium,  and  vice  versa.  ('Thus  a  ray  of  light  entering  the 
earth's  atmosphere  from  space  and  passing  through  layers  of  air  of 
steadily  increasing  density  must  be  continuously  bent  towards  the 

normal.     As  a  re- 
sult,   the    ray    of 
light     follows      a 
curved  path  as  in- 
dicated by  AO  in  Fig.  153. 
An  object  is  seen,  however, 
in  the  direction  from  which 
the  rays  enter  the  eye,  that 
is,  in  the  direction  OB  in  the 
figure.     The  effect  of  refrac- 

tion is  thus  to  raise  an  object  or  increase  its  altitude  above  the  horizon. 
The  amount  of  refraction  is  zero  at  the  zenith  or  point  directly  overhead, 
and  increases  steadily  toward  the  horizon,  where  it  has  a  maximum  value 
which,  on  the  average,  is  about  35'  of  arc,  or  a  little  more  than  half  a 
degree.  The  amount  of  refraction  is  not  constant,  but  depends  upon 
the  density  of  the  air.  This  in  turn  depends  upon  the  temperature, 
the  pressure,  and  the  amount  of  water  refraction  for  any 
given  altitude  and  a  known  condition  of  vapor  present  in  it. 
In  order  to  find  the  exact  value  of  the  atmosphere,  elabor- 
ate tables  or  formulae  are  used,  and  these  may  be  found  in 
books  on  Practical  Astronomy.  There  are  several  approxi- 
mate formulae  for  computing  rough  values  of  refraction.  The  best  one, 

due  to  Professor  Comstock,  is  probably  r  =  -  tan  £,  where  r  is 


FIG.  153.  —  The  Effect  of  Refraction. 


Amount  of 
refraction 
depends 
upon  three 
things. 


~\t 

the  value  of  refraction  in  seconds  of  arc,  b  is  the  barometric  pressure  in 
inches,  t  is  the  temperature  Fahrenheit,  and  £  is  the  zenith  distance, 
that  is,  the  angular  distance  from  the  zenith.  This  is  not  applicable  to 
an  object  very  near  to  the  horizon. 


ATMOSPHERIC  OPTICS  485 

The  angular  diameter  of  both  the  sun  and  the  moon  is  just  about 
half  a  degree,  while  the  value  of  refraction  at  the  horizon  is  a  little  more 
than  half  a  degree.     As  a  result,  both  sun  and  moon  come 
into  view  before  they  have  really  come  (geometrically  risen) 
above  the  horizon  and  are  still  visible  after  they  have  really  the  time  of 
set.     The  day  is  lengthened  in  these  latitudes  from  4  to  8 
minutes  by  this  effect  of  refraction.     The  value  of  refrac- 
tion is,  on  the  average,  35'  at  the  horizon,  while  at  an  altitude  of  only 
one  half  a  degree  above  it,  the  value  has  already  lessened  to  29'.     Thus 
when  the  sun  or  moon  is  on  the  horizon,  the  lower  edge  or 
limb  is  raised  35'  while  the  upper  limb  is  raised  only  29'.     As  refraction  on 
a  result,  the  disk  appears  not  circular,  but  decidedly  flattened,  *?e  *orm  of 
the  flattening  amounting  to  about  one  fifth-  of  the  diameter. 

Briefly  put,  the  effects  of  refraction  are  to  raise  all  objects,  to  lengthen 
the  day,  and  to  flatten  the  disk  of  the  sun  and  moon  when  rising  and 
setting. 

446.    Twinkling    (steadiness   of    the    atmosphere).  —  The   twinkling 
or,  as  it  is  technically  called,  the  scintillation  of  the  stars  is  a  well-known 
phenomenon  which  is  particularly  conspicuous  on  cold  winter  Of  what 
nights,  and  near  the  horizon.     When  critically  considered,  twinkling 
it  consists  of  a  change  in  position,  a  change  in  brightness,  c 
and  a  change  in  color.     These  three  components  will  be  considered  in 
order. 

The  atmosphere  is  always  far  from  homogeneous,  but  consists  of 
numerous  layers  and  pockets  of  air  of  very  different  temperature  and 
moisture  content,  and  thus  with  different  densities.  These  The  cause 
layers  and  pockets  of  air  are  moved  about  and  mixed  by  the  of  change  in 
wind.  As  a  result,  the  condition  of  the  air  through  which  pos 
a  ray  of  light  comes  to  the  eye  of  an  observer  is  different  each  succeeding 
moment  and  the  amount  of  refraction  is  constantly  changing.  It  is 
this  constantly  changing  amount  of  refraction  which  causes  the  small 
change  in  position  which  is  one  of  the  components  of  twinkling.  When 
a  star  is  viewed  through  a  telescope,  this  change  in  position  sometimes 
becomes  so  marked  that  the  star  fairly  "  dances  "  in  the  field  of  view. 

The  change  in  brightness  is  due  to  what  may  be  called  the  lens  effect 
of  the  atmosphere.     If  at  night  the  light  from  an  arc  light 
shines  through  a  window  pane  upon  the  opposite  white  wall  Of  changes 
of  a  dark  room,  it  will  be  found  that  the  wall  is  not  uni-  "*  brisht- 

ness. 

formly  illuminated,  but  that  it  is  covered  with  dark  and 

light  mottlings.     This  is  because  the  window  pane  is  not  perfectly 


486  METEOROLOGY 

smooth  and  of  the  same  thickness  at  all  points.  It  acts  like  a  jumble 
of  convex  and  concave  lenses  concentrating  the  light  at  some  points, 
and  diverting  it  from  others.  The  atmosphere  acts  in  exactly  the  same 
way  on  the  light  which  passes  through  it.  As  the  various  layers  and 
pockets  of  air  are  wafted  past  the  line  of  sight  of  the  observer,  at  one 
moment  the  light  is  concentrated,  while  the  next  it  may  be  diverted. 
This  constant  change  in  brightness  is  the  result. 

The  change  in  color  is  due  to  optical  interference.     The  rays  of  light 
which  reach  the  eye  of  the  observer  at  the  same  instant  may  have  come 
by  paths  of  slightly  different  length.     As  a  result,  the  ether 
of  the  waves  are  out  of  phase  and  may  interfere,  thus  causing  the 

change  in       destruction  of  certain  wave  lengths  or  colors.     Since  the 
colors  eliminated  by  interference  will  be  different  at  succes- 
sive moments,  the  star  will  appear  to  change  color. 

Twinkling  is  much  more  violent  near  the  horizon,  because  the  thick- 
ness of  the  air  through  which  the  rays  of  light  come  is  there  much 
Why  planets  grea^er-  The  planets,  except  when  near  the  horizon,  seldom 
do  not  appear  to  twinkle.  The  reason  is  because  they  have  disks 

twinkle'  and  are  not  mere  points  of  light  like  the  stars.  Each  point 
on  the  disk  twinkles,  but  the  twinklings  do  not  synchronize,  so  that  the 
average  condition  of  the  whole  disk  is  much  more  nearly  constant. 

The  steadiness  of  the  atmosphere  is,  in  a  certain  sense,  the  antithesis 
of  twinkling.  On  a  cold  winter  night  when  the  sky  is  brilliant  and  the 
steadiness  stars  sparkle,  but  little  astronomical  work  can  be  done, 
oftheatmos-  Steadiness  on  the  part  of  the  atmosphere  is  the  great  desid- 
eratum for  astronomical  work.  This  means  an  atmosphere 
which  is  as  uniform  and  constant  as  possible.  It  is  a  condition  just  the 
opposite  of  this  which  causes  the  greatest  twinkling. 

447.  Mirage  and  looming.  —  A  description  and  explanation  of 
mirage  has  already  been  given  in  section  52.  It  was  treated  there  as 
The  cause  illustrating  the  fact  that  conditions  were  then  suitable  for 
of  a  mirage.  ioca]  convection.  A  layer  of  very  warm  air  is  next  to  the 
surface  of  the  ground,  and  above  it  is  a  layer  of  colder  and  thus  denser 
air.  If  an  observer  is  situated  a  little  distance  above  this  warm  layer, 
the  rays  of  light  may  be  so  bent  by  refraction  that  total  reflection  finally 
takes  place,  and  the  observer  sees  an  inverted  image  of  the  object  as  if 
reflected  from  a  horizontal  body  of  water,  and  all  intervening  objects 
are  invisible.  This  appearance  is  called  a  mirage  and  occurs  chiefly 
during  the  hot  hours  of  the  day  when  the  air  is  quiet,  and  in  level  desert 
regions.  It  occurs  also  over  water  surfaces  near  the  land.  The  height 


ATMOSPHERIC  OPTICS  487 

of  the  observer  above  the  warm  layer  usually  makes  a  great  difference 
in  the  mirage. 

Looming  is,  in  a  certain  sense,  the  opposite  of  mirage.     Here  the  cold, 
dense  layer  of  air  is  next  the  surface  of  the  ground  and  the  warmer,  less 
dense  layer  is  above.     The  rays  of  light  passing  upward  from  The  expla_ 
an  object  may  be  so  bent  by  refraction  that  total  reflection  nation  of 
again  takes  place,  and  the  observer  sees  an  inverted  image  looming- 
above  the  object.     Objects  even  below  the  horizon  may  be  brought 
into  view  in  this  way,  and  nearer  objects  seem  much  raised  and  elon- 
gated.    For  this  reason  the  term  looming  is  applied  to  the  phenomenon. 
It  occurs  chiefly  over  the  ocean  near  the  seashore,  and  in  the  Arctic 
regions. 

THE  OPTICAL  PHENOMENA  DUE  TO  THE  PARTICLES  SOMETIMES 
PRESENT  IN  THE  ATMOSPHERE 

448.    Halos  and  related  phenomena.  —  The  sun  during  the  day  and 
the  moon  during  the  night  are  often  surrounded  by  rings  or  circles  of 
light  which  are  of  different  diameters,  and  are  sometimes  Halos  and 
colored.     These  rings  may  be  divided  into  two  classes  :   the  coronas  con- 
coronae    and    the    halos   (Greek  aAws  =  disk).      These    two  * 
classes  of  rings  differ  not  only  in  size  and  coloring,  but  are  formed  in 
entirely  different  ways.     The  coronse  are  due  to  water  drops,  and  are 
caused  by  diffraction  and  interference,  while  the  halos  are  due  to  ice 
crystals  and  are  caused  by  refraction  and  reflection. 

The  coronae  are  the  smallest  rings  which  may  appear  around  the  sun 
or  moon,  and  several  concentric  rings  may  be  visible  at  the  same  time. 
The  radius  of  the  circle  varies  from  1°  to  10°,  and  they  are  Description 
usually  colored  with  the  red  on  the  outside  and  shading  off  of  a  corona- 
to  a  whitish  blue  on  the  inside.      Portions  only  of  a  coronal  ring  are 
usually  seen ;   a  full  colored  circle  is  of  very  rare  occurrence.     A  corona 
is  formed  when  a  thin  cloud  covers  the  sun  or  moon,  or  when  one  is  very 
near  to  them.     The  light  is  diffracted  by  the  water  drops  and  its  expia- 
by  interference  causes  the  colored  rings  to  appear.     The  nation> 
larger  the  drops,  the  smaller  the  ring.     Thus,  when  several  rings  are  seen 
at  the  same  time,  water  drops  of  several  sizes  must  be  present.      The 
characteristics  and  explanation  of  the  corona  are  exactly  the  same  as  in 
the  case  of  the  colored  rings  which  are  seen  when  a  strong  source  of  light 
is  viewed  through  a  piece  of  glass  which  has  been  coated  with  moisture 
by  breathing  upon  it. 


488 


METEOROLOGY 


Two  kinds  of  halos  are  recognized  ;  one  has  a  radius  of  about  21°  50' 
and  the  other  a  radius  of  45°  46'.  These  are  usually  spoken  of  as  the 
The  two  22°  and  46°  halo.  The  radius  of  a  halo  may  be  measured 
halos.  by  means  of  a  sextant  or  surveyor's  transit.  If  a  star  or 

planet  happens  to  be  on  the  edge  of  a  halo  and  the  time  of  observation  is 
noted,  the  angular  distance  between  the  moon  and  the  stars  can  be  com- 
puted and  thus  the  radius  determined.  The  different  determinations 
vary  by  perhaps  20'.  A  halo  about  the  moon  is  usually  so  little  colored 
that  it  appears  essentially  white,  while  one  about  the  sun  is  generally 
colored  with  the  red  on  the  inside  and  shading  off  to  a  whitish  blue  on 
the  outside.  Halos  are  much  more  common  than  corona,  and  the  22° 
halo  is  far  more  common  than  the  46°  halo.  Halos  are  formed  when 
the  sky  is  covered  with  a  thin  veil  of  cirro-stratus  or  alto-stratus  clouds. 
The  cause  These  clouds  consist  of  snowflakes  and  ice  needles  as  has  been 
of  the  halo,  abundantly  proved  by  observations  made  on  mountains  and 
in  balloons.  It  is  the  refraction  and  reflection  of  light  from  these  ice 
particles  which  cause  the  halo.  This  correct  explanation  was  first 
given  by  Mariotte  in  1686,  but  it  was  long  neglected  and  another  expla- 
nation even  supplanted  it  in  part  for  a  time.  During  recent  times,  the 
theory  has  been  much  improved,  and  numerous  investigations  have  been 
carried  out  to  determine  how  the  ice  particles  must  be  oriented  to  give 
rise  to  the  various  phenomena. 

There  are  several  other  optical  phenomena  which  are  closely  related 
with  halos  and  are  occasionally  seen.  Sometimes  a  ring  of  white  light 
is  observed  parallel  to  the  horizon  and  at  the  altitude  of  the  sun. 

Related  Where  this  circle 

phenomena.     crosses     the     halos, 

patches  of  light,  sometimes 
colored,  appear  which  are 
known  as  sun  dogs  or  mock 
suns.  Arcs  tangential  to  the 
halos  and  convex  to  the  sun 
are  also  occasionally  seen. 
Also  columns  of  light  and 
crosses  extending  vertically 
and  horizontally  through  the 

gun    are    some  times    glimpsed. 

A  ,,  ,t  ^4-nfJ  ;„  "&:„ 

All  tllCSC  are  represented  in  T  Ig. 

-,  e  A         rrn  11  i  i    •        j 

1  54.        I  hey  Can  all  DC  explained 

as  the  refraction  and  reflection 


FIG.  154.  —  Halos  and  Related  Phenomena. 

ABD,  horizon;  ZSD,  vertical  circle  through  the  sun; 
Z,  zenith  ;  O,  position  of  the  observer  ;  S,  the  position  of 
the  sun;  1,22°  halo;  2,  46°  halo;  3,  white  ring  parallel 
to  the  horizon  ;  4,  tangential  arcs  ;  5,  mock  suns  or  sun 

c°lumns  °f  Hght  extending  vertically  and  h°ri" 


ATMOSPHERIC  OPTICS  489 

of  light  from  the  ice  particles  which  are  either  haphazard  in  arrange- 
ment, or  are  oriented  in  a  definite  way  because  they  are  rising  or  falling. 

449.  Rainbow.  —  The  rainbow,  or  arc  of  prismatic  colors,  is  too  well 
known  to  need  description.  It  is  formed  if  the  sun  is  shining  and  at  the 
same  time  it  is  raining  in  a  direction  opposite  to  the  sun.  The  location 
The  sun,  the  observer's  .eye,  and  the  center  of  the  circle  of  of  a  rainbow- 
which  the  bow  is  a  part  are  always  in  a  straight  line.  As  a  result,  if 
the  sun  is  exactly  on  the  horizon,  the  length  of  the  bow  is  180°, 
while  in  most  cases  less  than  a  semicircle  is  seen.  Furthermore,  each 
observer  sees  his  own  rainbow  and  in  a  slightly  different  place.  The  six 
spectrum  colors  (violet,  blue,  green,  yellow,  orange,  and  red)  are  so 
arranged  that  the  red  is  on  the  outside  and  the  violet  on  the  inside.  The 
radius  of  the  red  part  is  42°  2',  while  the  radius  of  the  violet  The  size  of 
part  is  40°  17'.  Thus,  if  the  sun  is  more  than  42°  above  the  a  rainbow, 
horizon,  no  rainbow  can  be  visible.  Rainbows,  therefore,  always  occur 
during  the  early  morning  hours  or  during  the  hours  of  the  late  afternoon. 
Rainbows  are  by  far  the  most  common  during  the  later  part  of  a  summer 
afternoon.  The  reason  is  because  they  are  nearly  always  when  they 
associated  with  thundershowers.  The  steady  cyclonic  rains  occur- 
of  winter  are  followed  by  large  cloud  areas,  so  that  the  sun  seldom  shines 
shortly  after  it  has  ceased  to  rain.  The  summer  thundershowers,  on 
the  contrary,  usually  occur  during  the  afternoon,  and  the  clouds  often 
break  through  quickly,  allowing  the  necessary  sunshine. 

There  is  often  a  fainter  secondary  bow  concentric  with  the  first  and 
somewhat  larger.  The  red  is  here  on  the  inside,  and  the  violet  on  the 
outside.  The  radius  of  the  red  part  is  50°  59',  while  the  The  second- 
radius  of  the  violet  part  is  54°  9'.  «y  bow- 

The  rainbow  is  caused  by  the  refraction  and  reflection  of  sunlight  in 
the  falling  drops  of  water.  In  the  case  of  the  primary  bow,  there  are 
two  refractions  and  one  total  reflection.  In  the  case  of  the  secondary 
bow,  there  are  two  refrac-  The  expla. 

tions  and  two  total  reflec-  nation  of  the  PR'^,RY  SECONDARY 

mi  f  .LI       rainbow.  BOW  Bow 

tions.     The  course  of  the  8I 

sun's  rays  is  pictured  in  Fig.  155  in 
the  case  of  both  the  primary  and  the 
secondary  bow.     It  will  be  seen  that 
the  raindrops  which  send  any  par- 
ticular wave  length  of  light  to  the          T0  OBSERVER 
eye  of  the  observer  are  all  located  in    FlQ  155>_The  Formation  of  the  Rain- 
a  conical  surface  extending  from  the  bow. 


490  METEOROLOGY 

observer  to  the  particular  arc  of  color  in  the  rainbow.  Light  of  this 
wave  length  sent  back  by  other  raindrops  will  not  be  perceived  by  the 
observer,  as  it  will  pass  above  or  below  his  eye.  The  first  theory  of  the 
rainbow  was  given  by  Descartes  in  1637  and  seems  very  simple.  If  all 
the  factors  are,  however,  taken  into  account,  the  theory  is  by  no  means 
so  simple  and,  in  its  modern  form,  it  dates  from  Airy  in  1836. 1  The 
truth  of  the  theory  concerning  rainbows  can  be  tested  experimentally 
by  means  of  glass  balls  or  glass  globes,  filled  with  water. 

Three,  four,  or  even  five  or  more  total  reflections  are  possible  as  wel? 
as  one  or  two.  Thus  more  rainbows  are  a  possibility  and  more  have 
occasionally  been  glimpsed.  The  purity  of  color  of  a  rainbow  depends 
upon  the  size  of  the  raindrops  and  their  uniformity.  When  examined 
critically,  it  will  be  found  that  rainbows  differ  in  the  width  and  promi- 
nence of  certain  colors. 

450.  Cloud  shadows.  —  In  connection  with  cloud  shadows  only  the 
shadows  of  clouds  on  the  atmosphere  need  consideration.  These  occur 
Cloud  shad-  wnen  the  atmosphere  is  hazy  or  misty,  that  is,  when  it  con- 
ows  and  the  tains  an  unusual  number  of  dust  or  moisture  particles.  If 
STwhfch*06  the  gky  ig  covered  with  broken  clouds,  the  sun  shines  through 
they  give  the  openings  in  the  clouds  and  illuminates  the  dust  and  mois- 
ture particles  beneath.  Light  streaks  radiating  from  the 
sun  and  extending  down  from  the  clouds  are  then  seen.  Between  the 
light  streaks  are  dark  bands  due  to  the  unilluminated  parts  of  the  atmos- 
phere. These  dark  bands  are  thus  simply  cloud  shadows  on  the  atmos- 
phere. This  whole  phenomenon  is  called  popularly  the  "  sun  drawing 
water."  It  is  caused  simply  by  the  sun  shining  through  rifts  in  the 
clouds  and  illuminating  portions  of  the  atmosphere  while  other  portions 
are  in  the  shadows  of  the  clouds.  This  is  illustrated  in  Fig.  156. 


THE  OPTICAL  PHENOMENA  DUE  TO  THE  SMALL  PARTICLES  ALWAYS 
PRESENT  IN  THE  ATMOSPHERE 

451.    The  blue  color  of  the  sky.  —  The  colors  of  the  clear  sky  when 

the  sun  is  not  near  the  horizon  are  deeper  or  lighter  shades  of  blue. 

.   .        Near   the   sun  the  blue  fades  out  and  becomes  more  and 

more  whitish.     Near  the  horizon  the  blue   color   becomes 

so  faint  that  it  practically  disappears  and  is  replaced  by  gray.     The 

clearer  the  sky,  the  purer  and  more  intense  the  blue.     If  the  atmosphere 

1  Monthly  Weather  Review,  November,  1904,  p.  506. 


FIG.  156.  —  Cloud  Shadows;  "  the  Sun  drawing  Water." 


ATMOSPHERIC  OPTICS  491 

becomes  gradually  hazy,  the  blue  becomes  more  and  more  whitish  and 
is  finally  almost  lost. 

The  blue  color  of  the  sky  is  due  primarily  to  the  selective  scattering 
of  sunlight  by  the  myriads  of  particles  which  are  always  present  in  the 
atmosphere.     Some  of  these  particles  are  smaller  than  the 
wave  length  of  light,  and  some  are  larger.     If  the  atmos-  Of  the  blue 
phere  were  entirely  gaseous  and  contained  no  particles,  there  c£lor  of  the 
would  be  practically  no  scattering,  and  but  little  light  would 
come  to  us  from  the  sky.     The  sky  would  then  appear  nearly  black,  and 
the  sun,  moon,  planets,  and  stars  would  be  seen  resplendent  against  a 
dark  background  even  in  the  daytime.     Since  the  atmosphere  contains 
so  many  particles,  it  must  be  considered  a  turbid  medium,  and  the 
problem  is  to  determine  the  BLUE  AND  GREEN 

influence  of  this  turbid  me- 
dium on  the  light  passing 
through  it.  This  is  illus- 
trated in  Fig.  157.  The  longer 
ether  waves,  that  is,  the  red 
and  yellow  waves,  get  through 
the  turbid  medium  with  the 
greatest  facility,  while  the 
shorter  ether  waves  are  more  BLUE  AND  GREEN 

Scattered.      Diffuse  reflection,  FlG-  157.  —  Selective  Scattering  by  a  Turbid 

diffraction,    and    perhaps    a 

certain  amount  of  interference  play  the  chief  part  in  causing  this 
scattering.  It  will  be  seen  that  this  scattering  is  selective.  The 
reds  and  yellows  are  allowed  to  pass  through,  while  the  blues  and 
greens  predominate  in  the  light  which  is  scattered.  This  can  be 
easily  illustrated  experimentally  by  filling  a  flask  with  a  turbid 
medium  (possibly  a  soap  solution).  On  viewing  a  source  of  light 
through  the  flask  reds  and  yellows  predominate,  while  on  viewing 
the  flask  from  the  side  blues  and  greens  are  chiefly  in  evidence. 
Exactly  this  same  thing  occurs  in  connection  with  the  atmosphere. 
If  one  looks  in  a  direction  away  from  the  sun,  the  light  that  is 
received  is  the  light  which  has  been  scattered  by  the  myriads  of  particles 
which  happen  to  be  in  the  line  of  sight  at  the  moment,  and  the  pre- 
dominating color  is  then  blue.  On  looking  in  a  direction  nearer  the  sun 
much  white  light  which  has  been  reflected  by  the  dust  particles  near  the 
line  of  sight  is  added  to  the  blue,  so  that  it  becomes  whitish  and  pale. 
Near  the  horizon  the  thickness  of  the  atmosphere  through  which  one  is 


492  METEOROLOGY 

looking  is  very  great,  and  the  dust  particles  again  add  much  white  light. 
The  smaller  the  particles,  the  smaller  the  amount  of  light  that  is  scattered, 
but  the  purer  will  be  the  blue.  The  larger  the  particles,  the  greater 
the  amount  of  scattered  light,  but  all  wave  lengths  will  be  present  and 
the  selection  will  be  much  less  definite. 

The  first  explanation  of  the  blue  color  of  the  sky  was  attempted  by 
Leonardo  da  Vinci.  The  modern  correct  explanation  begins  with  Lord 
Rayleigh  in  1871. 

452.  Sunrise  and  sunset  colors.  —  As  the  sun  approaches  the  horizon 
in  setting,  its  color  often  becomes  yellow  or  orange.  In  fact,  if  the 
The  color  of  atmosphere  is  particularly  dusty  and  hazy,  its  color  may  be 
the  sun  at  decidedly  red.  The  reason  for  this  is  evident  from  the 
explanation  of  the  blue  color  of  the  clear  sky  which  has  just 
been  given.  The  atmosphere  contains  myriads  of  particles,  and  these 
exercise  a  selective  scattering  on  the  sunlight  which  passes  through  it. 
The  short  waves  are  scattered,  while  the  long  waves  (red,  orange,  and 
yellow)  predominate  in  the  light  which  is  allowed  to  pass  through. 
When  the  sun  is  near  the  horizon,  the  thickness  of  the  atmosphere 
through  which  the  light  must  come  is  large,  so  that  the  sorting  of  the 
wave  lengths  has  been  particularly  efficient,  and  only  the  red,  or  the  red, 
orange,  and  yellow  light  gets  through. 

From  the  time  the  sun  gets  near  the  horizon  until  it  is  some  16°  below, 
a  long  series  of  changing  sunset  glows  and  colors  are  seen.  The  presence 
Sunset  °f  a  tittle  more  or  a  little  less  haze,  or  the  presence  of  a  few 
glows  and  clouds,  makes  a  tremendous  difference  in  the  sunset  colors. 
In  order  to  determine  exactly  the  sequence  of  changes  in 
the  sunset  colors  and  glows,  it  would  be  necessary  to  observe  critically 
sunset  colors  during  many  absolutely  clear  sunsets.  This  has  been 
done  by  a  few  observers,  and  the  reader  must  be  referred  to  their  work  for 
detailed  descriptions.  The  colors  are  mostly  reds,  yellows,  and  purples, 
and  selective  scattering  is  the  agency  which  is  the  cause  of  most  of  the 
coloring. 

Sunset  colors  are  also  visible  in  the  east  as  the  sun  is  setting.  These 
are  due  to  the  colored  light  coming  from  the  west  and  reflected  to  the 
observer  by  the  dust  particles  in  the  east.  The  pink  twilight  arch  is 
seen  rising  in  the  east  as  the  sun  goes  farther  and  farther  below  the 
horizon.  Beneath  this  arch  is  a  blue  black  patch.  This  is  the  shadow 
of  the  earth  on  its  atmosphere. 

Sunrise  During  sunrise  the  colors  are  essentially  the  same  as  dur- 

coiors.  ing  sunset,  only  they  occur  in  the  inverse  order. 


ATMOSPHERIC  OPTICS  493 

453.  Twilight.  —  Twilight  has  already  been  briefly  treated  in  section 
24  in  connection  with  the  height  of  the  atmosphere.      It  is  caused  by 
the   reflection    and    diffraction   of   sunlight   by  the    many  The  cause 
particles  in  the  upper  atmosphere  after  the  sun  has  gone  of  t^sk*- 
below  the  horizon  of  the  observer  and  is  no  longer  directly  visible. 
This  was  illustrated  in  Fig.  2.     By  noting  the  duration  of  twilight  at 
a  certain  place  and  on  a  given  date,  it  is  possible  to  compute  the 
height  of  the  particles  which  sent  the  light  to  the  observer.     It  has 
been  found  there  are  enough  particles  above  a  height  of  100  miles  to 
send  a  perceptible  amount  of  light  to  an  observer. 

The  duration  of  twilight  or  the  transition  period  from  daylight  to 
darkness  is  very  different  at  different  times  of  year  and  at  different 
places.     It  is  ordinarily  stated  that  twilight  lasts  until  the  Duration  of 
sun  has  gone  18°  below  the  horizon.     This  means  that  at  the  twili8ht- 
equator,  where  the  sun  always  sets  perpendicularly  to  the  horizon,  the 
twilight  would  last  an  hour  and  ten  minutes.     In  higher  latitudes,  where 
the  sun  makes  a  small  angle  with  the  horizon  when  rising  and  setting, 
the  duration  would  be  much  longer.     In  the  polar  regions  it  lasts  for 
months. 

MISCELLANEOUS  OPTICAL  PHENOMENA 

454.  Cloud  colors.  —  So  much  light  is  returned  to  the  observer  by 
reflection  and  diffraction  in  connection  with  the  myriads  of  particles 
which  constitute  a  cloud,  that  the  color  of  a  cloud  is  usually  cloud 
that  of  the  light  by  which  it  is  illuminated.     Thus  a  cloud  colors- 
upon  which  the  light  of  the  sun  is  falling  usually  appears  white.     If  a 
thick  cloud  comes  between  the  sun  and  an  observer,  the  central  portion 
which  is  in  the  shadow  usually  appears  gray  or  even  black.     The  bril- 
liant white  edge  is  due  chiefly  to  diffraction.     Clouds  seen  near  the 
horizon  or  at  the  time  of  sunrise  or  sunset  are  usually  reddish  in  color. 
This  is  because  the  light  which  falls  upon  them  contains  chiefly  the  long 
wave  lengths. 

455.  Transparency  of  the  atmosphere  —  haze.  —  Haze  has  already  been 
fully  treated  in  section  229,  and,  as  was  there  stated,  there  are  two  kinds 
of  haze.     The  haze  which  exists  high  up  in  the  atmosphere  The  two 
and  gives  the  sky  a  whitish  appearance  by  day  and  dims  the  kinds  of 
stars  at  night  is  due  to  the  presence  of  ice  particles  in  the 

upper  atmosphere.  The  low  haze  which  renders  distant  objects  indis- 
tinct in  outline  is  optical  as  well  as  due  to  foreign  particles  in  the  atmos- 
phere. The  mixture  of  layers  and  pockets  of  air  of  different  temperature 


494  METEOROLOGY 

and  moisture  content,  and  thus  of  different  density,  renders  distant 
objects  indistinct.  This  is  purely  an  optical  effect  and  may  be  called 
optical  haze.  The  dust  and  moisture  particles  present  also  cut  down 
the  distance  to  which  one  can  see. 

It  is  a  fact  of  everyday  observation,  that  the  air  is  much  more  trans- 
parent at  certain  times  than  others.     Distant  hills  and  mountains  are 

sometimes  said  to  look  near  because  they  are  so  sharp  and 

distinct.  When  distant  objects  are  indistinct,  it  is  usually 
distinctness  said  that  it  is  hazy.  Now  the  light  coming  from  a  distant 
objects.0*  object  is  weakened  in  two  ways.  Some  of  the  light  waves 

are  absorbed  by  the  gases  of  the  atmosphere.  This  absorp- 
tion is  usually  selective,  as  certain  wave  lengths  are  much  more  readily 
absorbed  than  others.  The  light  is  also  scattered  by  the  dust  and 
moisture  particles  which  are  present.  The  distance  to  which  one  can 
see  from  a  smoky  city  is  usually  very  much  greater  in  the  direction  from 
which  the  wind  comes  than  that  towards  which  it  is  blowing.  But  the 
indistinctness  of  distant  objects  is  not  due  alone  to  the  weakening  of 
light  by  absorption  and  scattering,  but  to  the  optical  haze  as  well. 
Transparency  then  depends,  not  only  on  the  absence  of  dust  and  moisture 
particles,  but  also  on  the  steadiness  and  uniformity  of  the  atmosphere. 
Several  ways  of  measuring  the  transparency  of  the  atmosphere  have 
been  devised.  The  usual  method  makes  use  of  two  white  disks  with 

black  crosses  of  unequal  size,  say,  in  the  ratio  of  12  to  1. 
urementof  If  the  air  were  perfectly  transparent,  the  two  disks  would 
transpar-  appear  the  same  to  the  eye  when  the  distance  of  the  larger  one 

was  12  times  the  distance  of  the  smaller  one.  It  will  be  found 
by  observation  that  the  ratio  of  distance,  when  the  two  appear  the  same, 
is  always  less  than  12,  and  from  the  observed  ratio  values  of  the  trans- 
parency can  be  computed.  From  such  measurements  it  has  been  found 
that  the  transparency  is  greater  in  mountains  than  in  the  lowlands,  in 
the  morning  than  in  the  evening,  and  during  the  winter  than  during  the 
summer. 

In  order  to  determine  the  exact  cause  of  changes  in  transparency,  the 
values  of  the  meteorological  elements  at  the  earth's  surface  and  above, 
and  the  number  of  dust  particles  per  cubic  centimeter,  should  be  deter- 
mined as  well  as  the  transparency. 

TOPICS   FOR   INVESTIGATION 

(1)  Approximate  formulae  for  the  values  of  refraction. 

(2)  The  sequence  of  colors  during  sunset. 


ATMOSPHERIC   OPTICS  ,  495 

PRACTICAL   EXERCISES 

(1)  If  familiar  with  astronomical  apparatus  and  methods,  determine  the  alti- 
tude of  a  star,  compute  its  value,  and  thus  determine  the  amount  of  refraction. 

(2)  Measure  the  radius  of  a  halo. 

(3)  Study  critically  the  width  and  pureness  of  the  colors  of  a  rainbow. 

(4)  Note  the  duration  of  twilight. 

REFERENCES 

BESSON,  Louis,  Sur  la  theorie  des  halos,  Paris,  1909. 

DAVIS,  WILLIAM  M.,  Elementary  Meteorology,  Ginn  &  Co.,  1894.     (Chapter  IV 

deals  with  atmospheric  optics.) 

MASCART,  E.,  Traite  d'optique,  Paris,  1889-1894  (especially  Vol.  3). 
PERNTER-EXNER,  Meteor 'ologische  Optik,  Wien  und  Leipzig,  1910.     (This  is  an 

extensive  treatise  and  an  invaluable  compendium  of  information.) 


CHAPTER  XIII 

ATMOSPHERIC  ACOUSTICS 

SOUND  —  ITS  ORIGIN,  NATURE,  AND  PROPAGATION,  455 
THUNDER,  457 

THE  EFFECT  OF  THE  METEOROLOGICAL  ELEMENTS  ON  THE  CHARACTER 
AND  CARRYING  DISTANCE  OF  SOUNDS,  458 

SOUND  —  ITS  ORIGIN,  NATURE,  AND  PROPAGATION 

456.  Sound  may  be  defined  as  the  sensation  produced  when  a  dis- 
turbance or  wave  motion  in  the  air  reaches  the  ear  and  excites  the 
auditory  nerves.  Sound  is  caused  by  a  vibrating  body  and 
conveyed  to  the  ear  by  some  elastic  medium,  usually  air. 
That  sound  is  always  produced  by  a  vibrating  body  can  be  readily 
determined.  If  the  finger  is  placed  lightly  on  the  edge  of  a  bell  which 
The  origin  has  been  struck  and  which  is  emitting  a  sound,  the  tremulous 
of  sound.  motion  of  the  edge  can  be  felt.  If  greater  pressure  is  applied, 
the  vibration  of  the  edge  is  stopped  and  the  sound  ceases.  In  the  case 
of  the  piano,  or  harp,  or  any  stringed  instrument,  the  actual  vibration 
of  the  string  which  is  emitting  the  musical  sound  can  be  seen.  That  an 
elastic  medium  is  necessary  for  the  conveyance  of  sound  can  also  be 
readily  proved  experimentally.  If  a  sound-emitting  body,  like  a  watch 
or  a  bell,  is  placed  under  the  receiver  of  an  air  pump  and  the  air  is  ex- 
hausted, the  sound  grows  much  fainter  and  would  cease  entirely  if  it 
were  not  conveyed  to  some  extent  by  the  imperfect  vacuum  and  the 
supports  of  the  sound-emitting  body.  If  the  air  is  again  admitted,  the 
sound  returns  to  its  customary  loudness.  The  action  of  the  vibrating 
The  propa-  body  and  the  elastic  medium  in  producing  and  conveying 
gation  of  sound  is  thus  as  follows.  The  vibrating  body  moves  in  and 
out  and  thus  causes  rarefactions  and  compressions  in  the 
elastic  medium.  This  wave  motion  is  then  conveyed  by  the  elastic 
medium  with  a  speed  which  depends  on  the  density  and  elasticity.  The 
velocity  of  transmission  increases  with  greater  elasticity  and  smaller 

496 


ATMOSPHERIC  ACOUSTICS  497 

density.  For  air,  the  velocity  is  1090  feet  per  second  at  a  temperature 
of  32°  F.  For  water,  the  velocity  is  a  little  more  than  four  times  as  large. 
When  these  waves  of  compression  and  rarefaction  reach  the  ear,  the 
sensation  of  sound  is  produced. 

Since  sound  is  a  wave  motion,  it  may  be  reflected  by  a  rigid  barrier, 
just  as  a  water  wave  is  turned  back  from  a  wall,  or  a  light  wave  is  re- 
flected from  a  mirror.     The  echo  is  the  familiar  example  of  The  reflec_ 
the  reflection  of  sound  waves,  and  many  places  have  become  tion  of 
famous  on  account  of  their  echoes.     In  mountainous  regions, 
where  there  are  many  reflecting  surfaces,   the  reverberations  which 
follow  any  sharp  report  are  particularly  noticeable.     Sound  waves  may 
be  reflected  by  a  cloud,  or  by  the  bounding  surface  of  two  layers  of  air 
of  different  density,  as  well  as  by  a  barrier.     In  this  last  instance,  the 
process  is  very  similar  to  the  cause  of  mirage  and  looming  where  the 
light  waves  are  finally  totally  reflected  from  such  a  bounding  surface 
between  media  of  different  temperature. 


THUNDER 

457.   A  near-by  lightning  flash  is  always  followed  by  thunder,  which  is 
simply  the  familiar  snap  of  the  electric  spark  in  a  much  modified  form. 
If  the  observer  is  very  near  the  lightning  flash,  only  a  single  The  cause 
tremendous  crash  is  heard.     The  lightning  flash  suddenly  of  thunder, 
heats  the  filament  of  air  which  marks  its  path  and  causes  it  to  expand 
quickly.     This  causes  a  wave  of  compression  which  travels  outward  in 
every  direction  from  the  path  of  the  flash.     Thus  the  observer  near  the 
flash  receives  but  the  single  impulse  and  hears  but  a  single 
crash.     If  an  observer  is  at  a  considerable  distance  from  the 
lightning  flash,  the  well-known  rolling  and  reverberation  of  the  thunder  and 
thunder  is  heard,  and  the  sound  may  continue  for  some  time,  jj^m!!  * 
There  are  two  main  causes  which  contribute  to  this.     In  the 
first  place,  the  observer  is  at  very  different  distances  from  different  parts 
of  the  lightning  flash.     This  is  particularly  the  case  if  the  lightning  flash 
is  several  miles  long.     Since  sound  travels  but  1090  feet  per  second,  it 
will  require  a  very  different  time  for  the  sound  to  reach  the  observer  from 
different  parts  of  the  flash.     Thus  the  long  continuance  of  the  thunder 
is  in  part  explained.     In  the  second  place,  the  sound  is  reflected  by  sur- 
rounding objects,  particularly  in  mountainous  regions,  and  also  by  the 
clouds  and  the  bounding  surface  of  layers  of  air  of  different  density. 
All  the  characteristics  of  thunder  can  be  explained  as  a  combination  of 
2x 


498  METEOROLOGY 

these  two  things,  varying  distance  from  different  parts  of  the  flash,  and 

the  reflection  of  sound. 

Since  sound  travels  1090  feet  per  second,  it  requires  very  nearly  five 
seconds  to  cover  a  mile.  By  counting  the  number  of  seconds 
which  elapse  between  a  lightning  flash  and  the  resulting 

lightning        thunder,  and  then  dividing  by  five,  the  distance  in  miles 

thunder.        of  the  lightning  is  at  once  determined. 

THE  EFFECT  OF  THE  METEOROLOGICAL  ELEMENTS  ON  THE  CHARACTER 
AND  CARRYING  DISTANCE  OF  SOUNDS 

458.   It  is  a  well-known  fact  of  observation  that  certain  sounds,  such 

as  those  coming  from  a  distant  whistle  or  foghorn,  or  a  railway  train 

passing  over  a  bridge,  are  much  more  distinctly  heard  and 

ments  which  are  heard  at  much  greater  distances  at  certain  times  than 

affect  the       ^t  others.     In  fact,  this  is  such  a  common  matter  of  expe- 

distinctness  '  .  j  i     i 

and  carry-  nence  that  these  things  are  sometimes  used  popularly  as 
ing  distance  weather  prognostics.  The  wind  direction,  of  course,  plays 
the  major  part  in  this.  A  sound  can  be  much  better  heard 
if  the  sound-emitting  body  is  in  the  direction  from  which  the  wind  comes. 
If  the  sound  waves  must  make  their  way  against  the  moving  air  to  the 
observer,  they  are  more  mixed  up  and  are  reflected  away  from  the 
observer.  This  reflection  is  due  to  the  fact  that  the  upper  layers  of  air 
are  moving  at  a  higher  velocity  than  those  nearer  the  ground.  They 
thus  act  in  the  same  way  in  reflecting  sound  as  if  they  were  at  a  different 
temperature.  The  presence  of  a  small  amount  of  fog  also  serves  to  make 
sounds  more  distinctly  heard.  This  may  be  due  partly  to  the  fact  that 
the  air  is  usually  very  quiet  at  the  time  of  fog.  A  fog  also  usually  occurs 
when  there  is  an  inversion  of  temperature,  and  this  means  layers  of  air  of 
different  temperature  and  thus  density.  Sound  waves  are  readily 
reflected  from  the  bounding  surfaces  of  these  layers.  Critical  observa- 
tion of  the  distinctness  and  carrying  distance  of  sounds  and  the  values 
of  the  meteorological  elements  would  lead  to  interesting  generaliza- 
tions. 

It  has  also  been  found  that  near  a  foghorn,  for  example,  there  are 
patches  where  no  sound  can  be  heard  and  these  areas  of  silence  or  sound 
Sound  shadows  may  be  differently  located  at  different  times.  They 

shadows.  are  probably  caused  by  a  pocket  of  air  of  peculiar  character- 
istics which  reflects  away  the  sound  waves  and  thus  causes  a  sound 
shadow. 


ATMOSPHERIC  ACOUSTICS  499 

There  are  certain  sounds  produced  in  the  atmosphere  by  its  passage 
over  obstacles  as,  for  example,  the  howling  of  the  wind  and  the  singing 
of  telegraph  wires.     Here  compressions  and  rarefactions  are 
produced  in  the  moving  air  and  these  constitute  the  sound,  duced  byPr° 
The  phenomena  are  analogous  to  the  formation  of  waves  in  air  V1 
a  stream  passing  over  a  rough  bed.     In  the  case  of  telegraph 
wires,  it  is  usually  a  wind  of  a  particular  velocity,  not  necessarily  high, 
which  is  especially  efficacious  in  producing  the  sound.     The  direction 
of  the  wind  also  plays  a  part.     The  pitch  of  the  sound  depends  on  the 
size  of  the  wire,  the  length  between  the  poles,  and  also  the  pole  which 
through  resonance  emphasizes  certain  tones.1 


TOPICS   FOR   INVESTIGATION 


(1)  Sound  shadows. 

(2)  Famous  echoes. 


PRACTICAL   EXERCISES 


(1)  Observe  critically  the  characteristics  of  thunder  and  attempt  to  explain 
them  in  each  case. 

(2)  Observe  critically  the  distinctness  and  carrying  distance  of  a  particular 
sound  and  attempt  to  correlate  it  with  the  values  of  the  meteorological  ele- 
ments. 

(3)  Determine  the  distance  at  which  thunder  is  audible. 

REFERENCES 

ARRHENIUS,  SVANTE  AUGUST,  Lehrbuch  der  kosmischen  Physik,  Leipzig,  1903. 
(Chapter  XIV  deals  with  meteorological  acoustics.) 

i  Monthly  Weather  Review,  March,  1904,  p.  230. 
Meteorologische  Zeitschrift,  1902,  p.  525. 


APPENDICES 

Appendix  I 
THE  METRIC  SYSTEM 

Appendix  II 

A  COMPARISON  OF  THE  THREE  THERMOMETRIC  SCALES 

Appendix  III 

A  GRAPHICAL  COMPARISON  OF  THE   ENGLISH  AND  METRIC  BAROMETRIC 
SCALES  AND  THE  FAHRENHEIT  AND  CENTIGRADE  THERMOMETRIC  SCALES 

Appendix  IV 
THE  REDUCTION  OF  BAROMETRIC  READINGS  TO  SEA  LEVEL 

Appendix  V 
THE  TABLES  IN  THE  TEXT 

Appendix  VI          « 
THE  ALBANY  DATA  GIVEN  IN  THE  TEXT 

Appendix  VII 

THE    REGULAR    STATIONS    OF    THE    U.  S.  WEATHER  BUREAU  AND  THE 
CANADIAN  STATIONS 

Appendix  VIII 
TEACHING  METEOROLOGY 

Appendix  IX 
THE  LITERATURE  OF  METEOROLOGY 

(A)  General  directions. 

(B)  A  list  of  books. 

(C)  The  U.  S.  government  publications  on  meteorology. 

(D)  Bibliography  of  Bibliography. 

(E)  Digest  of  literature. 

(F)  Periodicals. 

(1)  Where  lists  may  be  found. 

(2)  A  partial  list. 

501 


APPENDIX  I 
THE  METRIC  SYSTEM 


Length 

10      millimeters  =  1  centimeter. 
10      centimeters  =  1  decimeter. 
10      decimeters    =  1  meter. 
1000  meters          =  1  kilometer. 

1  centimeter  =  0.393700  inch. 


Mass 

10      milligrams  =  1  centigram. 
10      centigrams  =  1  decigram. 
10      decigrams    =  1  gram. 
1000  grams          =  1  kilogram. 

1  inch     =  2.54000  centimeters. 


1  meter          =  39.3700  inches  1 

or  3.28083  feet.          1 
1  kilometer    =  3280.83  feet  .  1 

or  0.621370  mile.         1 
1  centigram    =  0.154323  grain. 
1  gram  =  15.4323  grains 

or  0.0352739  ounces. 
1  kilogram     =  35.2739  ounces 

or  2.20462  pounds  (av.). 


foot     =  0.304801  meter, 
mile     =  1.60935  kilometers, 
ounce  =  28.3495  grams, 
pound  =  0.453592  kilogram. 


For  rough  ratios  it  is  sufficient  to  remember  that : 


1  centimeter  =  .4  inch. 
1  meter  =  3£  feet. 
1  kilometer  =  .6  mile. 
1  gram  =  ^  ounce. 

1  kilogram     =  2\  pounds. 


1  inch     =  2J  centimeters. 
1  foot      =  .3  meter. 
1  mile     =1.6  kilometers. 
1  ounce  =  28  grams. 
1  pound  =  |  kilogram. 


503 


APPENDIX  II 
A  COMPARISON  OF  THE  THREE  THERMOMETRIC  SCALES 

C  =  F  -  32°  =  R 
594* 


FAHREN- 
HEIT 

CENTI- 
GRADE 

REAUMUR 

FAHREN- 
HEIT 

CENTI- 
GRADE 

REAUMUR 

FAHREN- 
HEIT 

CENTI- 
GRADE 

REAUMUR 

-459f 

-273 

-218f 

-16 

-26! 

-211 

92 

331 

26! 

-148 

-100 

-80 

-13 

-25 

-20 

95 

35 

28 

-139 

-95 

-76 

-10 

-23| 

-18! 

98 

36! 

291 

-130 

-90 

-72 

-7 

-171 

101 

381 

30f 

-121 

-85 

-68 

-4 

-203 

-16 

104 

40 

32 

-112 

-80 

-64 

-1 

—  181 

-14! 

107 

41! 

331 

-103 

-75 

-60 

2 

—16! 

-131 

110 

431 

34! 

5 

-15 

-12 

113 

45 

36 

-100 

-731 

-58! 

8 

-131 

-10! 

116 

46! 

371 

-97 

-71! 

-571 

11 

-HI 

-91 

119 

481 

38! 

-94 

-70 

-56 

14 

-10 

-8 

122 

50 

40 

-91 

—681 

-54| 

17 

-81 

-6| 

125 

51! 

411 

-88 

-66! 

20 

-6! 

128 

531 

42f 

-85 

-65 

-523 

23 

-5 

-43 

131 

55 

44 

-82 

-631 

-50! 

26 

-31 

-2! 

134 

56! 

451 

-79 

—  61f 

-491 

29 

-If 

-H 

137 

581 

46! 

-76 

-60 

-48 

32 

0 

0 

140 

60 

48 

-73 
-70 

=81 

-46! 
-451 

35 

If 

11 

149 

65 

52 

-67 

-55 

-44 

38 

2! 

158  " 

70 

56 

-64 

-53| 

-42! 

41 

5* 

4 

167 

75 

60 

-61 

-411 

44 

6f 

51 

176 

80 

64 

-58 

-503 

-40 

47 

gi 

6! 

185 

85 

68 

-55 

—481 

-38! 

50 

10 

8 

194 

90 

72 

-52 

-46! 

-371 

53 

iif 

91 

203 

95 

76 

-49 

-45 

-36 

56 

131 

10! 

212 

100 

80 

-46 

-43| 

-34! 

59 

15 

12 

-43 

-331 

62 

16! 

131 

392 

200 

160 

-40 

-403 

-32 

65 

181 

14! 

572 

300 

240 

68 

20 

16 

752 

400 

320 

-37 

—381 

-30! 

71 

21! 

171 

1832 

1000 

800 

-34 

-36! 

-291 

74 

231 

18! 

3632 

2000 

1600 

-31 

-35 

-28 

77 

25 

20 

5432 

3000 

2400 

-28 

-331 

80 

26! 

211 

7232 

4000 

3200 

-25 

-31! 

-25| 

83 

281 

22f 

9032 

5000 

4000 

-22 

-30 

-24 

86 

30 

24 

10832 

6000 

4800 

-19 

-281 

-22! 

89 

31! 

251 

12632 

7000 

5600 

504 


APPENDIX  II 
CENTIGRADE  SCALE  TO  FAHRENHEIT 


505 


CENTI- 

QHADE 

.0 

.1 

.2 

.3 

.4 

.5 

.6 

.7 

.8 

.9 

oF. 

oF. 

oF. 

oF. 

oF. 

cF. 

oF. 

oF. 

oF. 

oF. 

-  6 

+32.00 

+31.82 

+31.64 

+31.46 

+31.28 

+31.10 

+30.92 

+30.74 

+30.5'. 

+30.38 

i 

30.20 

30.02 

29.84 

29.66 

29.48 

29.30 

29.12 

28.94 

28.76 

28.58 

2 

28.40 

28.22 

28.04 

27.86 

27.68 

27.50 

27.32 

27.14 

26.96 

26.78 

3 

26.60 

26.42 

26.24 

26.06 

25.88 

25.70 

25.52 

25.34 

25.16 

24.98 

4 

24.80 

24.62 

24.44 

24.26 

24.08 

23.90 

23.72 

23.54 

23.36 

23.18 

-  5 

+23.00 

+22.82 

+22.64 

+22.46 

+22.28 

+22.10 

+21.92 

+21.74 

+21.56 

+21.38 

6 

21.20 

21.02 

20.84 

20.66 

20.48 

20.30 

20.12 

19.94 

19.76 

19.58 

7 

19.40 

19.22 

19.04 

18.86 

18.68 

18.50 

18.32 

18.14 

17.96 

17.78 

8 

17.60 

17.42 

17.24 

17.06 

16.88 

16.70 

16.52 

16.34 

16.16 

15.98 

9 

15.80 

15.62 

15.44 

15.26 

15.08 

14.90 

14.72 

14.54 

14.36 

14.18 

-10 

+14.00 

+13.82 

+13.64 

+13.46 

+13.28 

+13.10 

+12.92 

+12.74 

+12.56 

+12.38 

11 

12.20 

12.02 

11.84 

11.66 

11.48 

11.30 

11.12 

10.94 

10.76 

10.58 

12 

10.40 

10.22 

10.04 

9.86 

9.68 

9.50 

9.32 

9.14 

8.96 

8.78 

13 

8.60 

8.42 

8.24 

8.06 

7.88 

7.70 

7.52 

7.34 

7.16 

6.98 

14 

6.80 

6.62 

6.44 

6.26 

6.08 

5.90 

5.72 

5.54 

5.36 

5.18 

-15 

+  5.00 

+  4.82 

+  4.64 

+  4.46 

+  4.28 

+  4.10 

+  3.92 

+  3.74 

+  3.56 

+  3.38 

16 

+  3.20 

+  3.02 

+  2.84 

+  2.66 

+  2.48 

+  2.30 

+  2.12 

+   1.94 

+  1.76 

+  1-58 

17 

+  1.40 

+  1.22 

+  1.04 

+  0.86 

+  0.68 

+  0.50 

+  0.32 

+  0.14 

—  0.04 

—  0.22 

18 

-  0.40 

-  0.58 

-  0.76 

-  0.94 

—  1.12 

-   1.30 

—   1.48 

—   1.66 

-  1.84 

—  2.02 

19 

-  2.20 

-  2.38 

-  2.56 

-  2.74 

-  2.92 

-  3.10 

—  3.28 

—  3.46 

—  3.64 

-  3.82 

-20 

—  4.00 

—  4.18 

-  4.36 

-  4.54 

-  4.72 

-  4.90 

-  5.08 

-  5.26 

-  5.44 

-  5.62 

21 

5.80 

5.98 

6.16 

6.34 

6.52 

6.70 

6.88 

7.06 

7.24 

7.42 

22 

7.60 

7.78 

7.96 

8.14 

8.32 

8.50 

8.68 

8.86 

9.04 

9.22 

23 

9.40 

9.58 

9.76 

9.94 

10.12 

10.30 

10.48 

10.66 

10.84 

11.02 

24 

11.20 

11.38 

11.56 

11.74 

11.92 

12.10 

12.28 

12.46 

12.64 

12.82 

-25 

-13.00 

-13.18 

-13.36 

-13.54 

-13.72 

-13.90 

-14.08 

-14.26 

—14.44 

-14.62 

26 

14.80 

14.98 

15.16 

15.34 

15.52 

15.70 

15.88 

16.06 

16.24 

16.42 

27 

16.60 

16.78 

16.96 

17.14 

17.32 

17.50 

17.68 

17.86 

18.04 

18.22 

28 

18.40 

18.58 

18.76 

18.94 

19.12 

19.30 

19.48 

19.66 

19.84 

20.02 

29 

20.20 

20.38 

20.56 

20.74 

20.92 

21.10 

21.28 

21.46 

21.64 

21.82 

-30 

-22.00 

-22.18 

—22.36 

-22.54 

-22.72 

-22.90 

-23.08 

—23.26 

-23.44 

-23.62 

31 

23.80 

23.98 

24.16 

24.34 

24.52 

24.70 

24.88 

25.06 

25.24 

25.42 

32 

25.60 

25.78 

25.96 

26.14 

26.32 

26.50 

26.68 

26.86 

27.04 

27.22 

33 

27.40 

27.58 

27.76 

27.94 

28.12 

28.30 

28.48 

28.66 

28.84 

29.02 

34 

29.20 

29.38 

29.56 

29.74 

29.92 

30.10 

30.28 

30.46 

30.64 

30.82 

-35 

-31.00 

—31.18 

-31.36 

-31.54 

—31.72 

-31.90 

-32.08 

-32.26 

-32.44 

-32.62 

36 

32.80 

32.98 

33.16 

33.34 

33.52 

33.70 

33.88 

34.06 

34.24 

34.42 

37 

34.60 

34.78 

34.96 

35.14 

35.32 

35.50 

35.68 

35.86 

36.04 

36.22 

38 

36.40 

36.58 

36.76 

36.94 

37.12 

37.30 

37.48 

37.66 

37.84 

38.02 

39 

38.20 

38.38 

38.56 

38.74 

38.92 

39.10 

39.28 

39.46 

39.64 

39.82 

-40 

—40.00 

—40.18 

—40.36 

-40.54 

-40.72 

-40.90 

-41.08 

-41.26 

-41.44 

-41.62 

41 

41.80 

41.98 

42.16 

42.34 

42.52 

42.70 

42.88 

43.06 

43.24 

43.42 

42 

43.60 

43.78 

43.96 

44.14 

44.32 

44.50 

44.68 

44.86 

45.04 

45.22 

43 

45.40 

45.58 

45.76 

45.94 

46.12 

46.30 

46.48 

46.66 

46.84 

47.02 

44 

47.20 

47.38 

47.56 

47.74 

47.92 

48.10 

48.28 

48.46 

48.64 

48.82 

-45 

-49.00 

-49.18 

—49.36 

-49.54 

—49.72 

-49.90 

—50.08 

—50.26 

-50.44 

—50.62 

46 

50.80 

50.98 

51.16 

51.34 

51.52 

51.70 

51.88 

52.06 

52.24 

52.42 

47 

52.60 

52.78 

52.96 

53.14 

53.32 

53.50 

53.68 

53.86 

54.04 

54.22 

48 

54.40 

54.58 

54.76 

54.94 

55.12 

55.30 

55.48 

55.66 

55.84 

56.02 

49 

56.20 

56.38 

56.56 

56.74 

56.92 

57.10 

57.28 

57.46 

57.64 

57.82 

—50 

-58.00 

—58.18 

—58.36 

-58.54 

-58.72 

-58.90 

-5908 

-59.26 

-59.44 

-59.62 

506 


APPENDIX  II 
CENTIGRADE  SCALE  TO  FAHRENHEIT 


CENTI- 
GRADE 

.0 

.1 

.2 

.3 

.4 

.5 

.6 

.7 

.8 

.9 

0F. 

o  F. 

0F. 

0F. 

0F. 

0F. 

F 

oF. 

oF. 

0F. 

+50 

+122.00 

+  122.18 

+122.36 

+122.54 

+122.72 

+122.90 

+123.08 

+123.26 

+123.44 

+123.62 

49 

120.20 

120.38 

120.56 

120.74 

120.92 

121.10 

121.28 

121.46 

121.64 

121.82 

48 

118.40 

118.58 

118.76 

118.94 

119.12 

119.30 

119.48 

119.66 

119.84 

120.02 

47 

116.60 

116.78 

116.96 

117.14 

117.32 

117.50 

117.68 

117.86 

118.04 

118.22 

46 

114.80 

114.98 

115.16 

115.34 

115.52 

115.70 

115.88 

116.06 

116.24 

116.42 

+45 

+113.00 

+113.18 

+113.36 

+113.54 

+113.72 

+113.90 

+114.08 

+114.26 

+114.44 

+114.62 

44 

111.20 

111.38 

111.56 

111.74 

111.92 

112.10 

112.28 

112.46 

112.64 

112.82 

43 

109.40 

109.58 

109.76 

109.94 

110.12 

110.30 

110.48 

110.66 

110.84 

111.02 

42 

107.60 

107.78 

107.96 

108.14 

108.32 

108.50 

108.68 

108.86 

109.04 

109.22 

41 

105.80 

105.98 

106.16 

106.34 

106.52 

106.70 

106.88 

107.06 

107.24 

107.42 

+40 

+104.00 

+104.18 

+104.36 

+104.54 

+104.72 

+104.90 

+105.08 

+105.26 

+105.44 

+105.62 

39 

102.20 

102.38 

102.56 

102.74 

102.92 

103.10 

103.28 

103.46 

103.64 

103.82 

38 

100.40 

100.58 

100.76 

100.94 

101.12 

101.30 

101.48 

101.66 

101.84 

102.02 

37 

98.60 

98.78 

98.96 

99.14 

99.32 

99.50 

99.68 

99.86 

100.04 

100.22 

36 

96.80 

96.98 

97.16 

97.34 

97.52 

97.70 

97.88 

98.06 

98.24 

98.42 

+35 

+  95.00 

+  95.18 

+  95.36 

+  95.54 

+  95.72 

+  95.90 

+  96.08 

+  96.26 

+  96.44 

+  96.62 

34 

93.20 

93.38 

93.56 

93.74 

93.92 

94.10 

94.28 

94.46 

94.64 

94.82 

33 

91.40 

91.58 

91.76 

91.94 

92.12 

92.30 

92.48 

92.66 

92.84 

93.02 

32 

89.60 

89.78 

89.96 

90.14 

90.32 

90.50 

90.68 

90.86 

91.04 

91.22 

31 

87.80 

87.98 

88.16 

88.34 

88.52 

88.70 

88.88 

89.06 

89.24 

89.42 

+30 

+  86.00 

+  86.18 

+  86.36 

+  86.54 

+  86.72 

+  86.90 

+  87.08 

+  87.26 

+  87.44 

+  87.62 

29 

84.20 

84.38 

84.56 

84.74 

84.92 

85.10 

85.28 

85.46 

85.64 

85.82 

28 

82.40 

82.58 

82.76 

82.94 

83.12 

83.30 

83.48 

83.66 

83.84 

84.02 

27 

80.60 

80.78 

80.96 

81.14 

81.32 

81.50 

81.68 

81.86 

82.04 

82.22 

26 

78.80 

78.98 

79.16 

79.34 

79.52 

79.70 

79.88 

80.06 

80.24 

80.42 

+25 

+  77.00 

+  77.18 

+  77.36 

+  77.54 

+  77.72 

+  77.90 

+  78.08 

+  78.26 

+  78.44 

+  78.62 

24 

75.20 

75.38 

75.56 

75.74 

75.92 

76.10 

76.28 

76.46 

76.64 

76.82 

23 

73.40 

73.58 

73.76 

73.94 

74.12 

74.30 

74.48 

74.66 

74.84 

75.02 

22 

71.60 

71.78 

71.96 

72.14 

72.32 

72.50 

72.68 

72.86 

73.04 

73.22 

21 

69.80 

69.98 

70.16 

70.34 

70.52 

70.70 

70.88 

71.06 

71.24 

71.42 

+20 

+  68.00 

+  68.18 

+  68.36 

+  68.54 

+  68.72 

+  68.90 

+  69.08 

+  69.26 

+  69.44 

+  69.62 

19 

66.20 

66.38 

66.56 

66.74 

66.92 

67.10 

67.28 

67.46 

67.64 

67.82 

18 

64.40 

64.58 

64.76 

64.94 

65.12 

65.30 

65.48 

65.66 

65.84 

66.02 

17 

62.60 

62.78 

62.96 

63.14 

63.32 

63.50 

63.68 

63.86 

64.04 

64.22 

16 

60.80 

60.98 

61.16 

61.34 

61.52 

61.70 

61.88 

62.06 

62.24 

62.42 

+15 

+  59.00 

+  59.18 

+  59.36 

+  59.54 

+  59.72 

+  59.90 

+  60.08 

+  60.26 

+  60.44 

+  60.62 

14 

57.20 

57.38 

57.56 

57.74 

57.92 

58.10 

58.28 

58.46 

58.64 

58.82 

13 

55.40 

55.58 

55.76 

55.94 

56.12 

56.30 

56.48 

56.66 

56.84 

57.02 

12 

53.60 

53.78 

53.96 

54.14 

54.32 

54.50 

54.68 

54.86 

55.04 

55.22 

11 

51.80 

51.98 

52.16 

52.34 

52.52 

52.70 

52.88 

53.06 

53.24 

53.42 

+10 

+  50.00 

+  50.18 

+  50.36 

+  50.54 

+  50.72 

+  50.90 

+  51.08 

+  51.26 

+  51.44 

+  51.62 

9 

48.20 

48.38 

48.56 

48.74 

48.92 

49.10 

49.28 

49.46 

49.64 

49.82 

8 

46.40 

46.58 

46.76 

46.94 

47.12 

47.30 

47.48 

47.66 

47.84 

48.02 

7 

44.60 

44.78 

44.96 

45.14 

45.32 

45.50 

45.68 

45.86 

46.04 

46.22 

6 

42.80 

42.98 

43.16 

43.34 

43.52 

43.70 

43.88 

44.06 

44.24 

44.42 

+  5 

+  41.00 

+  41.18 

+  41.36 

+  41.54 

+  41.72 

+  41.90 

+  42.08 

+  42.26 

+  42.44 

+  42.62 

4 

39.20 

39.38 

39.56 

39.74 

39.92 

40.10 

40.28 

40.46 

40.64 

40.82 

3 

37.40 

37.58 

37.76 

37.94 

38.12 

38.30 

38.48 

38.66 

38.84 

39.02 

2 

35.60 

35.78 

35.96 

36.14 

36.32 

36.50 

36.68 

36.86 

37.04 

37.22 

1 

33.80 

33.98 

34.16 

34.34 

34.52 

34.70 

34.88 

35.06 

35.24 

35.42 

+  0 

+  32.00 

+  32.18 

+  32.36 

+  32.54 

+  32.72 

+  32.90 

+  33.08 

+  33.26 

+  33.44 

+  33.62 

APPENDIX  III 

A  GRAPHICAL  COMPARISON   OP  THE   ENGLISH   AND  METRIC  BAROMETRIC 
SCALES  AND  THE  FAHRENHEIT  AND  CENTIGRADE  THERMOMETERS 


THERMOMETER 

SHOWING 

CENTIGRADE  AND 

FAHRENHEIT 

SCALES 

C? 
50 


507 


APPENDIX  IV 


THE  REDUCTION  OP  BAROMETRIC  READINGS  TO  SEA  LEVEL 

As  stated  in  section  111,  the  correction  to  be  applied  to  reduce  to  sea  level 
is  not  a  constant  but  depends  on  the  pressure,  temperature,  and  moisture.  In 
order  to  determine  this  correction  precisely,  elaborate  tables  must  be  used.  In 
the  following  table,  moisture  has  been  neglected,  or  better,  the  values  given  hold 
for  an  average  amount  of  moisture.  The  values  of  the  correction  are  given  for 
only  a  few  pressures  and  temperatures.  The  table  should  not  be  used  for  the 
reduction  of  a  long  series  of  well-taken  observations,  but  is  intended  simply  to 
show  roughly  the  amount  of  the  correction  in  a  few  instances.  Enough  values 
have,  however,  been  given  so  that  interpolation  is  not  troublesome.  The  table 
is  based  on  the  Smithsonian  Meteorological  Tables. 

REDUCTION  TO  SEA  LEVEL 
(correction  additive) 


g 
I 

PRESSURE  30  INCHES 

PRESSURE  29  INCHES 

H 

B 

Temperature  —  Fahrenheit  ' 

Temperature  —  Fahrenheit 

2 

(external  air) 

(external  air) 

5 

W 

-20 

0 

20 

40        60 

80 

100 

-20 

0 

20        40        60 

80 

100 

100 

0.13 

0.12 

0.12 

0.11     0.10 

0.10 

0.10 

0.12 

0.12 

0.11    0.11    0.10 

0.10 

0.10 

200 

0.26 

0.25 

0.23 

0.23     0.22 

0.21 

0.20 

0.25 

0.23 

0.22    0.22    0.21 

0.20 

0.19 

300 

0.38 

0.37 

0.35 

0.34     0.32 

0.31 

0.30 

0.37 

0.36 

0.34    0.33    0.31 

0.30 

0.29 

400 

0.51 

0.49 

0.47 

0.45     0.43 

0.42 

0.40 

0.50 

0.47 

0.46    0.44    0.42 

0.40 

0.38 

500 

0.65 

0.62 

0.59 

0.57    0.54 

0.52 

0.50 

0.63 

0.60 

0.57    0.55    0.53 

0.51 

0.48 

600 

0.78 

0.74 

0.71 

0.68    0.65 

0.63 

0.60 

0.75 

0.72 

0.69     0.66    0.63 

0.61 

0.58 

700 

0.91 

0.87 

0.83 

0.79    0.76 

0.73 

0.70 

0.88 

0.84 

0.80    0.76    0.73 

0.71 

0.68 

800 

1.04 

0.99 

0.95 

0.91     0.87 

0.83 

0.80 

1.00 

0.96 

0.92    0.88    0.84 

0.80 

0.77 

900 

1.17 

1.12 

1.07 

1.03    0.98 

0.94 

0.91 

1.13 

1.08 

1.03    0.99    0.95 

0.91 

0.87 

1000 

1.31 

1.24 

1.19 

1.14     1.10 

1.05 

1.01 

1.26 

1.20 

1.15     1.10     1.05 

1.01 

0.97 

1100 

1.44 

1.38 

1.32 

1.26    1.21 

1.15 

1.11 

1.39 

1.33 

1.27     1.22     1.17 

.11 

1.07 

1200 

1.57 

1.50 

1.43 

1.38  '  1.32 

1.26 

1.21 

1.52 

1.45 

.39       .33       .27 

.22 

1.17 

1300 

1.70 

1.64 

1.56 

1.49     1.43 

1.37 

1.32 

1.65 

1.58 

.51       .44       .38 

.32 

1.27 

1400 

— 

1.62     1.54 

1.48 

1.41 

1.78 

1.70 

.63       .56       .49 

.43 

1.37 

1500 

— 

— 

— 

—       — 

1.59 

1.52 

1.91 

1.82 

.74      .67       .60 

.53 

1.47 

1600 

— 

— 

— 

—       — 

— 

1.63 

2.04 

1.95 

.86      .79      .71 

1.64 

1.58 

1700 

2.17 

2.07 

1J9     1.90       .81 

1.74 

1.67 

1800 

2.31 

2.20 

2.11     2.02       .93 

1.84 

1.77 

1900 

2.44 

2.33 

2.23    2.13     2.04 

1.95 

1.88 

2000 

2.57 

2.45 

2.35    2.24    2.15 

2.06 

1.98 

2100 

— 

— 

2.47    2.36    2.26 

2.17 

2.07 

2200 

— 

— 

—     2.48    2.37 

2.27 

2.18 

2300 

— 

— 

—       —      2.49 

2.38 

2.29 

2400 

— 

— 

—       —       — 

2.49 

2.38 

2500 

— 

— 

—       —       — 

— 

2.49 

2600 

2700 

2800 

2900 

3000 

508 


APPENDIX  IV 


509 


REDUCTION  TO  SEA  LEVEL 
(correction  additive) 


1 

PRESSURE  28  INCHES 

PRESSURE  27  INCHES 

5 

i 

Temperature  —  Fahrenheit 

Temperature  —  Fahrenheit 

2 

(external  air) 

(external  air) 

• 
H 

-20 

0 

20 

40       60 

80 

100 

-20 

O 

2O       40       60 

80 

100 

100 

0.12 

0.11 

0.11 

0.10    0.10 

0.10 

0.10 

200 

0.24 

0.23 

0.22 

0.21    0.20 

0.19 

0.18 

300 

0.36 

0.34 

0.32 

0.31     0.30 

0.29 

0.28 

400 

0.48 

0.46 

0.44 

0.42    0.40 

0.39 

0.37 

500 

0.61 

0.58 

0.55 

0.53    0.51 

0.49 

0.47 

600 

0.73 

0.70 

0.66 

0.63    0.61 

0.59 

0.56 

700 

0.85 

0.81 

0.77 

0.74    0.71 

0.69 

0.65 

800 

0.97 

0.93 

0.88 

0.85    0.81 

0.77 

0.75 

900 

1.09 

1.04 

1.00 

0.96    0.92 

0.87 

0.84 

1000 

1.22 

1.16 

1.11 

1.06     1.02 

0.97 

0.94 

1.18 

1.12 

1.07      .02    0.98 

0.94 

0.90 

1100 

1.34 

1.29 

1.23 

1.17     1.13 

1.07 

1.03 

1.29 

1.24 

.18       .13       .08 

.03 

0.99 

1200 

1.46 

1.40 

1.34 

1.28     1.23 

1.17 

1.13 

1.41 

1.35 

.29       .24      .19 

.13 

1.09 

1300 

1.59 

1.52 

1.45 

1.39     1.33 

1.28 

1.23 

1.53 

1.47 

1.40      .34      .28 

1.23 

1.19 

1400 

1.72 

1.64 

1.57 

.50     1.43 

1.38 

1.32 

1.66 

1.59 

.51       .45       .39 

.33 

1.27 

1500 

1.84 

1.76 

1.68 

.61     1.54 

1.48 

.42 

1.78 

1.70 

.62       .55       .49 

.43 

1.37 

1600 

1.97 

1.88 

1.80 

.73     1.65 

.58 

.51 

1.90 

1.81 

.73       .67       .59 

.52 

1.46 

1700 

2  10 

2.00 

1.91 

.83     1.75 

68 

.61 

2.02 

1.93 

1.84       .76       .69 

.62 

1  55 

1800 

2.23 

2.13 

2.04 

.95     1.86 

1.78 

.71 

2.15 

2.05 

1.96     1.88       .80 

.72 

1.64 

1900 

2.36 

2.25 

2.15 

2.06     1.97 

1.88 

.81 

2.28 

2.17 

2.07     1.98       .90 

1.82 

1.74 

2000 

2.49 

2.37 

2.27 

2.17    2.07 

1.99 

1.91 

2.40 

2.29 

2.19    2.09    2.00 

1.92 

1.84 

2100 

2.62 

2.50 

2.38 

2.28    2.19 

2.09 

2.00 

2.52 

2.41 

2.30    2.20    2.11 

2.02 

1.93 

2200 

2.75 

2.62 

2.51 

2.39    2.29 

2.19 

2.11 

2.65 

2.53 

2.41     2.31     2.21 

2.12 

2.03 

2300 

2.88 

2.75 

2.62 

2.51     2.40 

2.30 

2.21 

2.78 

2.65 

2.53     2.42    2.31 

2.22 

2.13 

2400 

3.01 

2.87 

2.75 

2.62    2.51 

2.40 

2.30 

2.90 

2.77 

2.65    2.53    2.42 

2.32 

2.22 

2500 

3.14 

3.00 

2.86 

2.74    2.62 

2.51 

2.40 

3.03 

2.89 

2.76    2.64    2.52 

2.42 

2.32 

2600 

3.27 

3.12 

2.99 

2.85    2.73 

2.62 

2.51 

3.16 

3.01 

2.88    2.75    2.63 

2.52 

2.42 

2700 

3.41 

3.25 

3.10 

2.97    2.83 

2.72 

2.61 

3.28 

3.14 

2.99    2.86    2.73 

2.62 

2.51 

2800 

3.54 

3.38 

3.23 

3.08    2.95 

2.82 

2.71 

3.42 

3.26 

3.11     2.97    2.84 

2.72 

2.61 

2900 

3.68 

3.51 

3.34 

3.20    3.06 

2.93 

2.81 

3.55 

3.38 

3.22    3.09    2.95 

2.82 

2.71 

3000 

3.82 

3.63 

3.47 

3.32    3.17 

3.04 

2.90 

3.68 

3.50 

3.35    3.20    3.06 

2.93 

2.80 

APPENDIX  V 

THE  TABLES  IN  THE  TEXT 

The  following  tables  have  been  given  in  the  text  and  are  indexed  here  for 
convenience  of  reference : 

Section  9,  page  7. 


Percentage  composition  by  volume  and  by  weight  of 

pure  dry  air. 

Percentage  composition  of  inhaled  and  exhaled  air. 
Percentage  composition  of  the  atmosphere  at  various 

heights. 
Number  of  dust  particles  per  cubic  centimeter  under 

various  conditions. 

The  boiling  points  of  the  constituents  of  the  atmosphere. 
Greatest  possible   duration   of   insolation   at   various 

latitudes. 

Values  of  insolation  for  different  dates  and  latitudes. 
The  thickness  of  the  atmosphere  traversed  by   the 

sun's  rays  at  different  altitudes. 

The  amount  of  insolation  received  at  the  earth's  sur- 
face when  all  is  transmitted,  and  when  six  tenths  is 
transmitted. 
Normal  monthly  and  annual  temperatures  at  various 

places  in  the  United  States  and  abroad. 
Normal  values  of  variability  of  temperature  at  various 

stations  in  the  United  States. 
Section  101,  page  118.    Temperature  corrections  to  be  applied  to  a  mercury 

barometer. 
Section  101,  page  119.    Gravity    corrections    to    be    applied    to    a    mercury 

barometer. 

Section  113,  page  131.  The  number  of  feet  corresponding  to  a  difference  in 
pressure  of  ^  inch  for  various  pressures  and  tempera- 
tures. 

Section  114,  page  132.    The  barometric  pressure  at  various  elevations. 
Section  121,  page  137.     The  pressure  in  pounds  per  square  foot  of  the  wind  for 

several  velocities. 

Section  124,  page  140.    The  Beaufort  and  ten-point  wind  scales. 

510 


Section  10,  page  8. 
Section  11,  page  9. 

Section  13,  page  12. 

Section  20,  page  17. 
Section  36,  page  34. 

Section  38,  page  35. 
Section  43,  page  39. 

Section  44,  page  41. 


Section  74,  page  82. 
Section  79,  page  89. 


APPENDIX  V 


511 


Section  126,  page  143. 
Section  134,  page  153. 
Section  136,  page  154. 
Section  145,  page  161. 
Section  181,  page  195. 

Section  189,  page  202. 
Section  194,  page  205. 

Section  197,  page  209. 
Section  226,  page  230. 
Section  252,  page  248. 
Section  253,  page  252. 
Section  254,  page  253. 
Section  275,  page  277. 
Section  302,  page  304. 

Section  304,  page  307. 
Section  326,  page  327. 


The  corrected  velocities   corresponding  to  the  indi- 
cated velocities  of  a  Robinson  cup  anemometer. 
Normal  monthly  and  yearly  wind  velocities  for  many 

stations  in  the  United  States. 
Highest  observed  wind  velocities  at  many  stations  in 

the  United  States. 
Radius  of  curvature  for  frictionless  motion  on  the 

earth's  surface  for  several  velocities  and  latitudes. 
Vapor   content  and  mass  of  saturated  air  at  various 

temperatures. 
Psychrometer  table. 
Normal  monthly  and  yearly  values  of  absolute  humidity 

for  various  stations  in  the  United  States. 
Normal  monthly  and  yearly  values  of  relative  humidity 

for  various  stations  in  the  United  States. 
Normal  monthly  and  yearly  values  of   sunshine  for 

various  stations  in  the  United  States. 
Normal  monthly  and  annual  precipitation  for  various 

stations  in  the  United  States  and  elsewhere. 
Normal  monthly    and  annual    snowfall    for    various 

stations  in  the  United  States. 
Normal  number  of  days  with  precipitation  for  various 

stations  in  the  United  States. 
The  frequency  of  occurrence  at  different  times  of  year 

of  tropical  cyclones. 
The  number  and  velocity  of  motion  of  lows  on  the 

eleven  different  tracks  according  to  Russell. 
Velocity  of  lows  and  highs  according  to  von  Herrmann. 
The  normal  number  of  days  with  thundershowers  for 

the  various  months  and  for  the  year  for  various 

stations  in  the  United  States. 


APPENDIX  VI 
THE  ALBANY  DATA  GIVEN  IN  THE  TEXT 

Many  tables  of  data  for  the  Weather  Bureau  station  at  Albany,  N.Y.,  have 
been  given  in  the  text  to  illustrate  various  points.  These  are  indexed  here  for 
convenience  of  reference. 

Section  28,  page  22.  Number  of  thundershowers  each  year  from  1884  to 
1910. 

Section  73,  page  78.        The  adjusted  normal  daily  temperatures. 

Section  74,  page  79.        Average  and  normal  monthly  and  yearly  temperatures. 

Section  79,  page  87.  Average  and  normal  values  of  daily  range  of  tempera- 
ture. 

Section  79,  page  90.  Average  and  normal  values  of  variability  of  tem- 
perature. 

Section  79,  page  91.  The  number  of  zero  days,  days  above  90°,  and  days 
above  100°. 

Section  251,  page  247.  The  amount  of  precipitation  for  the  various  months  and 
for  the  year  for  several  years,  and  the  normal  values. 

Section  253,  page  251.  Snowfall  for  the  various  months  and  for  the  year  for 
several  years,  and  the  normal  values. 


512 


APPENDIX  VII 


THE  REGULAR  STATIONS  OF  THE  U.  S.  WEATHER  BUREAU  AND  THE 
CANADIAN  STATIONS 

The  following  list  contains  the  regular  stations  of  the  U.  S.  Weather  Bureau 
during  1911.    But  little  change  occurs  from  year  to  year. 


Abilene,  Tex. 
Albany,  N.Y.  M 
Alpena,  Mich.  M 
Amarillo,  Tex. 
Anniston,  Ala. 
Asheville,  N.C. 
Altanta,  Ga.*f 
Atlantic  City,  N.J.f 
Augusta,  Ga. 

Baker  City,  Ore. 
Baltimore,  Md.f 
Bentonville,  Ark. 
Binghamton,  N.Y.  M 
Birmingham,  Ala. 
Bismarck,  N.  Dak.f 
Block  Island,  R.I. 
Boise,  Idaho,  f 
Boston,  Mass.fM 
Buffalo,  N.Y.  M 
Burlington,  Vt.  M 

Cairo,  111.  M 
Canton,  N.Y. 
Cape  Henry,  Va. 


Cape  May,  N.J. 
Charles  City,  Iowa. 
Charleston,  S.C. 
Charlotte,  N.C. 
Chattanooga,  Tenn. 
Cheyenne,  Wyo.fM 
Chicago,  Ill*M 
Cincinnati,  Ohio.  M 
Cleveland,  Ohio.  M 
Columbia,  Mo.f 
Columbia,  S.C.f 
Columbus,  Ohio.fM 
Concord,  N.H.  M 
Concordia,  Kan. 
Corpus  Christi,  Tex.  M 

Davenport,  Iowa. 
Del  Rio,  Tex. 
Denver,  (7o/o.*fM 
Des  Moines,  Iowa.*fM 
Detroit,  Mich.  M 
Devils  Lake,  N.  Dak. 
Dodge  City,  Kan. 
Dubuque,  Iowa. 
Duluth,  Minn. 
Durango,  Colo. 


*  Center  of  one  of  the  twelve  climatological  districts, 
t  Center  of  one  of  the  old  forty-five  sections. 
M  The  station  issues  weather  maps. 
The  six  forecast  centers  are  printed  in  italic  type. 
2L  513 


514 


APPENDIX  VII 


Eastport,  Me. 
Elkins,  W.  Va. 
El  Paso,  Tex. 
Erie,  Pa.  M 
Escanaba,  Mich. 
Eureka,  Cal. 
Evansville,  Ind. 

Flagstaff,  Ariz. 
Fort  Smith,  Ark. 
Fort  Worth,  Tex.  M 
Fresno,  Cal.  M 

Galveston,  Tex. 
Grand  Haven,  Mich. 
Grand  Junction,  Colo.  M 
Grand  Rapids,  Mich.fM 
Green  Bay,  Wis. 

Hannibal,  Mo. 
Harrisburg,  Pa. 
Hartford,  Conn.  M 
Hatteras,  N.C. 
Havre,  Mont. 
Helena,  Mont.fM 
Honolulu,  Hawaii,  f 
Houghton,  Mich. 
Houston,  Tex.*f 
Huron,  S.  Dak.f 

Independence,  Cal. 
Indianapolis,  Ind.f 
lola,  Kan. 
Jthaca,  N.Y.*fM 

Jacksonville,  Fla.f 
Jupiter,  Fla. 

Kalispell,  Mont. 
Kansas  City,  Mo.  M 
Keokuk,  Iowa. 
Key  West,  Fla. 
Knoxville,  Term. 


La  Crosse,  Wis. 
Lander,  Wyo. 
Lansing,  Mich. 
La  Salle,  111. 
Lewiston,  Idaho.  M 
Lexington,  Ky. 
Lincoln,  Neb.f 
Little  Rock,  Ark.fM 
Los  Angeles,  Cal.  M 
Louisville,  Ky.*f 
Lynchburg,  Va. 

Macon,  Ga.  M 
Madison,  Wis. 
Manteo,  N.C. 
Marquette,  Mich. 
Memphis,  Tenn. 
Meridian,  Miss. 
Miles  City,  Mont. 
Milwaukee,  Wis.fM 
Minneapolis,  Minn.fM 
Mobile,  Ala. 
Modena,  Utah. 
Montgomery,  Ala.fM 
Moorhead,  Minn. 
Mount  Tamalpais,  Cal. 

(Through  San  Francisco  Sta.) 
Mount  Weather,  Va. 

(Via  Bluemont,  Va.) 

Nantucket,  Mass. 
Narragansett  Pier,  R.I. 
Nashville,  Tenn.f 
New  Haven,  Conn.  M 
New  Orleans,  La.*fM 
New  York,  N.Y.  M 
Norfolk,  Va.  M 
Northfield,  Vt.  M 
North  Head,  Wash. 

(Via  Ilwaco,  Wash.) 
North  Platte,  Nebr. 

Oklahoma,  Okla.fM 
Omaha,  Neb. 
Oswego,  N.Y. 


APPENDIX  VII 


515 


Palestine,  Tex. 
Parkersburg,  W.  Va.f  M 
Pensacola,  Fla. 
Peoria,  111.  M 
Philadelphia,  Pa.f 
Phoenix,  Ariz,  f  M 
Pierre,  S.  Dak. 
Pittsburg,  Pa.  M 
Pocatello,  Idaho. 
Point  Reyes  Light,  Cal. 

(Through  San  Francisco  Sta.) 
Port  Crescent,  Wash. 
Port  Huron,  Mich. 
Portland,  Me. 
Portland,  Ore*fa 
Providence,  R.I. 
Pueblo,  Colo. 

Raleigh,  N.C.f 
Rapid  City,  S.  Dak. 
Red  Bluff,  Cal. 
Reno,  Nev.f 
Richmond,  Va.fM 
Rochester,  N.Y. 
Roseburg,  Oreg. 
Roswell,  N.  Mex.  M 

Sacramento,  Cal. 
St.  Joseph,  Mo. 
St.  Louis,  MO.*M 
St.  Paul,  Minn. 
Salt  Lake  City,  Utah*t 
San  Antonio,  Tex. 
San  Diego,  Cal.  M 
Sand  Key,  Fla. 

(Through  Key  West  Sta.) 
Sandusky,  Ohio. 
San  Francisco,  Ca/.*fM 
San  Jose*,  Cal. 
San  Juan,  Porto  Rico,  W.I.f 


San  Luis  Obispo,  Cal. 
Santa  F6,  N.  Mex.  f 
Sault  Sainte  Marie,  Mich.  M 
Savannah,  Ga.  M 
Scranton,  Pa.  M 
Seattle,  Wash.f  M 
Sheridan,  Wyo. 
Shreveport,  La. 
Sioux  City,  Iowa.  M 
Southeast  Farallon,  Cal. 

(Through  San  Francisco  Sta.) 
Spokane,  Wash. 
Springfield,  111.  fat 
Springfield,  Mo. 
Syracuse,  N.Y. 

Tacoma,  Wash. 
Tampa,  Fla. 
Tatoosh  Island,  Wash. 
Taylor,  Tex.  M 
Thomasville,  Ga. 
Toledo,  Ohio. 
Tonopah,  Nev. 
Topeka,  Kan.f 

Valentine,  Neb. 
Vicksburg,  Miss.f 

Wagon  Wheel  Gap,  Colo. 
Walla  Walla,  Wash.  M 
Washington,  D.C.  M 
Wichita,  Kan. 
Wffliston,  N.  Dak. 
Wilmington,  N.C. 
Winnemucca,  Nev. 
Wytheville,  Va. 

Yankton,  S.  Dak. 
Yellowstone  Park,  Wyo. 
Yuma,  Ariz. 


516  APPENDIX  VII 

The  following  list  contains  the  Canadian  stations  from  which  observations 

are  received  by  telegraph  at  Toronto  for  the  construction  of  the  weather 

map:  • 

Dawson  City  Battleford               Pt.  Stanley  Yarmouth 

Atlin  Prince  Albert  Toronto  Halifax 

Prince  Rupert  Qu'Appelle              Kingston  Sydney 

Victoria  Minnedosa              Stonecliffe  Charlottetown 

New  Westminster          Winnipeg  Ottawa  Sable  Island 

Kamloops  The  Pas                   Montreal  St.  Johns 

Barkerville  Port  Arthur            Quebec  Burin 

Calgary  White  River  Father  Point  Port  Aux  Basques 

Edmonton  Cochrane  Anticosti  Fogo 

Medicine  Hat  Parry  Sound           Chatham  Belle  Isle 

Swift  Current  Southampton  St.  John 


APPENDIX  VIII 

TEACHING  METEOROLOGY 

The  method  of  teaching  meteorology  and  the  character  of  the  course  to  be 
given  depend  upon  the  age  and  advancement  of  the  students,  the  time  allotted 
to  the  subject,  and  the  standpoint  from  which  it  is  taught.  Instruction  in 
meteorology  is  given  in  grammar  and  high  schools  as  well  as  in  colleges  and 
universities.  The  time  allotted  to  the  subject  is  only  a  few  minutes  each  day 
or  week  in  some  schools,  while  in  many  colleges  and  universities  systematic 
semester  or  year  courses  are  given.  Meteorology  is  sometimes  taught  with  the 
ability  to  forecast  as  the  chief  aim ;  occasionally  it  is  given  with  the  emphasis 
on  the  mathematical  side ;  sometimes  the  application  of  the  laws  and  principles 
of  physics  is  chiefly  emphasized ;  and  often  the  laboratory  method  of  presenta- 
tion is  used.  Each  teacher  must  thus  work  out  for  himself  the  most  suitable 
course  and  the  best  way  of  teaching  it. 

Appendix  A,  pages  171  to  185,  in  WARD'S  Practical  Exercises  in  Elementary 
Meteorology,  contains  suggestions  to  the  teacher  who  is  giving  a  course  in  a 
grammar  or  high  school  from  the  laboratory  standpoint. 

The  New  York  State  Education  Department  at  Albany,  N.Y.,  issues  a  pam- 
phlet which  contains  an  outline  in  syllabus  form  of  the  topics  which  the  depart- 
ment considers  should  be  included  in  a  course  on  Physical  Geography.  Meteor- 
ology is  included  under  this  head,  and  the  pamphlet  is  entitled  Syllabus  for 
Secondary  Schools;  Physical  Geography.  Attention  should  also  be  called  to 
Bulletin  No.  3  (1906)  of  the  Geographic  Society  of  Chicago,  edited  by  Cox  and 
Goode,  and  entitled  Lantern  Slide  Illustrations  for  the  Teaching  of  Meteorology. 
The  National  Educational  Association  has  also  considered  from  time  to  time 
the  matter  of  teaching  Physical  Geography  (usually  including  meteorology), 
and  the  Report  of  the  Committee  of  Ten,  1894,  should  be  consulted. 


517 


APPENDIX  IX 

THE  LITERATURE  OF  METEOROLOGY 
(A)    General  Directions 

The  lists  of  books  and  periodicals,  and  the  bibliographical  material  here  added 
are  to  enable  two  entirely  different  classes  of  students  to  gain  more  information 
than  can  be  acquired  by  reading  this  book.  The  first  class  of  students  consists 
of  those  who  wish  to  gain  somewhat  more  detailed  information  on  any  given 
subject.  The  second  class  consists  of  the  research  students  who  wish  to  find 
every  word  which  has  been  written  on  a  certain  very  small  and  definite  topic. 

In  the  case  of  those  who  simply  desire  more  information  about  a  given  subject, 
the  best  method  of  procedure  is  probably  to  read  first  whatever  may  be  given  on 
the  subject  in  the  twenty-five  best  books  in  the  following  list  and  also  to  look  up 
all  of  the  references  to  the  literature  at  the  end  of  each  chapter  of  this  book  as 
far  as  they  apply  to  the  subject.  If  still  more  information  is  desired,  it  would 
probably  be  best  to  go  over  more  books  in  the  following  lists  and  also  to  look  up 
any  references  to  the  literature  which  may  be  given  in  what  has  been  read.  If 
still  more  information  is  desired,  the  student  is  essentially  in  the  same  position 
as  the  research  student. 

In  the  case  of  the  research  student  who  desires  to  find  every  word  which  has 
been  written  on  a  certain  definite  topic,  the  best  method  of  procedure  would 
probably  be  first  to  see  what  is  given  in  each  of  the  books  in  the  following  lists 
and  also  to  look  up  any  references  at  the  end  of  the  various  chapters  of  this 
book  which  bear  on  the  topic.  The  student  should  note  every  reference  to  the 
literature  in  the  books,  pamphlets,  and  articles  read,  and  follow  these  up.  Next 
the  student  would  naturally  look  in  the  bibliography  of  bibliography  to  see  if 
there  is  any  bibliography  on  the  topic  in  question  and,  if  so,  he  would  look  up  all 
the  references  contained  in  it.  The  student  should  then  take  the  two  digests  of 
meteorological  literature  and,  starting  with  the  current  year,  work  backwards 
to  the  beginning  of  the  publications,  or  as  far  as  seems  advisable.  If  all  the 
references  obtained  in  all  these  ways  are  followed  up,  the  student  will  probably 
have  found  nearly  every  word  on  the  topic,  and  will  have  a  clear  idea  how  much 
has  been  done  and  to  what  extent  the  periodical  literature  has  been  incorporated 
in  the  latest  books  on  meteorology. 

518 


APPENDIX  IX  519 

(B)   A  List  of  Books 

The  following  list  contains  nearly  three  hundred  books  which  cover  the  whole 
subject  of  meteorology  or  some  phase  or  part  of  it.  Publications  by  the  U.  S. 
government  and  publications  by  the  weather  bureaus  of  other  countries  have 
in  nearly  all  cases  been  excluded.  All  pamphlets  and  reprints  of  articles  in 
the  periodical  literature  have  also  been  excluded.  These  will  be  found  in  the 
references  at  the  end  of  each  chapter.  The  books  have  been  grouped.  The 
first  group  contains  general  works  on  the  whole  field.  The  second  group  con- 
tains books  which  cover  the  whole  field  of  meteorology,  but  from  a  special  stand- 
point, or  for  a  special  purpose.  The  remaining  groups  follow  the  chapters  of 
this  book.  If  a  library  of  exactly  one  hundred  books  on  meteorology  were  to  be 
chosen  from  the  list,  those  marked  with  one  star  would  be  suggested  as  most 
suitable.  If  a  library  of  exactly  twenty-five  were  to  be  chosen  from  the  list, 
those  marked  with  two  stars  would  be  suggested.  In  this  list  of  twenty-five, 
if  a  treatise  is  divided  into  several  parts  or  volumes,  it  is  considered  as  a  single 
treatise.  Books  in  foreign  languages  have  not  been  excluded,  as  this  list  con- 
tains six  German  and  one  French  book.  This  does  not  mean  that  the  beginner 
could  start  with  any  one  of  these  twenty-five  books  and  find  it  understandable. 
The  list  contains,  however,  several  elementary  books  which  start  at  the  begin- 
ning. It  rather  means  that  the  possessor  of  these  books  would  have  a  fairly 
complete  treatment  of  meteorology  from  the  foundation  up. 

t 

(i)  General  Books 

**ABBE,  CLEVELAND,  The  Aims  and  Methods  of  Meteorological  Work,  4°,  pp.  219 
to  330,  Baltimore,  1899.  (Part  Ilia  of  Vol.  I  of  Maryland  Weather  Service.) 

*ABBE,  CLEVELAND,  Treatise  on  Meteorological  Apparatus  and  Methods,  8°, 
392  pp.,  Washington,  1888.  (Annual  report  of  the  Chief  Signal  Officer 
for  1887 ;  Appendix  46.) 

ABBE,  CLEVELAND,  "Meteorology,"  in  the  Encyclopaedia  Britannica,Vol.  XXX, 
1902.  (Revised  by  him  for  1911  or  llth  edition.) 

*ABERCROMBY,  RALPH,  Weather,  12°,  xix  +  472  pp.,  London,  1887  (5th  impres- 
sion 1902).  (International  Scientific  Series.) 

*ALLINGHAM,  WILLIAM,  A  Manual  of  Marine  Meteorology,  12°,  xvi  +  182  pp., 
London,  1900. 

ANDRE,  CH.,  Relations  des  pMnomenes  meteor ologiques,  4°,  168  pp.,  Lyon,  1892. 

**ANGOT,  ALFRED,  Traite  elementaire  de  meteorologie,  8°,  vi  +  417  pp.,  Paris,  1899. 
(2d  ed.,  1907.) 

ARAGO,  FRANCOIS,  Meteorological  Essays,  8°,  540  pp.,  London,  1855. 

**ARCHIBALD,  DOUGLAS,  The  Story  of  the  Atmosphere,  16°,  210  pp.,  London,  1901. 

*ARRHENIUS,  SVANTE  AUGUST,  Lehrbuch  der  kosmischen  Physik,  8°,  viii  + 1026 
pp.,  Leipzig,  1903.  (pp.  473  to  925  deal  with  meteorology.) 

*BARNES,  HOWARD  T.,  Ice  Formation,  8°,  x  +  260  pp.,  New  York,  1906. 


520  APPENDIX  IX 

BEBBER,  W.  J.  VAN,  Katechismus  der  Meteorologie,  3d  ed.,  12°,  xii +  259  pp., 
Leipzig,  1891. 

*BEBBER,  W.  J.  VAN,  Lehrbuch  der  Meteorologie,  8°,  xii  +  391  pp.,  Stuttgart,  1890. 

*BERGET,  ALPHONSE,  Physique  du  globe  et  meteorologie,  8°,  v  +  353  pp.,  Paris, 
1904.  (pp.  162  to  343  deal  with  meteorology. f 

*BEZOLD,  WILHELM  VON,  Gesammelte  Abhandlungen  aus  den  Gebieten  der  Meteor- 
ologie und  des  Erdmagnetismus,  4°,  viii  +  448  pp.,  Braunschweig,  1906. 

*BLANFORD,  H.  F.,  Indian  Meteorologists  Vade  Mecum,  in  three  parts,  8°,  85, 
185,  81  pp.,  Calcutta,  1877. 

*BORNSTEIN,  R.,  Leitfaden  der  Wetterkunde,  2d  ed.,  8°,  xi  +  230  pp.,  Braun- 
schweig, 1906. 

BROCKLESBY,  JOHN,  Elements  of  Meteorology,  12°,  xii  +  240  pp.,  New  York,  1848. 

*BUCHAN,  ALEXANDER,  A  Handy  Book  of  Meteorology,  12°,  204  pp.,  London, 
1867. 

*BUCHAN,  ALEXANDER,  Report  on  Atmospheric  Circulation,  f°,  iv  +  263  pp., 
52  maps,  London,  1889.  (Report  on  the  scientific  results  of  the  voyage  of 
H.M.S.  Challenger.) 

BUTLER,  THOMAS  BELDEN,  The  Philosophy  of  the  Weather,  12°,  xviii  +  414  pp., 
New  York,  1856. 

"CHAMBERS,  G.  F.,  The  Story  of  the  Weather,  24°,  234  pp.,  London,  1897. 

CHASE,  PLINY  EARLE,  Elements  of  Meteorology,  2  vols.,  128  and  256  pp.,  Phila- 
delphia, 1884. 

CORDEIRO,  FREDERICK,  J.  B.,  The  Atmosphere;  its  Characteristics  and  Dynamics, 
4°,  viii  +  129  pp.,  New  York,  1910.  • 

CORNELIUS,  C.  S.,  Meteorologie,  8°,  x  -f  614  pp.,  Halle,  1863. 

Cyclopedia  of  American  Agriculture,  New  York,  1907.  Chap.  XVII,  "Weather 
Terms  and  Weather  Knowledge,"  by  WILFORD  M.  WILLSON  ;  Chap.  XVIII, 
"The  Atmosphere  and  its  Phenomena,"  by  CLEVELAND  ABBE,  JR. 

DANIELL,  J.  F.,  Elements  of  Meteorology,  3d  ed.,  2  vols.,  8°,  xxxv  +  341,  and  viii 
+  389  pp.,  London,  1845. 

**DAVIS,  WILLIAM  MORRIS,  Elementary  Meteorology,  8°,  xi  +  355,  New  York, 
1894. 

DICKSON,  H.  N.,  Meteorology,  12°,  viii  +  192  pp.,  London,  1893. 

DREW,  JOHN,  Practical  Meteorology,  12°,  xi  +  291  pp.,  London,  1855. 

DUCLAUX,  E.,  Cours  de  physique  et  de  meteorologie  professe  a  I'institut  agro- 
nomique,  8°,  iv  +  504  pp.,  Paris,  1891. 

*DUNN,  E.  B.,  The  Weather,  8°,  viii  +  356  pp.,  New  York,  1902. 

*FERREL,  WILLIAM,  Recent  Advances  in  Meteorology,  systematically  arranged  in 
the  Form  of  a  Text-book,  8°,  440  pp.,  Washington,  1886.  (Annual  Report  of 
the  Chief  Signal  Officer,  1885;  Appendix  71.) 

FINDLAY,  ALEXANDER  GEORGE,  Text-book  of  Ocean  Meteorology,  8°,  259  pp., 
London,  1887. 

FLAMMARION,  CAMILLE,  V 'Atmosphere,  Meteorologie  populaire,  8°,  453  pp., 
London,  1873.  (Translated  by  J.  Glaisher.) 


APPENDIX  IX  521 

FLAMMARION,  CAMILLE,  L' Atmosphere  et  les  grands  phenomenes  de  la  nature, 

4°,  370  pp.,  Paris,  1905. 

GEROSA,  GIUSEPPE,  Elementi  di  Meteorologia,  8°,  x  +  316  pp.,  Livorno,  1909. 
*GIBERNE,  AGNES,  The  Ocean  of  Air,  8°,  xiv  +  340  pp.,  London,  1903. 
GILBERT,   OTTO,   Die    meteorologischen   Theorien   des    Griechischen    Altertums, 

8°,  iv  +  746  pp.,  Leipzig,  1907. 

**GREELY,  A.  W.,  American  Weather,  8°,  xii  +  286  pp.,  New  York,  1888. 
**HANN,  JULIUS,  Lehrbuch  der  Meteorologie,  4°,  xi  +  642  pp.,  1st  ed.  Leipzig, 

1901 ;    2d  ed.,  Leipzig,  1906. 
"HARRINGTON,  MARK  W.,  About  the  Weather,  12°,  xx  +  246  pp.,  New  York, 

1899. 

HELLMANN,  G.,  Meteorologische  Volksbucher,  2d  ed.,  4°,  68  pp.,  Berlin,  1895. 
HELLMANN,  G.,  Neudrucke  von  Schriften  und  Karten  uber  Meteorologie  und  Erd- 

magnetismus,  15  vols.,  Berlin,  1893-1904. 
*HENKEL,  F.  W.,  Weather  Science,  8°,  336  pp.,  London,  1911. 
HERSCHEL,  SIR  J.,  Meteorology,  16°,  vii  +  288  pp.,  Edinburgh,  1862. 
HILDEBRANDSSON  AND  HELLMANN,  Codex  of  Resolutions  adopted  at  International 

Meteorological  Meetings,  1872-1907,  British  Meteorological  Office,  London, 

1909. 
*HILDEBRANDSSON,  H.,  ET  TEissERENC  DE  BORT,  LEON,  Les  bases  de  la  meteor- 

ologie  dynamique  historique,  4°,  3  vols.  (2  have  appeared),  Paris,  1898. 
HORNBERGER,  J.,  Grundriss  der  Meteorologie  und  Klimatologie,  8°,  ix  +  233  pp., 

Berlin,  1891. 
*HOUSTON,  EDWIN  J.,  The  Wonder  Book  of  the  Atmosphere,  8°,  x  +  326  pp., 

New  York,   1907. 
HOUZEAU,  J.  C.,  ET  LANCASTER,  A.,  Traite  elementaire  de  meteor ologie,  324  pp., 

Mons,  1883. 
KAMTZ,  LUDWIG  FRIEDRICH,  Lehrbuch  der  Meteorologie,  8°,  3  vols.,  526,  615, 

563  pp.,  Leipzig,  1831. 
KAMTZ,  LUDWIG  FRIEDRICH,  Complete  Course  of  Meteorology,  12°,  xxii  +  598  pp., 

London,  1845.     (Translated  by  C.  V.  Walker.) 
*KASSNER,  CARL,  Das  Wetter  und  seine  Bedeutung  fur  das  praktische  Leben,  12°, 

vi  +  148  pp.,  Leipzig,  1908. 
KASTNER,  K.  W.  G.,  Handbuch  der  Meteorologie,  8°,  3  vols.,  502,  655,  638  pp., 

Erlangen,  1823-1830. 

KINDLER,  P.  FINTAN,  Das  Wetter,  16°,  viii  +  142  pp.,  Koln,  1909. 
KLEIN,  HERMANN  F.,  Allgemeine  Witterungskunde,  12°,  259  pp.,  Leipzig,  1884. 

(2d  ed.,  Wien,  1905.) 

*KOPPEN,  W.,  Grundlinien  der  maritimen  Meteorologie,  12°,  vi  +  83  pp.,  Ham- 
burg, 1899. 

LOMMEL,  G.,  Wind  und  Wetter,  16°,  vii  +  344  pp.,  Miinchen,  1880. 
LOOMIS,  E.,  Contributions  to  Meteorology,  rev.  ed.,  4°,  232  pp.,  New  Haven, 

1885-1887. 
*LOOMIS,  ELIAS,  A  Treatise  on  Meteorology,  8°,  viii  +  305  pp.,  New  York,  1868. 


522  APPENDIX  IX 

MEYERS,  Konversationslexikon.    (See  the  various  articles  on  meteorological  sub- 
jects.) 
M'PHERSON,  J.  G.,  Meteorology;   or  Weather  Explained,  12°,  126  pp.,  London, 

1905. 
*MOHN,  H.,  Grundzuge  der  Meteorologie,  5th  ed.,  8°,  xii  +  419  pp.,  Berlin, 

1898. 
**MOORE,  JOHN  WILLIAM,  Meteorology,  8°,  xvi  +  445  pp.,  London,  1894.     (New 

edition,  revised,  enlarged,  and  somewhat  changed,  1910.) 
**MOORE,  WILLIS  L.,  Descriptive  Meteorology,  8°,  xviii  +  344  pp.,  New  York, 

1910. 
*MULLER,  JOH.,  Lehrbuch  der  kosmischen  Physik.  (5th  ed.  by  C.  F.  W.  Peters), 

8°,  xxiii  +  907  pp.,  Braunschweig,  1894.     (Atlas  as  separate  vol.) 
OLIVER,  MIGUEL  CORREA,  Tratado  elemental  de  Meteorologia,  8°,  303  pp.  (a  vol. 

of  plates),  Madrid,  1909. 
*PHIPSON,  THOMAS  LAMB,  Researches  on  the  Past  and  Present  History  of  the 

Earth's  Atmosphere,  12°,  xii  +  194  pp.,  London,  1901. 
POWERS,  EDWARD,  War  and  Weather,  8°,  202  pp.,  Delavan,  Wis.,  1890. 
RENK,  FRIEDRICH,  Die  Luft,  8°,  vi  +  242  pp.,  Leipzig,  1886. 
**RUSSELL,  THOMAS,  Meteorology,  8°,  xxiii  +  277  pp.,  New  York,  1895. 
*SALISBURY,  ROLLIN  D.,  Physiography,  8°,  New  York,  1907.    (pp.  506-705  treat 

of  meteorology.) 
SCHMID,  ERNST  ERHARD,  Lehrbuch  der  Meteorologie,  8°,  xvi  +  1009  pp.,  Leipzig, 

1860. 

*SCOTT,  ROBERT  H.,  Elementary  Meteorology,  4th  ed.,  12°,  xiv  +  410  pp.,  Lon- 
don, 1887  (reprinted  1903).     (International  Science  Series.) 
*SPRUNG,  A.,  Lehrbuch  der  Meteorologie,  8°,  xii  +  407  pp.,  Hamburg,  1885. 
STEINMETZ,    ANDREW,    Sunshine    and   Shadows,  8°,  xvi  +  432  pp.,   London, 

1867. 
SYMONS,  G.  J.,  The  Eruption  of  Krakatoa  and  Subsequent  Phenomena,  4°,  xvi  -f 

494,  London,  1888. 
TAYLOR  INSTRUMENT  Co.,   Weather  and  Weather  Instruments,   12°,   175  pp., 

Rochester,  1908. 

TISSANDIER,  GASTON,  UOcean  aerien,  8°,  viii  +  312  pp.,  Paris,  1883. 
*TRABERT,  WILHELM,  Meteorologie  (Sammlung  Goschen),  24°,  150  pp.,  Leipzig, 

1896  (3d  ed.,  Leipzig,  1909). 
TRABERT,  WILHELM,  Lehrbuch  der  kosmischen  Physik,  8°,  x  +  662  pp.,  Leipzig, 

1911. 

UMLAUFT,  FRIEDRICH,  Das  Luftmeer,  8°,  viii +  488  pp.,  Wien,  1891. 
**WALDO,  FRANK,  Elementary  Meteorology,  12°,  373  pp.,  New  York,  1896. 
**WALDO,     FRANK,    Modern    Meteorology,     12°,    xxiii  +  460    pp.,     London, 

1893. 
**WARD,  ROBERT  DE  COURCY,  Practical  Exercises  in  Meteorology,  8°,  xiii  + 

199  pp.,  Boston,  1899. 
WEBER,  LEONHARD,  Wind  und  Wetter,  12°,  v  +  130  pp.,  Leipzig. 


APPENDIX  IX  523 

(2)   Miscellaneous  Books  covering  the  Whole  Field  of  Meteorology,  but  with  a 
Special  Point  in  View 

(A)  Composition  of  the  Atmosphere 

*RAMSAY,  SIR  WILLIAM,  The  Gases  of  the  Atmosphere;  the  History  of  their  Dis- 
covery, 3d  ed.,  8°,  xii  +  296  pp.,  London,  1905. 

(B)  Instructions  to  Observers 

*ANGOT,  ALFRED,  Instructions  meteor  ologiques,  5th  ed.,  8°,  vi  +  161  pp.,  Paris,  1911. 
JELINEK,  CARL,  Jelinek's  Anleitung  zur  Ausfuhrung  meteorologischer  Beobach- 

tungen  nebst  einer  Sammlung  von  Hilfstafeln,  5th  ed.,  4°,  124  and  94  pp., 

Wien,  1905  and  1910. 

*LOISEL,  JULIEN,  Guide  de  I 'amateur  meteor ologiste,  8°,  vi  +  101  pp.,  Paris,  1906. 
*MARRIOTT,  WILLIAM,  Hints  to  Meteorological  Observers,  8°,  69  pp.,  London,  1906. 
Prussia  (K.  preussisches  met.  Institut),  Anleitung  zur  Anstellung  und  Berech- 

nung  meteorologischer  Beobachtungen,  2  pts.,  2d  ed.,  4°,  Berlin,  1904-1905. 
*The  Observer's  Handbook,  pub.  by  the  Meteorological  Office,  London.    (Revised 

almost  annually.) 

(C)  Tables 

Aspirations  —  Psychrometer  —  Tafeln     (vom     Kon.    Preussischen    meteorolo- 

gischen  Institut),  4°,  xiv  +  90  pp.,  Braunschweig,  1908. 

HAZEN,  Handbook  of  Meteorological  Tables,  8°,  vi  +  127  pp.,  Washington,  1888. 
JELINEK,  CARL,  Jelinek's  Psychrometer  Tafeln,  5th  ed.,  f°,  xiii  + 107  pp., 

Leipzig,  1903. 
**Smithsonian  Meteorological   Tables.     (Smithsonian  misc.  collections — 1032.) 

Rev.  ed.,  lix  +  274  pp.,  Washington,  1896.     (3d  revised  edition,  1907.) 
Tables  Meteorologiques  Internationales,  4°,  Paris,  1890. 

(D)   Upper  Air  Investigation 

*ASSMANN,  R.,  BERSON,  A.,  (and  others),  Wissenschaftliche  Luftfahrten,  3  vols.,  f°, 

150,  706,  and  313  pp.,    Braunschweig,  1899-1900. 
*HILDEBRANDT,  A.,  Airships  Past  and  Present,  8°,  xvi  +  364  pp.,  London,  1908. 

(Translated  by  W.  H.  Story.) 

LINKE,  FRANZ,  Aeronautische  Meteorologie,  8°,  viii  +  133  pp.,  Frankfurt,  1911. 
*MOEDEBECK,  HERRMANN  W.  L.,  Pocket-book  of  Aeronautics,  16°,  xiii  +  496  pp., 

London,  1907.     (Translated  by  W.  Mansergh  Varley.) 
NIMFUHR,  RAIMUND,  Leitfaden  der  Luftschiffart  und  Flugtechnik,  2d  ed.,  8°, 

xvi  +  528  pp.,  Wien,  1910. 
**ROTCH,  A.  LAWRENCE,  Sounding  the  Ocean  of  Air,  16°,  viii  +  184  pp.,  London, 

1900. 
ROTCH,  A.  L.,  and  PALMER,  ANDREW  H.,  Charts  of  the  Atmosphere  for  Aeronauts 

and  Aviators,  4°,  96  pp.  +  24  chts.,  New  York,  1911. 
ZAHM,  ALBERT  F.,  Aerial  Navigation,  8°,  xvii  +  497  pp.,  New  York,  1911. 


524  APPENDIX  IX 

(E)  Charts 

**BABTHOLOMEW,  J.  G.,  AND  HERBERTSON,  A.  J.,  Physical  Atlas;  Meteorology, 
f°,  40  +  xiv  pp.,  34  plates,  London,  1899.  (3d  vol.  of  Physical  Atlas.) 

*BERGHAUS,  Physical  Atlas,  f°,  Gotha,  1887.  (The  met'l  part  by  Hann  can 
be  bought  separately.) 

DENISON,  CHARLES,  Climates  of  the  United  States,  4°,  47  pp.,  Chicago,  1893. 

ELIOT,  SIR  JOHN,  Climatological  Atlas  of  India,  f°,  xxxii  pp.  and  120  maps,  Edin- 
burgh, 1906. 

Russia,  Atlas  Climatologique  de  I 'empire  de  Russie,  f°,  xiv  +  61  pp.,  (text), 
St.  Petersbourg,  1900. 

(F)  Relation  of  meteorology  to  plants  and  animals  —  phenology 

GUNTHER,  SIEGMUND,  Die  Phdnologie,  ein  Grenzgebiet  zwischen  Biologie  und 
Klimakunde,  8°,  51  pp.,  Mlinster,  1895. 

(G)  Relation  of  Meteorology  to  Medicine 

*BEBBER,  W.  J.  VAN,   Hygienische  Meteorologie,  8°,  x  +  330  pp.,  Stuttgart, 

1895. 
BELL,  AGRIPPA  NELSON,  Climatology  and  Mineral  Waters  of  the  United  States, 

8°,  vii  +  386  pp.,  New  York,  1885. 
*DEXTER,  EDWIN  GRANT,  Weather  Influences,  8°,  xxxi  +  286  pp.,  New  York, 

1904. 
GILES,  G.  M.,  Climate  and  Health  in  Hot  Countries,  8°,  xviii  +  188,  109  pp., 

New  York,  1905. 
*HUGGARD,  WILLIAM  R.,  A  Handbook  of  Climatic  Treatment,  8°,  xiii  +  536  pp., 

London,  1906. 
*SOLLY,  S.   EDWIN,  A  Handbook  of  Medical  Climatology,  8°,   xii  +  470  pp., 

Philadelphia,  1897. 
WEBER,  F.  PARKER,  AND  HINSDALE,  GUY,  Climatology;  Health  Resorts;  Mineral 

Springs,  2  vols.,  8°,  ix  +  (10)  +  336  and  x  +  (11)  +  420  pp.,  Philadelphia, 

1902. 
*WEBER,  SIR  HERRMANN,  AND  WEBER,  F.  PARKER,  Climatotherapy  and  Balne- 

otherapy,  4°,  833  pp.,  London,  1907. 
ZUNTZ,  N.  (and  others),  Hohenklima  und  Bergwanderungen  in  ihrer  Wirkung  auf 

den  Menschen,  4°,  xvi  +  494  pp.,  Berlin,  1906. 

(3)  The  Observation  and  Distribution  of  Temperature 

*BOLTON,  HENRY  CARRINGTON,  Evolution  of  the  Thermometer,  1592-1743,  16°,  98 

pp.,  Easton,  Pa.,  1900. 
GUILLAUME,  CH.  ED.,  TraitS  practique  de  la  thermometrie  de  Precision,  8°,  xv  + 

336,  Paris,  1889. 


APPENDIX  IX  525 

(4)  The  Pressure  and  Circulation  of  the  Atmosphere  —  including  the  Winds  and  the  General 
Theory  of  Atmospheric  Circulation 

*ABBE,  CLEVELAND.   The  Mechanics  of  the  Earth's  Atmosphere,  8°,  324  pp., 

Washington,      1891.     (Smithsonian     Miscellaneous     Collections — 843.) 

(Second  Collection  of  Translations.) 
*ABBE,  CLEVELAND,  The  Mechanics  of  the  Earth's  Atmosphere,  8°,  iv  +  616  pp., 

Washington,  1910.     (Third  Collection  of  Translations.) 
ANSART-DEUSY,  A.,  Theorie  des  mouvements  de  I' atmosphere  et  de  V ocean,  8°, 

272  pp.,  Paris,  1877. 

ASSMANN,  RICHARD,  Die  Winde  im  Deutschland. 
*BRILLOUIN,  MARCEL,  Memoires  originaux  sur  la  circulation  generate  de  I'atmos- 

phere,  8°,  xx  +  165  pp.,  Paris,  1900. 

BUYS-BALLOT,  Les  courants  de  I' air  et  de  V atmosphere,  8°,  39  pp.,  Bruges,  1891. 
CHATLEY,  HERBERT,  The  Force  of  the  Wind,  8°,  viii  +  83  pp.,  London,  1909. 
*COFFIN,  J.  H.,  The  Winds  of  the  Globe,  4°,  768  pp.,  Washington,  1876. 
Deutsche    Seewarte,  Segelhandbuch  fur  den   Atlantschen   Ozean;    Hierzu   ein 

Atlas.     (A  similar  treatise  is  also  issued  for  the  Pacific  and  Indian  Oceans.) 
**FERREL,  WILLIAM,  Popular  Treatise  on  the  Winds,  2d  ed.,  8°,  vii  +  505  pp., 

New  York,  1889. 
GILBERT,  G.  K.,  A  New  Method  of  Measuring  Heights  by  Means  of  the  Barometer. 

(U.  S.  Geological  Survey  —  annual  report,  1881  —  pp.  403  to  566.) 
LIZNAR,  J.,  Die  Barometrische  Hohenmessung,  8°,  48  pp.,  Leipzig,  1904. 
LOISEL,  JULIEN,  Le  Barometre  aneroide,  12°,  24  pp.,  Paris,  1905. 
MOHN,  H.,  AND  GULDBERG,  C.  M.,  Etudes  sur  les  mouvements  de  I'atmosphere. 

4°,  39  and  53  pp.,  2  parts,  Christiania,  1876-1880. 

PLYMPTON,  GEORGE  W.,  The  Aneroid  Barometer,  8°,  126  pp.,  New  York,  1907. 
SCHREIBER,  PAUL.  Handbuch  der  barometrischen  Hohenmessung  en,  2d  ed.,  8°, 

xiv  +  480  pp.,  London,  1863. 

SHAW,  WILLIAM  H.,  The  Life  History  of  Surface  Air  Currents,  4°,  107  pp.,  Lon- 
don, 1906. 

SUPAN,  A.,  Statistik  der  unteren  Luftstromungen,  8°,  vii  +  296  pp.,  Leipzig,  1881. 
WEGENER,  ALFR.,  Thermodynamik  der  Atmosphdre,  8°,  viii  +  331  pp.,  Leipzig, 

1911. 
WHYMPER,  EDWARD,  How  to  use  the  Aneroid  Barometer,  8°,  61  pp.,  London,  1891. 

(5)  The  Moisture  of  the  Atmosphere  —  Dew,  Frost,  Fog,  Clouds,  Precipitation 

ABERCROMBY,  R.,  Seas  and  Skies  in  Many  Latitudes,  xvi  +  447  pp.,  London, 

1888. 

*BARBER,  SAMUEL,  The  Cloud  World,  8°,  xii  +  139  pp.,  London,  1903. 
BARUS,  CARL,  A  Continuous  Record  of  Atmospheric  Nucleation,  f°,  xvi  +  226 

pp.,  Washington,  1905. 
BIGELOW,  FRANK  HAGAR,  Report  on  the  International  Cloud  Observations,  4°, 

787  pp.,  Washington,  1900.     (Vol.  Ill  of  the  Report  of  the  Chief  of  the 

Weather  Bureau,  1898-1899.) 


526  APPENDIX  IX 

**CLAYDEN,  ARTHUR  W.,  Cloud  Studies,  8°,  xiii  +  184  pp.,  New  York,  1905. 
Cloud  Crystals,  wide  8°,  158  pp.,  New  York,  1864. 

COLLINSON,  JOHN,  Rainmaking  and  Sunshine,  12°,  xvi  +  280  pp.,  London,  1894. 
FRITZSCHE,  RICHARD,  Niederschlag,  Abfluss,  und  Verdunstrung  auf  den  Land- 

fldchen  der  Erde,  8°,  54  pp.,  Halle,  1906. 
HELLMANN,  G.,  Die  Niederschldge  in  den  norddeutschen  Stromgebieten,  4°,  3  vols., 

Berlin,  1906. 

HELLMANN,  G.,  Schneekrystalle,  4°,  66  pp.,  Berlin,  1893. 
HERBERTSON,  The  Distribution  of  Rainfall  over  the  Land,  8°,  70  pp.,  London, 

1901. 

HOUDAILLE,  F.,  Les  orages  a  grele  et  le  tir  des  canans,  244  pp.,  Paris,  1901. 
* International  Cloud  Atlas,  (Atlas  International  des  Nuages),  by  HILDERBRANDS- 

SON,  and  others,  f°,  30  pp.,  xiv  plates,  Paris,   1896.      (A  new   slightly 

changed  edition,  4°,  viii  +  24  pp.,  appeared  in  1911.) 
*KASSNER,  CARL,  Das  Reich  der  Wolken  und  Niederschldge,  12°,  160  pp.,  Leipzig, 

1909. 

LANCASTER,  A.,  La  Pluie  en  Belgique,  8°,  224  pp.,  Bruxelles,  1894. 
*LEY,  CLEMENT,  Cloudland,  8°,  ix  +  208  pp.,  London,  1894. 
McAoiE,  ALEX.,  The  Clouds  and  Fogs  of  San  Francisco,  8°,  106  pp.,  San  Fran- 
cisco, 1912. 

RUSSELL,  ROLLO,  On  Hail,  8°,  xv  +  224  pp.,  London,  1893. 
SCHARF,  EDMUND,  Der  Hagel,  12°,  vi  +  195  pp.,  Halle  a.  S.,  1906. 
*SUPAN,  ALEXANDER,  Die  Verteilung  des  Niederschlags  auf  der  festen  Erdober- 

fldche,  4°,  iv  +  103  pp.,  Gotha,  1898. 
THOMPSON,  MRS.  JEANETTE  MAY,  Water  Wonders  Every  Child  Should  Know, 

xvii  +  233  pp.,  New  York,  1907. 

VINCENT,  J.,  Atlas  des  nuages,  f°,  29  pp.,  Bruxelles,  1907. 
Voss,  ERNST  LUDWIG,  Die  Niederschlagsverhdltnisse  von  Sudamerika,  4°,  iv  + 

59  pp.,  Gotha,  1907. 

WETHERELL,  HENRY  EMERSON,  Hygromedry,  82  pp.  (pub.  by  author). 
WILD,  H.,  Die  Regenverhdltnisse  des  Russischen  Reiches,  f°,  120  +  95  +  286  pp., 

St.  Petersburg,  1887. 
WILSON-BARKER,  D.,  Clouds  and  Weather  Signs,  8°,  31  pp.,  London. 

(6)  The  Secondary  Circulation  of  the  Atmosphere  —  Storms 

*ALGUE,  JOSE,  The  Cyclones  of  the  Far  East,  2d  ed.,  4°,  283  pp.,  Manila,  1904. 

*BERGHOLZ,  PAUL,  Die  Orkane  des  fernen  Ostens,  xii  +  260  pp.,  Bremen,  1900. 

BLASIUS,  WILLIAM,  Storms,  8°,  342  pp.,  Philadelphia,  1875. 

DAVIS,  WILLIAM  MORRIS,  Whirlwinds,  Cyclones,  and  Tornadoes,  24°,  90  pp., 
Boston,  1884. 

DE  PENNING,  GEORGE  A.,  Meteorology  and  the  Laws  of  Storms,  276  pp.,  Calcutta, 
1897. 

DOBERCK,  W.,  The  Law  of  Storms  in  the  Eastern  Seas,  4th  ed.,  8°,  44  pp.,  Hong- 
kong, 1904. 


APPENDIX  IX  527 

DOVE,  HEINRICH  WILHELM,  Law  of  Storms,  8°,  331  pp.,  London,  1862. 
*ELIOT,  JOHN,  Hand-book  of  Cyclonic  Storms  in  the  Bay  of  Bengal,  iv  +  212  pp., 

Calcutta,  1890.     (2d  ed.,  1900-1901,  2  vols.) 
ESPY,  JAMES,  The  Philosophy  of  Storms,  8°,  592  pp.,  Boston,  1841. 
FA  YE,  HERVE  A.  E.  A.,  Nouvelle  etude  sur  les  tempetes,  cyclones,  trombes  ou  tor- 
nados, 8°,  142  pp.,  Paris,  1897. 

FINLEY,  JOHN  P.,  Tornadoes,  12°,  196  pp.,  New  York,  1887! 
FISCHER,  ALFRED,  Die  Hurricanes  oder  Drehsturme  Westindiens,  4°,  70  pp., 

Gotha,  1908.     (Petermann's  Mitteilungen ;    Erganzungscheft  Nr.  159.) 
**GOCKEL,  ALBERT,  Das  Gewitter,  8°,  204  pp.,  Koln,  1905. 
HANZLIK,  STANISLAV,  Die  rdumliche  Verteilung  der  meteorologischen  elemente  in 

den  Antizyklonen,  4°,  94  pp.,  Wien,  1898. 

*HAZEN,  H.  A.,  The  Tornado,  12°,  ii  +  143  pp.,  New  York,  1890. 
HOUDAILLE,  F.,  Les  orages  a  grele  et  le  tir  des  canons,  8°,  244  pp.,  Paris,  1901. 
LOCKYER,  WILLIAM  J.  S.,  Southern  Hemisphere  Surface  Air  Circulation,  4°,  iii  + 

109  pp.,  1910.1 

PERLEY,  SIDNEY,  Historic  Storms  in  New  England,  8°,  x  +  341  pp.,  Salem,  1891. 
PIDDINGTON,  HENRY,  The  Sailor's  Horn-book  for  the  Law  of  Storms,  6th  ed.,  8°, 

xviii  +  408  pp.,  London,  1876. 

PLUMADON,  J.  R.,  Les  orages  et  la  grele,  8°,  192  pp.,  Paris,  1901. 
REID,  WILLIAM,  Attempt  to  develop  the  Law  of  Storms,  8°,  538  pp.,  London,  1850. 
*STREIT,  A.,  Das  Wesen  der  Cyklonen,  vi  +  125  pp.,  Wien,  1906. 
TOMLINSON,  CHARLES,  The  Thunder-storm,  3d  ed.,  16°,  xii  +  381  pp.,  London, 

1877. 

(7)   Weather  Bureaus  and  their  Work 

POLIS,  P.,  Der  Wetterdienst  und  die  Meteorologie  in  den  Vereinigten  staaten  von 
America  und  in  Canada,  4°,  43  pp.,  Berlin,  1908. 

(8)  Weather  Prediction  —  including  Weather  Proverbs  and  Prognostics 

ABBE,  CLEVELAND,  Preparatory  Studies  for  Deductive  Methods  in  Storm  and 

Weather  Predictions,  8°,  165  pp.,  Washington,  1890.     (Annual  Report  of 

Chief  Signal  Officer  for  1889 ;  Appendix  15.) 
ABERCROMBY,  RALPH,  Principles  of  Forecasting  by  Means  of  Weather  Charts, 

8°,  122  pp.,  London,  1885. 
BEBBER,  W.  J.  VAN,  Beurteilung  des  Welters  auf  mehrere  Tage  varaus,  8°,  32  pp., 

Stuttgart,  1896. 

**BEBBER,  W.  J.  VAN,  Die  Wettervorhersage,  2d  ed.,  xvi  +  219  pp.,  Stuttgart,  1898. 
*BEBBER,  W.  J.  VAN,  Handbuch  der  ausubenden  Witterungskunde,  8°,  2  parts, 

x  +  392  and  x  +  503  pp.,  Stuttgart  1885-1886. 
BENDEL,  T.,  Wetterpropheten,  8°,  166  pp.,  Regensburg,  Manz,  1904. 
DALLET,  G.,  La  Prevision  du  temps,  16°,  336  pp.,  Paris. 
*DUNWOODY,  H.    H.    C.,  Weather  Proverbs,   8°,  148  pp.,  Washington,  1883. 

(Signal  Service  Notes,  No.  IX.) 


528  APPENDIX  IX 

FITZROY,  ROBERT,  The  Weather  Book,  2d  ed.,  8°,  xiv  +  480  pp.,  London,  1863. 
*FREYBE,  OTTO,  Praktische  Wetterkunde,  8°,  vi  +  173  pp.,  Berlin,  1906. 
FRIESENHOF,  GREGOR,  Wetterlehre  oder  praktische  Meteorologie,  8°,  viii  +  680 

pp.,  Nedanocz,  1883. 

GRANGER,  FRANCIS  S.,  Weather  Forecasting,  8°,  xii  +  121  pp.,  Nottingham,  1909. 
*GUILBERT,   GABRIEL,  Nouvelle  methode  de  prevision   du   temps,  8°,   xxxiii  + 

343  pp.,  Paris,  1909. 

**!NWARDS,  RICHARD,  Weather  Lore,  8°,  xii  +  233  pp.,  London,  1898. 
KINDLER,  FINTAN,  Das  Wetter,  24°,  142  pp.,  Koln,  1909. 

KLEIN,  H.,  Wettervorhersage  fur  Jedermann,  12°,  vi  +  164  pp.,  Stuttgart,  1907. 
KUHLENBAUMER,  TH.,  Unser  Wetter  und  seine  Vorherbestimmung,  12°,  x  +  164 

pp.,  Minister,  1909. 

PERNTER,     J.    M.,   Wetterprognose   in  Osterreich,    16°,   61   pp.,    Wien,    1907. 
SCOTT,  A.  C.,  Notes  on  Meteorology  and  Weather  Forecasting,  8°,  40  pp.,  London, 

1909. 
*SCOTT,  ROBERT  H.,  Weather  Charts  and  Storm  Warnings,  3d  ed.,  8°,  vi  +  229 

pp.,  London,  1887. 

*SHAW,  W.  N.,  Forecasting  Weather,  8°,  xxvii  +  380  pp.,  London,  1911.,' 
STEINMETZ,  ANDREW,  Weather  Casts  and  Storm  Prognostics,  8°,  208  pp.,  London, 

1866. 
*SWAINSON,   REV.  C.,  A    Handbook  of   Weather   Folk-lore,  12°,  x  +  275  pp., 

London,  1873. 
TIMM.,  H.,  Wie  gestaltet  sich  das  Wetter?    8°,  viii  +  175  pp.,  Leipzig,  1892. 

(9)   Climate 

ALGUE,  JOSE  S.  J.,  The  Climate  of  the  Philippines,  8°,  103  pp.,  Manila,  1904. 

ARMAND,  Traite  de  climatologie  generate  du  globe,  8°,  xx  +  868  pp.,  Paris,  1873. 

BEHRE,  OTTO,  Das  Klima  von  Berlin,  8°,  158  pp.,  Berlin,  1908. 

BLANFORD,  S.  M.,  Climates  and  Weather  of  India,  8°,  382  pp.,  London,  1889. 

*BLODGET,  LORIN,  Climatology  of  the  United  States,  4°,  xvi  +  536  pp.,  Phila- 
delphia, 1857. 

BONACINA,  L.  C.  W.,  Climatic  Control,  viii  +  167  pp.,  London,  1911. 

BRUCKNER,  EDWARD,  Klimaschwankungen  seit  1700, 8°,  viii  +^324  pp.,  Wien,  1890. 

CROLL,  JAMES,  Climate  and  Time,  8°,  xvi  +  577  pp.,  New  York,  1887. 

CROLL,  JAMES,  Discussions  on  Climate  and  Cosmology,  8°,  xii  +  327  pp.,  New 
York,  1886. 

CULLIMORE,  D.  H.,  The  Book  of  Climate,  12°,  x  +  279  pp.,  London,  1891. 

DAVIS,  WALTER  G.,  Climate  of  the  Argentine  Republic,  8°,  154  pp.,  Buenos  Aires, 
1910.  (Argentina  Dept.  of  Agriculture.) 

ECKARDT,  WILHELM  R.,  Das  Klimaproblem  der  geologischen  Vergangenheit  und 
historischen  Gegenwart,  8°,  xi  +  183  pp.,  Braunschweig,  1909. 

*FASSIG,  OLIVER  L.,  The  Climate  and  Weather  of  Baltimore,  8°,  515  pp.,  Balti- 
more, 1907. 


APPENDIX  IX  529 

FRITSCHE,  H.,  The  Climate  of  Eastern  Asia.     (Journal  of  the  North-China  Branch 

of  the  Royal  Asiatic  Society,  Vol.  XII,  1877,  pp.  127-335.) 
**HANN,  JULIUS,  Hand-book  of  Climatology,  (translated  by  Robert  De  Courcy 

Ward),  8°,  xiv  +  437  pp.,  New  York,  1903. 
**HANN,  JULIUS,  Handbuch  der  Klimatologie,  2d  ed.,  3  vols.,  404,  384,  and 

576  pp.,  Stuttgart,  1897;   3d  ed.,  vol.  1,  xiv  +  394  pp.,  Stuttgart,  1908; 

3d  ed.,  vol.  2,  xii  +  426  pp.,  Stuttgart,  1910. 
HERBERTSON,  A.  J.  and  F.  D.,  Man  and  his  Work,  London,  1899. 
HERZ,  NORBERT,  Die  Eiszeiten  und  ihre  Uhrsachen,  4°,  iv  +  306  pp.,  Leipzig,  1909. 
KNOX,  ALEXANDER,  The  Climate  of  the  Continent  of  Africa,  8°,  xii  +  552  pp., 

Cambridge,  1911. 

KOPPEN,  W.,  Klimakunde,  2d  ed.,  16°,  132  pp.,  Leipzig,  1906. 
MARCHI,  L.  DE,  Climatologia,  8°,  x  +  204  pp.,  Milano,  1890. 
*MEYER,  HUGO,  Anleitung  zur  Bearbeitung  meteorologischer  Beobachtungen  fur 

die  Klimatologie,  8°,  viii  +  187  pp.,  Berlin,  1891. 

MUHY,  A.,  Klimatographische   Ubersicht  der  Erde,  8°,  xvi  +  744  pp.  (supple- 
ment xii  +  320  pp.),  Leipzig,  1862. 
QUETELET,  A.,  Sur  la  climat  de  la  Belgique,  4°,  2  vols.,  about  500  pp.,  Bruxelles, 

1849. 

RATZEL,  Anthropogeographie,  2d  ed.,  Stuttgart,  1899. 
ROSTER,  GIORGIO,  Climatologia  dell  Italia  nelle  sue  attinenze  con  I'  ingiene  e  con 

agricoltura,  8°,  xxix  +  1040  pp.,  Torino,  1909. 
SUPAN,  A.,  Grundzuge  der   physischen   Erdkunde,  4th  ed.,  8°,   ix  +  934  pp., 

Leipzig,  1908. 
Die  Veranderungen  des  Klimas  seit  dem  Maximum  des  letzten  Eisgeit,  4°,  Iviii  + 

459  pp.,  Stockholm,  1910.   (Pub.  by  llth  International  Geological  Congress.) 
**WARD,  ROBERT  DE  COURCY,  Climate,  8°,  xiv  +  372  pp.,  London,  1909. 
WOEIKOF,  A.,  Die  Klimate  der  Erde,  2  parts,  396,  445  pp.,  Jena,  1887. 

(10)   Atmospheric  Electricity 

ANDERSON,  RICHARD,  Lightning  Conductors,  8°,  xv  -f  470  pp.,  London,  1885. 

ANGOT,  ALFRED,  The  Aurora  Borealis,  8°,  xii  +  264  pp.,  New  York,  1897. 

CAPRON,  J.  RAND,  Auroras:  their  Characters  and  Spectra,  4°,  xv  +  207  pp., 
London,  1879. 

CHAUVEAU,  A.  B.,  L' Electricite  Atmospherique,  f°,  70  pp.,  Paris,  1902. 

FLAMMARION,  CAMILLE,  Thunder  and  Lightning,  12°,  281  pp.,  Boston,  1906. 
(Translated  by  Walter  Mostyn.) 

*GOCKEL,  ALBERT,  Die  Luftelektrizitat,  vi  +  206  pp.,  Leipzig,  1908. 

GREELY,  A.  W.,  Chronological  List  of  Auroras  1870  to  1879,  4°,  76  pp..  Washing- 
ton, 1881.  (Prof.  Papers  of  the  Signal  Service,  No.  3.) 

HARRIS,  WILLIAM  SNOW,  On  the  Nature  of  Thunderstorms,  xvi  +  226  pp., 
London,  1843. 

*HEDGES,  KILLINGWORTH,  Modern  Lightning  Conductors,  4°,  vi  +  119  pp., 
London,  1905.  (New  ed,  1910.). 

2M 


530  APPENDIX  IX 

LEMSTROM,  L'aurore  boreale,  xii  +  179  pp.,  Paris,  1886. 

LODGE,  SIR  OLIVER  J.,  Lightning  Conductors  and  Lightning  Guards,  12°,  xii  +  544 

pp.,  London,  1892. 
**MACHE,  H.,  and    SCHWEIDLER,   E.  v.,  Die  Atmospherische  Elektrizitdt,   8°, 

xi  +  247  pp.,  Braunschweig,  1909. 
SCHROETER,  J.  FR.,  Catalog  der  in  Norwegen  biz  Juni  1878  beobachteten  Nord- 

lichter,  f°,  422  pp.,  Kristiania,  1902. 
SPANG,  HENRY  W.,  A  Practical  Treatise  on  Lightning  Protection,  12°,  63  pp., 

New  York,  1883. 

(n)   Atmospheric  Optics 

BESSON,  Louis,  Sur  la  theorie  des  halos,  8°,  89  pp.,  Paris,  1909. 
MASCART,  E.,  Traite  d'optique  (especially  vol.  3),  8°,  Paris,  1889-1894. 
**PERNTER,  J.  M.,  UND  EXNER,  FELIX  M..,Meteorologische  Optik,  8°,  xvii  +  799 
pp.,  Leipzig,  1910. 

(C)   The   U.   S.   Government  Publications  on  Meteorology 

The  publications  issued  by  the  U.  S.  Weather  Bureau  may  be  divided  into  two 
groups,  the  periodical  publications,  and  those  which  appear  at  irregular  intervals. 

The  periodical  publications  are :  — 

Daily  Weather  Map,  from  Jan.,  1871,  to  date.  These  are  now  published  once 
daily,  based  on  the  8  A.M.  observations  at  Washington  and  many  regular 
stations.  Many  newspapers  also  publish  daily  weather  maps  based  on 
both  the  8  A.M.  and  the  8  P.M.  observations.  The  maps  were  formerly 
issued  twice  and  three  times  daily.  Maps  for  Sunday  and  the  holidays  are 
issued  at  Washington  only. 

Daily  Forecast  Cards.  These  cards  contain  the  forecast  only,  and  are  issued  at 
Washington,  by  many  regular  stations,  and  from  some  post  offices  on  receipt 
of  telegraphic  information  from  some  Weather  Bureau  station. 

Monthly  Weather  Review,  4°,  1872  to  date.  Prior  to  July,  1891,  it  was  published 
by  the  U.  S.  Signal  Service.  Originally  it  was  only  a  bulletin  of  current 
meteorological  conditions  in  the  United  States  and  Canada,  but  later  it 
included  all  the  features  of  a  general  meteorological  journal.  January, 
v  1908,  the  brief  summaries  of  the  observations  at  the  cooperative  stations  were 
discontinued.  Since  July,  1909,  its  form  has  again  been  changed.  Few 
research  articles  are  now  published  and  full  monthly  climatological  reports 
from  all  stations  have  been  included. 

Bulletin  of  the  Mount  Weather  Observatory,  8°,  1908  to  date.  This  contains  the 
results  of  the  observations  at  the  Mount  Weather  Observatory,  and  also 
research  articles.  Research  articles  are  now  being  published  in  this 
Bulletin  rather  than  in  the  Monthly  Weather  Review. 

National  Weather  Bulletin.  This  is  a  large  single  sheet  (19  by  24  inches),  printed 
on  one  side  only  and  issued  weekly  during  the  summer  and  monthly  during 


APPENDIX  IX  531 

the  winter.  In  addition  to  the  text,  it  contains  these  four  charts :  the 
average  temperature,  the  amount  of  precipitation,  and  the  departure  from 
normal  in  the  case  of  both  temperature  and  precipitation. 

Snow  and  Ice  Bulletin,  1893  to  date.  This  is  a  single  sheet  (12  by  19  inches), 
printed  on  one  side  only,  and  issued  weekly  during  the  winter.  It  contains, 
in  addition  to  the  tables  and  text,  a  chart  showing  the  depth  of  snow  on  the 
ground. 

Meteorological  Chart  of  the  Great  Lakes,  October,  1897,  to  date. 

Annual  Report  of  the  Chief,  1870  to  date.     This  is  a  large  volume  and  contains, 

in  addition  to  the  administrative  report,  a  summary  of  the  observations  of 

the  year.     Special  reports  and  research  articles  have  been  added  frequently 

as  appendices. 

All  regular  stations  of  the  Weather  Bureau  publish  a  monthly  and  yearly 

summary  of  their  observations. 

The  publications  which  appear  from  time  to  time  are :  — 

Numbered  Bulletins.  These  are  special  articles  and  nearly  fifty  have  now  ap- 
peared. The  list  is  given  below. 

Lettered  Bulletins.  These  are  usually  large  volumes  and  contain  very  valuable 
material.  Bulletin  V  has  recently  appeared.  The  list  is  given  below. 
W .  B.  Publications.  Nearly  all  of  the  publications  of  the  U.  S.  Weather  Bureau 
now  receive  a  serial  number.  The  Monthly  Weather  Review,  the  Bulletin 
of  the  Mount  Weather  Observatory,  the  numbered  bulletins,  and  the  lettered 
bulletins  are  included  in  these.  There  are  many  other  publications,  how- 
ever, which  are  not  in  any  one  of  the  four  series  which  have  a  W.  B.  number. 
These  numbers  have  now  reached  nearly  500. 

Miscellaneous  Publications.  There  are  many  publications  of  the  U.  S.  Weather 
Bureau  which  unfortunately  are  not  numbered  or  designated  in  any  way. 
Summary  of  the  Climatological  Data  for  the  United  States  by  Sections.  One  hun- 
dred and  six  summaries  are  to  be  published,  and,  when  complete,  this  will 
be  a  veritable  mine  of  information  about  the  climate  of  the  various  parts  of 
the  United  States:  It  will  be  complete  in  1911. 

Many  of  the  publications  have  changed  their  form,  name,  and  characteristics, 
but  a  full  account  of  these  changes  is  impossible  here.  There  are  also  publica- 
tions which  have  been  discontinued.  Climate  and  Health  was  published  from 
July,  1895,  to  March,  1896.  Forty-four  of  the  forty-five  sections  published  a 
monthly  climatological  report  until  July,  1909,  and  a  weekly  weather  bulletin 
during  the  summer  until  1909.  Only  three  sections,  Iowa,  Hawaiian  Islands, 
and  Porto  Rico,  continue  these  publications  at  present.  They  all,  however, 
continue  to  publish  an  annual  summary. 

Previous  to  1891,  while  the  weather  service  was  part  of  the  U.  S.  Signal  Ser- 
vice, the  meteorological  articles  were  published  as :  — 

Signal  Service  Notes;  Professional  Papers  of  the  Signal  Service;  Reports  of  the 
Chief  Signal  Officer;  Publication  without  any  special  designation. 


532  APPENDIX  IX 

A  complete  list  of  the  meteorological  publications  of  the  U.  S.  Signal  Service 
will  be  found  in  the  Report  of  the  Chief  Signal  Officer  for  1891.  This  bibli- 
ography covers  pages  389  to  409,  and  lists  119  publications  by  the  Signal  Service. 
This  includes  the  18  Professional  Papers  and  the  23  Signal  Service  Notes.  This 
bibliography  also  contains  all  books,  pamphlets,  or  articles  published  anywhere 
by  any  person  while  he  was  connected  with  the  Signal  Service. 

The  four  following  lists  contain  the  Professional  Papers  of  the  Signal  Service, 
the  Signal  Service  Notes,  the  lettered  bulletins  of  the  U.  S.  Weather  Bureau,  and 
the  numbered  bulletins  of  the  U.  S.  Weather  Bureau : 


U.  S.  Signal  Service  Professional  Papers 

No.  1  ABBE,  CLEVELAND,  Report  on  the  Solar  Eclipse  of  July,  1878.    4°,  186  pp., 

34  pis.,  Wash.,  1881. 
No.  2  GREELY,  A.  W.,  Isothermal  Lines  of  the  United  States,  1871-1880,  4°,  1  p., 

12  pis.,  Wash.,  1881. 
No.  3  GREELY,  A.  W.,  Chronological  List  of  Auroras  Observed  from  1870  to 

1879,  4°,  76  pp.,  Wash.,  1881. 
No.  4  FINLEY,  J.  P.,  Report  of  the  Tornadoes  of  May  29  and  30,  1879,  in  Kansas, 

Nebraska,  Missouri,  and  Iowa.    4°,  116  pp.,  29  chs.,  Wash.,  1881. 
No.  5  Information  Relative  to  the  Construction  and  Maintenance  of  Timeballs, 

4°,  31  pp.,  5  pis.,  Wash.,  1881. 

No.  6  HAZEN,  H.  A.,  The  Reduction  of  Air-pressure  to  Sea  Level  at  Elevated  Sta- 
tions West  of  the  Mississippi  River,  4°,  42  pp.,  20  maps,  Wash.,  1882. 
No.  7  FINLEY,  J.  P.,  Report  on  the  Character  of  Six  Hundred  Tornadoes,  4°, 

29  pp.,  3  chs.,  Wash.,  1884. 
No.  8  FERREL,  WILLIAM,  Recent  Mathematical  Papers  Concerning  the  Motions 

of  the  Atmosphere,  Part  I,  "  The  Motions  of  Fluids  and  Solids  on  the  Earth's 

Surface/'  reprinted  with  notes  by  Frank  Waldo,  4°,  51  pp.,  Wash.,  1882. 
No.  9  DUNWOODY,  H.  H.  C.,  Charts  and  Tables  showing  Geographical  Distribu- 
tion of  Rainfall  in  the  United  States,  4°,  29  pp.,  3  chs.,  Wash.,  1883. 
No.  10  Tables  of  Rainfall  and  Temperature  Compared  with  Crop  Production, 

4°,  15  pp.,  Wash.,  1882. 
No.  11  SHERMAN,  0.  T.,  Meteorological  and  Physical  Observations  on  the  East 

Coast  of  British  America,  4°,  202  pp.,  1  ch.,  Wash.,  1883. 
No.  12  FERREL,  WILLIAM,  Popular  Essays  on  the  Movements  of  the  Atmosphere, 

4°,  59  pp.,  Wash.,  1882. 
No.  13  FERREL,  WILLIAM,  Temperature  of  the  Atmosphere  and  Earth's  Surface, 

4°,  69  pp.,  Wash.,  1884. 
No.  14  FINLEY,  J.  P.,  Charts  of  Relative  Storm  Frequency  for  a  Portion  of  the 

Northern  Hemisphere,  4°,  9  pp.,  13  chs.,  Wash.,  1884. 
No.  15  LANGLEY,  S.  P.,  Researches  on  Solar  Heat  and  its  Absorption  by  the 

Earth's  Atmosphere.     (A  Report  of  the  Mount  Whitney  Expedition.)     4°, 

139  pp.,  22  pis.,  Wash.,  1884. 


APPENDIX  IX  533 

No.  16  FINLEY,  J.  P.,  Tornado  Studies  for  1884,  4°,  15  pp.,  72  chs.,  72  tables, 

Wash.,  1885. 
No.  17  FERREL,    WILLIAM,    Recent   Advances   in   Meteorology.    Published    as 

Part  2,  Appendix  No.  71,  of  the  annual  report  of  the  Chief  Signal  Officer 

for  1885,  8vo,  440  pp.,  Wash.,  1886. 
No.  18  HAZEN,  H.  A.,  Thermometer  Exposure,  4°,  32  pp.,  Wash.,  1885. 


U.  S.  Signal  Service  Notes 

JSTo.  1  BAILEY,  W.  O.,  Report  on  the  Michigan  Forest  Fires  of  1881,  8vo,  16  pp., 

6  chs.,  Wash.,  1882. 
No.  2  BIRKHIMER,  W.  E.,  Memoir  on  the  Use  of  Homing  Pigeons  for  Military 

Purposes,  8vo,  27  pp.,  Wash.,  1882. 

No.  3.  ALLEN,  JAMES,  To  Foretell  Frost,  8vo.  11  pp.,  Wash.,  1882. 
No.  4  UPTON,  WINSLOW,  The  Use  of  the  Spectroscope  in  Meteorological  Observa- 
tions, 8vo,  7  pp.,  3  chs.,  Wash.,  1883. 
No.  5  Work  of  the  Signal  Service  in  the  Arctic  Regions,  8vo,  40  pp.,  1  ch.,  Wash., 

1883. 
No.  6  HAZEN,  H.  A.,  Report  on  the  Wind  Velocities  at  the  Lake  Crib  and  at  Chicago, 

8vo,  20  pp.,  1  ch.,  Wash.,  1883. 
No.  7  HAZEN,  H.  A.,  Variation  of  the  Rainfall  West  of  the  Mississippi  River, 

8vo,  8  pp.,  Wash.,  1883. 
No.  8  WALDO,  FRANK,  The  Study  of  Meteorology  in  the  Higher  Schools  of  Germany, 

Switzerland,  and  Austria,  8vo,  148  pp.,  9  pp.,  Wash.,  1883. 
No.  9  DUNWOODY,  H.  H.  C.,  Weather  Proverbs,  8vo,  148  pp.,  1  map,  Wash., 

1883. 
No.  10  GARLINGTON,  E.  A.,  Report  on  Lady  Franklin  Bay  Expedition  of  1883, 

8vo,  52  pp.,  1  map,  Wash.,  1883. 

No.  11  WARD,  F.  K,  The  Elements  of  the  Heliograph,  8vo,  12  pp.,  Wash.,  1883. 
No.  12  FINLEY,  J.  P.,  The  Special  Characteristics  of  Tornadoes,  with  Practical 

Direction  for  the  Protection  of  Life  and  Property,  8vo,  19  pp.,  Wash.,  1884. 
No.  13  CURTIS,  G.  E.,  The  Relation  between  Northers  and  Magnetic  Disturbances 

at  Havana,  Cuba,  8vo,  16  pp.,  Wash.,  1885. 
No.  14  LAMAR,  W.  H.,  Jr.,  AND  ELLIS,  F.  W.,  Physical  Observations  During  the 

Lady  Franklin  Bay  Expedition  of  1883,  8vo,  62  pp.,  14  pis.,  1  map,  Wash., 

1884. 
No.  15  HAZEN,  H.  A.,  Danger  Lines  and  River  Floods  of  1882,  8vo,  30  pp., 

Wash.,  1884. 
No.  16  CURTIS,  G.  E.,  The  Effect  of  Wind  Currents  on  Rainfall,  8vo,  11  pp., 

2  pis.,  Wash.,  1884. 
No.  17  MORRILL,   PARK,   A   First  Report  upon  Observations  of  Atmospheric 

Electricity  at  Baltimore,  Md.,  8vo,  8  pp.,  6  chs.,  Wash.,  1884. 
No.  18  McAoiE,  ALEXANDER,  The  Aurora  in  its  Relations  to  Meteorology,  8vo, 

21  pp.,  14  chs.,  Wash.,  1885. 


534  APPENDIX  IX 

No.  19  GLENN,  S.  W.,  Report  on  the  Tornado  of  August  28,  1884,  near  Huron, 

Dak.,  8vo,  10  pp.,  11  chs.,  Wash.,  1885. 
No.  20  HAZEN,  H.  A.,  Thunderstorms  of  May,  1884,  8vo,  8  pp.,  2  chs.,  Wash., 

1885. 

No.  21  How  to  Use  Weather  Maps.     Not  published  as  Signal  Service  Notes. 
No.  22  RUSSELL,  THOMAS,  Corrections  of  Thermometers,  8vo,  11  pp.,  Wash.,  1885. 
No.  23  WOODRUFF,  T.  M.     Cold  Waves  and  their  Progress,  A  preliminary  study, 

8vo,  21  pp.,  Wash.,  1885. 

The  Lettered  Bulletins  of  the  U.  S.  Weather  Bureau 

A  DUNWOODY,  H.  C.     Summary  of  International  Meteorological  Observations 

(1878-1887),  x  pp.  and  59  charts,  1893. 
B  HARRINGTON,  MARK  WALROD,  Surface  Currents  of  the  Great  Lakes,  xiv  pp., 

1895. 
C  HARRINGTON,  MARK  W.,  Rainfall  and  Snow  of  the  United  States  compiled  to 

the  end  of  1891,  80  pp.,  1894. 

D  HENRY,  ALFRED  J.,  Rainfall  of  the  United  States,  58  pp.,  1897. 
E  MORRILL,  PARK,  Floods  of  the  Mississippi  River,  79  pp.,  1897. 
F  FRANKENFIELD,  H.  C.,  Report  on  the  Kite  Observations  of  1898,  71  pp.,  1899. 
G  VERY,  FRANK  W.,  Atmospheric  Radiation,  132  pp.,  1900. 
H  GARRIOTT,  E.  B.,  West  Indian  Hurricanes,  69  pp.,  1900. 
I    BIGELOW,  FRANK  H.,  Eclipse  Meteorology  and  Allied  Problems,  166  pp.,  1902. 
J    HENRY,  ALFRED  J.,  Wind  Velocity  and  Fluctuations  of  Water  Level  on  Lake 

Erie,  22  pp.,  1902. 

K  GARRIOTT,  E.  B.,  Storms  of  the  Great  Lakes,  9  pp.  and  968  charts,  1903. 
L   McAoiE,  ALEXANDER  G.,  Climatology  of  California,  270  pp.,  1903. 
M  FRANKENFIELD,  H.  C.,  The  Floods  of  the  Spring  of  1903  in  the  Mississippi 

Watershed,  63  pp.,  1904. 
N  STOCKMAN,  WILLIAM  B.,  Periodic  Variation  of  Rainfall  in  the  Arid  Region, 

15  pp.,  1905. 

O  STOCKMAN,  WILLIAM  B.,  Temperature  and  Relative  Humidity  Data,  29  pp.,  1905. 
P  GARRIOTT,  EDWARD  B.,  Cold  Waves  and  Frosts  in  the  United  States,  22  pp. 

and  328  charts,  1906. 

Q  HENRY,  ALFRED  JUDSON,  Climatology  of  the  United  States,  1012  pp.,  1906. 
R  BIGELOW,  FRANK  H.,  The  Daily  Normal  Temperature  and  the  Daily  Precipi- 
tation in  the  United  States,  186  pp.,  1908. 
S  BIGELOW,  FRANK  H.,  Report  on  the  Temperatures  and  Vapor  Tensions  in  the 

United  States,  302  pp.,  1909. 
T  Cox,  HENRY  C.,  Frost  and  Temperature  Conditions  in  the  Cranberry  Marshes 

of  Wisconsin,  121  pp.,  1910. 
U  BIGELOW,  FRANK  H.,  Temperature  Departures,  Monthly  and  Annual,  in  the 

United  States,  January,  1873,  to  June,  1909,  inclusive,  5  pp.,  474  charts,  1911. 
V  DAY,  P.  C.,  Frost  Data  of  the  United  States  and  Length  of  the  Crop  Growing 

Season,  5  pp.,  5  charts,  1911. 


APPENDIX  IX  535 

Numbered  Bulletins  of  the  U.  S.  Weather  Bureau 

No.  1  HARRINGTON,  MARK  W.,  Notes  on  the  Climate  and  Meteorology  of  Death 

Valley,  California,  50  pp.,  1892. 

No.  2  BIGELOW,  FRANK  H.,  Notes  on  a  New  Method  for  the  Discussion  of  Mag- 
netic Observations,  40  pp.,  1892. 
No.  3  HILGARD,  E.  W.,  A  Report  on  the  Relations  of  Soil  to  Climate,  59  pp., 

1892. 
No.  4  WHITNEY,  MILTON,  Some  Physical  Properties  of  Soils  in  their  Relation  to 

Moisture  and  Crop  Distribution,  90  pp.,  1892. 
No.  5  KING,  FRANKLIN  H.,  Fluctuations  in  the  Level  and  Rate  of  Movement  of 

Ground-water,  75  pp.,  1892. 
No.  6  COLE,  FRANK  N.,  The  Daily  Variation  of  Barometric  Pressure,  32  pp., 

1892. 
No.  7  Report  of  the  First  Annual  Meeting  of  the  American  Association  of  State 

Weather  Services,  49  pp.,  1893. 

No.  8  MELL,  P.  H.,  Report  on  the  Climatology  of  the  Cotton  Plant,  68  pp.,  1893. 
No.  9  CONGER,  N.  B.,  Report  on  the  Forecasting  of  Thunderstorms  during  the 

Summer  of  1892,  54  pp.,  1893. 

No.  10  HAZEN,  HENRY  A.,  The  Climate  of  Chicago,  137  pp.,  1893. 
No.  11  FASSIG,  OLIVER  L.,  Report  of  the  International  Meteorological  Congress 

Held  at  Chicago,  III,  Aug.  21-24,  1893,  1896. 
No.  12  BARUS,  CARL,  Report  on  the   Condensation  of  Atmospheric  Moisture, 

104  pp.,  1895. 
No.  13  WILLIAMS,  H.  E.,  Temperatures  injurious  to  Food  Products  in  Storage 

and  during  Transportation,  20  pp.,  1894. 
No.  14  Report  of  the  Third  Annual  Meeting  of  the  American  Association  of  State 

Weather  Services,  31  pp.,  1894. 

No.  15  McADiE,  ALEXANDER,  Protection  from  Lightning,  26  pp.,  1895. 
No.  16  JEWELL,  L.  E.,  The  Determination  of  the  Relative  Quantities  of  Aqueous 

Vapor  in  the  Atmosphere  by  Means  of  Absorption  Lines  of  the  Spectrum, 

12  pp.,  1896. 
No.  17  MOORE,  WILLIS  L.,  The  Work  of  the  Weather  Bureau  in  Connection  with 

the  Rivers  of  the  United  States,  106  pp.,  1896. 
No.  18  Report  of  the  Fourth  Annual  Meeting  of  the  American  Association  of  State 

Weather  Services,  55  pp.,  1896. 
No.  19  HENRY,  ALFRED  J.,  Report  on  the  Relative  Humidity  of  Southern  New 

England  and  Other  Localities,  23  pp.,  1896. 
No.  20  BIGELOW,  FRANK  H.,  Storms,  Storm  Tracks,  and  Weather  Forecasting, 

87  pp.,  1897. 

No.  21  BIGELOW,  FRANK  H.,  Abstract  of  a  Report  on  Solar  and  Terrestrial  Magnet- 
ism in  their  Relations  to  Meteorology,  176  pp.,  1898. 
No.  22  PHILLIPS,  W.  F.  R.,  Climate  of  Cuba,  23  pp.,  1898. 
No.  23  HAMMON,  W.  H.,  Frost:  When  to  Expect  it,  and  How  to  Lessen  the  Injury 

therefrom,  37  pp.,  1899. 


536  APPENDIX  IX 

No.  24  Proceedings  of  the  Convention  of  Weather  Bureau  Officials  held  at  Omaha, 
Neb.,  October  13-14,  1898,  184  pp.,  1899. 

No.  25  MOORE,  WILLIS  L.,  Weather  Forecasting :  Some  Facts  Historical,  Practi- 
cal and  Theoretical,  16  pp.,  1899. 

No.  26  McAoiE,  ALEXANDER  G.,  AND  HENRY,  ALFRED  J.,  Lightning  and  the 
Electricity  of  the  Air,  74  pp.,  1899. 

No.  27  BIGELOW,  FRANK  H.,  The  Probable  State  of  the  Sky  along  the  Path  of 
Total  Eclipse  of  the  Sun,  May,  28,  1900,  Observations  of  1899,  23  pp., 
1899. 

No.  28  McAoiE,  ALEXANDER  G.,  AND  WILLSON,  GEORGE  H.,  The  Climate  of  San 
Francisco,  California,  30  pp.,  1899. 

No.  29  McAoiE,  ALEXANDER  G.,  Frost  Fighting,  15  pp.,  1900. 

No.  30  HENRY,  ALFRED  J.,  Loss  of  Life  in  the  United  States  by  Lightning,  21  pp., 
1901. 

No.  31  BERRY,  JAMES,  AND  PHILLIPS,  W.  F.  R.,  Proceedings  of  the  Second  Con- 
vention of  Weather  Bureau  Officials,  246  pp.,  1902. 

No.  32  ALEXANDER,  WILLIAM  H.,  Hurricanes,  79  pp.,  1902. 

No.  33  GARRIOTT,  EDWARD  B.,  Weather  Folk-lore  and  Local  Weather  Signs, 
153  pp.,  1903. 

No.  34  MOORE,  WILLIS  L.,  Climate:  its  Physical  Basis  and  Controlling  Factors, 
19  pp.,  1904. 

No.  35  GARRIOTT,  E.  B.,  Long-range  Weather  Forecasts,  68  pp.,  1904. 

No.  36  ABBE,  CLEVELAND,  A  First  Report  on  the  Relations  between  Climates 
and  Crops,  386  pp.,  1905. 

No.  37  HENRY,  ALFRED  J.,  Recent  Practice  in  the  Erection  of  Lightning  Conduc- 
tors, 20  pp.,  1906. 

No.  38  EMERY,  SAMUEL  C.,  Mississippi  River  Levees  and  their  Effect  on  River 
Stages  during  Flood  Periods,  21  pp.,  1910. 


(D)   Bibliography  of  Bibliography1 

Group  I.    General 

ABBE,  CLEVELAND,  A  First  Report  on  the  Relation  Between  Climates  and  Crops, 
Bulletin  36,  U.  S.  Weather  Bureau.  (Pages  365-375  contain  a  bibli- 
ography.) 

BARTHOLOMEW,  J.  G.,  AND  HERBERTSON,  A.  J.,  Physical  Atlas;  Meteorology. 
(It  contains  a  general  bibliography.) 

BORNSTEIN,  R.,  Leitfaden  der  Wetterkunde.  (Pages  209-222  contain  references 
to  the  literature.) 

Brussels  (Observatoire  Royal  de  Belgique),  Catalogue  des  ouvrages  d'astro- 
nomie  et  de  meteorologie  qui  se  trouvent  dans  les  principales  bibliotheques  de 
la  Belgique,  8°,  xxiii  +  645  pp.,  Bruxelles,  1878. 

1See  Appendix  IX,  B,  for  further  details  about  the  books  mentioned. 


APPENDIX  IX  537 

FASSIG,  OLIVER,  L.,  Bibliography  of  Meteorology:  A  Classified  Catalogue  of  the 
Printed  Literature  of  Meteorology  to  1887,  Washington,  1889-1891.  (Mimeo- 
graphed; 4  parts  only  have  been  published.  These  parts  cover  tempera- 
ture, moisture,  wind,  and  storms.) 

HARDING,  J.  S.,  JR.,  Catalogue  of  the  Library  of  the  Royal  Meteorological  Society 
to  1890,  8°,  viii  +  214  pp.,  London,  1891. 

HELLMANN,  G.,  Repertorium  der  deutschen  Meteorologie  bis  1881,  8°,  xxii  + 
995  pp.,  Leipzig,  1883. 

HELLMANN,  G.,  "  Contribution  to  the  Bibliography  of  Meteorology  and  Ter- 
restrial Magnetism  in  the  15th,  16th,  and  17th  Centuries/'  Rep.  of  Chicago 
Meteor.  Congress,  1893,  Part  II,  Washington,  1894. 

HERSCHEL,  SIR  J.,  Meteorology,  Edinburgh,  1862.     (Critical  bibl.  to  date.) 

Index  to  the  Publ.  of  the  English  Meteorological  Societies,  1839-1881,  32  pp., 
London,  1881. 

Katalog  der  Bibliothek  der  Deutschen  Seewarte  zu  Hamburg,  8°,  x  +  619  pp., 
also  Nachtrag,  1-8,  Hamburg,  1890. 

Library  of  Congress.     (The  Card  Catalogue  of  this  Library  can  be  obtained.) 

LOOMIS,  ELIAS,  A  Treatise  on  Meteorology.  (Pages  296-300  contain  a  list  of  the 
works  on  meteorology  before  1868.) 

Meteorological  Annuaire  of  the  Royal  Observatory  of  Belgium  for  1905.  (A 
bibliography  by  J.  Vincent  of  treatises  on  meteorology,  containing  about 
200  books.) 

Meteorologische  Zeitschrift  Namen-  und  Sachregister,  Vols.  1-25,  1884-1908, 
4°,  231  pp.,  Braunschweig,  1910. 

MOORE,  JOHN  WILLIAM,  Meteorology.  (Pages  410-419  contain  publications  of 
the  U.S.  Weather  Bureau  and  Signal  Service.  Omitted  in  the  new  edition.) 

MOORE,  WILLIS  L.,  Descriptive  Meteorology.  (A  bibliography  at  the  end  of 
each  chapter.) 

Quarterly  Journal  of  the  Royal  Meteorological  Society,  Index  to  vols.  VIII- 
XXVI,  1882-1900,  8°,  37  pp.,  London,  1901. 

SYMONS,  G.  J.,  "English  Meteorological  Literature,  1337-1699,"  Report  of  the 
Chicago  International  Meteorological  Congress,  1893,  Part  II,  Washington, 
1894. 

SYMONS'S  Monthly  Meteorological  Magazine,  Index  to  vols.  I-XXX,  1886-1895, 
8°,  iv  4-  84  pp.,  London,  1897. 

SZALAY,  LADISLAUS  v.,  Namen-  und  Sachregister  der  Bibliothek  der  Kon.  Ungar- 
ischen  Reichsanstalt  fur  Meteorologie  und  Erdmagnetismus,  8°,  viii  +  423  pp., 
Budapest,  1902. 

TALMAN,  G.  FITZHUGH,  Brief  List  of  Meteorological  Text-books  and  Reference 

Books,  8°,  18  pp.,  U.  S.  Weather  Bureau. 

TRABERT,  WILHELM,  Meteorologie.     (Pages  7  and  8  are  on  the  literature.) 
WALDO,  FRANK,  Modern  Meteorology,  London,  1893.     (Pages  6-19  have  to  do 

with  meteorological  publications.) 

WEBER,  SIR  HERMANN,  AND  WEBER,  F.  PARKER,  Climatherapy  and  Balneo- 
therapy.  (Pages  745-774  extensive  bibliography.) 


538  APPENDIX  IX 

Group  II.    Temperature 

BOLTON,  HENRY  C.,  Evolution  of  the  Thermometer.  (Pages  92-96  contain  a 
bibliography  on  the  history  of  the  thermometer.) 

Group  III.    The  Moisture  of  the  Atmosphere  —  Dew,  Frost,  Fog,  Clouds,  Precipitation 

HELLMANN,  G.,  Die  Niederschldge  in  den  norddeutschen  Stromgebieten.  (Pages 
31-36  bibliography.) 

HELLMANN,  G.,  Schnee  Krystalle,  1893.     (A  bibliography  of  the  subject.) 

JELINEK,  CARL,  Jelinek's  Psychromter  =  Tofeln.  (Pages  xi— xiii  bibliograph 
of  psychrometer  and  hair  hygrometer.) 

HERBERTSON,  ANDREW  J.,  The  Distribution  of  Rainfall  over  the  Land.  (Bibli- 
ography of  the  subject.) 

Monthly  Weather  Review.  (An  annotated  bibliography  of  Evaporation,  by 
Mrs.  Grace  J.  Livingston,  June,  1908,  to  June,  1909.  Also  reprinted  in 
1910  as  a  whole,  121  pp.) 

Voss,  ERNST  LUDWIG,  Die  Niederschlagsverhdltnisse  von  Sudamerika.  (Bib- 
liography of  the  subject.) 

Group  IV.    The  Secondary  Circulation  of  the  Atmosphere  —  Storms 

ALGUE,  JOSE,  The  Cyclones  of  the  Far  East.     (A  bibliography  at  the  end  of  each 

chapter.) 
FERREL,  WILLIAM,  Popular  Treatise  on  the  Winds.     (Pages  480-483  contain 

a  list  of  books  and  articles  on  this  general  subject.) 
POEY  Y  AGUIRRE,  ANDRES,  Bibliographic  Cyclonique,  8°,  96  pp.,  Paris,  1866. 

Group  V.    Weather  Prediction  —  Including  Weather  Proverbs  and  Prognostics 

BEBBER,  W.  J.  VAN,  Handbuch  der  ausubenden  Witter ungskunde.  (Part  I,  pp. 
367-392,  Part  II,  pp.  480-494,  references  to  articles  on  various  meteor- 
ological subjects  mentioned  in  the  text.) 

INWARDS,  RICHARD,  Weather  Lore.  (Pages  206-212  contain  a  bibliography 
of  weather  lore). 

SWAINSON,  REV.  C.,  A  Handbook  of  Weather  Folk-lore.  (A  short  list  of  works 
consulted.) 

Group  VI.    Climate 

ECKHARDT,  WILHELM,   R.,  Das  Klimaproblem  der  geologischen  Vergangenheit 

und    historischen    Gegenwart,     Braunschweig,     1909.     (Literaturangaben, 

pp.  176-183.) 
RAMSAY,  A.,  A  Bibliography,  Guide,  and  Index  to  Climate,  8°,  449  pp.,  London, 

1884. 
SUPAN,  ALEXANDER,  Grundzuge  der  physichen  Erdkunde,  4th  ed.,  Leipzig,  1908. 

(Bibliography  on  climatic  changes,  pp.  229-353.) 


APPENDIX  IX  539 

Group  VII.    Rivers  and  Floods 

Pittsburg  (Carnegie  Library),  Floods  and  Flood  Protection;  References  to  Books 
and  Magazine  Articles,  48  pp.,  Pittsburg,  1908. 

Group  VIII.    Atmospheric  Electricity 

CHAUVEAU,  A.  B.,  L'Electricite  Atmospherique.     (Pages  61-70  bibliography  of 

the  subject.) 
MACHE,  H.,  UND  SGHWIDLER,  E.  v.,  Die  atmosphdrische  Elektrizitat.     (Pages 

237-247  articles  and  books  on  atmospheric  electricity.) 

Group  IX.    Atmospheric  Optics 

Monthly  Weather  Review,  Sept.,  1900,  pp.,  386-389.     (A  bibliography  of  works 
on  the  intensity,  color,  and  polarization  of  sky  light.) 

(E)   Digest  of  Literature 

The  current  literature  on  meteorology  in  the  form  of  books,  pamphlets,  serial 
publications,  or  articles  in  scientific  magazines,  is  now  completely  listed  or 
abstracted  in -two  very  valuable  publications.  These  are  Die  Fortschritte  der 
Physik  (3  volumes  each  year),  and  The  International  Catalogue  of  Scientific 
Literature,  Section  F,  Meteorology.  In  each  of  these,  the  material  is  so  classified 
and  subdivided  that  it  is  comparatively  easy  to  find  the  material  on  any  given 
topic. 

Die  Fortschritte  der  Physik  was  first  published  in  1845  and  the  67th  volume 
covers  the  literature  of  1911.  Part  III  of  the  annual  issue  of  this  publication 
covers  cosmical  physics,  and,  of  course,  includes  the*  whole  of  meteorology. 
The  International  Catalogue  of  Scientific  Literature  was  begun  in  1901-1902,  with 
the  literature  for  1901,  and  the  8th  annual  issue  covers  the  literature  of  1908 
(published  in  1910). 

The  literature  of  meteorology  is  also  partially  abstracted  in  Science  Abstracts 
(v.  14  for  1911),  and  Beibldtter  zu  den  Annalen  der  Physik  (v.  35  in  1911),  but 
these  cannot  be  relied  upon  for  completeness.  The  Monthly  Weather  Review, 
the  Meteor vlogische  Zeitschrift,  and  the  Quarterly  Journal  of  the  Royal  Meteor- 
ological Society  publish  in  each  issue  valuable  comments  on  the  current  litera- 
ture, but  they  do  not  aim  at  completeness.  For  popular  articles  on  meteor- 
ological subjects  the  indices  of  Poole,  Fletcher,  and  Reader's  Guide  should  be 
consulted. 

(F)   Periodicals 

In  an  appendix  to  BARTHOLOMEW  AND  HERBERTSON'S  Physical  Atlas  — 
Meteorology,  (v.  3)  will  be  found  a  list  of  the  serial  publications  of  the  weather 
bureaus  and  weather  services  of  the  various  countries.  In  the  International 
Catalogue  of  Scientific  Literature,  (F,  Meteorology)  is  given  a  list  of  scientific 
magazines  and  publications  containing  articles  on  meteorological  subjects. 

In  the  first  of  the  following  lists  are  given  the  more  important  periodicals 


540  APPENDIX  IX 

which  are  devoted  almost  entirely  to  meteorology.  In  the  second  list  are  given 
those  periodicals  which  regularly  or  occasionally  contain  meteorological 
articles.  Periodicals  in  English,  French,  or  German  are  the  only  ones  which 
have  been  considered. 

(i)   Periodicals  devoted  entirely  to  Meteorology 

Monthly  Weather  Review,  4°,  July,  1872-date,  Washington,  D.C.  1  volume 
each  year;  v.  39  during  1911. 

Bulletin  of  the  Mount  Weather  Observatory,  8°,  1908-date,  Washington,  D.C. 
1  volume  each  year;  v.  4  during  1911. 

American  Meteorological  Journal,  8°,  May,  1884r-April,  1896.  v.  1,  Detroit; 
v.  2-8,  Ann  Arbor;  v.  9-12,  Boston. 

Quarterly  Journal  of  the  Royal  Meteorological  Society,  8°,  November,  1871-date, 
London.  1  volume  each  year;  v.  37  during  1911. 

Symons's  Meteorological  Magazine,  8°,  1866-date,  London.  1  volume  each  year ; 
v.  46  during  1911. 

Journal  of  the  Scottish  Meteorological  Society,  8°,  3d  series,  1864-date,  Edin- 
burgh and  London.  3  years  in  one  volume;  v.  16  during  1911-1913. 

Die  Meteor ologische  Zeitschrift,  4°,  January,  1884-date,  Braunschweig  (formerly 
Berlin  and  Wien).  1  volume  each  year;  v.  28  during  1911. 

Zeitschrift  der  Oesterreichischen  Gesellschaft  fur  Meteorologie,  4°,  1866-1884,  Wien. 
1  volume  each  year ;  19  volumes  in  all. 

Das  Wetter,  8°,   1885-date.  Berlin.     1  volume  each  year;   v.  28  during  1911. 

Himmel  und  Erde,  8°,  1888-date,  Berlin.  1  volume  each  year ;  v.  22  during 
1911. 

Beitrage  zur  Physik  der  freien  Atmosphare,  f°,  later  4°,  1904-date,  Strassburg. 
Several  years  in  each  volume. 

Beobachtungen  mil  bemannten  und  unbemannten  Ballons  und  Drachen  sowie  auf 
Berg-  und  Wolkenstationen  (published  by  Internationale  Remission  fur 
Wissenschaftliche  Luftschiffahrten),  4°,  1900-date,  Strassburg.  1  vol- 
ume each  year. 

Annalen  der  Hydrographie  und  maritimen  Meteorologie,  8°,  1873-date,  Berlin. 
1  volume  each  year;  v.  39  during  1911. 

Ciel  et  terre,  8°,  1880-date,  Bruxelles.     1  volume  each  year ;  v.  31  in  1911-1912. 

Annuaire  de  la  Societe  meteorologique  de  France,  large  8°,  1849-date,  Paris. 
1  volume  each  year;  v.  59  during  1911. 

La  Revue  Nephologique,  8°,  1906-date. 

(The  publications  of  the  weather  services  of  the  various  countries  should, 

perhaps,  be  added  here.) 

(n)   Periodicals  which  regularly  or  occasionally  contain  Meteorological  Articles 

The  Astrophysical  Journal,  large  8°,  1895-date,  Chicago.  2  volumes  each 
year;  v.  33  during  the  first  half  of  1911. 


APPENDIX  IX  541 

Science,  8°.     New  series,  1895-date,  New  York.    2  volumes  each  year ;  v.  33 

during  first  half  of  1911. 
Terrestrial  Magnetism  and  Atmospheric  Electricity,  8°,  1896-date,   Baltimore. 

1  volume  each  year;  v.  16  during  1911. 

Scientific  American,  f°,  1845-date,  New  York.     2  volumes  each  year;   v.  104 

during  first  half  of  1911. 
Scientific  American  Supplement,  f°,   1876-date,  New  York.     2  volumes  each 

year;  v.  71  during  first  half  of  1911. 
London,  Edinburgh,  and  Dublin  Philosophical  Magazine,  8°,  1798-date,  London. 

2  volumes  each  year ;  v.  21,  of  6th  series,  during  first  half  of  1911. 
Nature,  large   8°,  1869-date,   London.     3  volumes  each  year;   v.  86  during 

first  third  of  1911. 
Annalen  der  Physik,  8°,    1790-date,  Leipzig.      3  volumes   each  year;    v.  34 

(4th  series)  during  first  third  of  1911. 
Physikalische  Zeitschrift,  large  8°,  1899-date,  Leipzig.    1  volume  each  year;  v.  13 

during  1911. 
Petermanns  Mitteilungen,  4°,   1855-date,  Gotha.      1  volume  each  year ;  v.   57 

during  1911.     (Many  Erganzungshefte,  numbering  162  in  1908.) 
Das    Weltall,  4°,    1900-date,    Treptow-Berlin.      1  volume    each  year;    v.  11 

during  1910-1911. 
Comtes  rendus,  4°/  1835-date,  Paris.    Two  volumes  each  year;  v.  152  during  first 

half  of  1911. 
Bulletin  de  la  societe  astronomique  de  France,  8°,  1887-date,  Paris.     1  volume 

each  year;   v.  25  during  1911. 


INDEX 


[The  numbers  refer  to  pages.] 


Abbe,  C.,  48,  354. 

Abbot,  C.  G.,  solar  constant,  40. 

Absorption,  effects  of,  37. 

of  insolation,  37.    . 
Actinometry,  38. 
Adiabatic  cooling,  ^j.  -, 
Air  (see  atmosphere) . 
Aitken,  J.,  11. 
Albany,  N.Y.,  86,  88,  125,  148. 

daily  range  of  temperature  at,  87. 

daily  variation  in  temperature  at,  84. 

graphical  representation  of  station  nor- 
mals of  temperature  at,  80. 

normal  absolute  humidity  at,  205. 

normal  daily  temperature  at,  78. 

normal  monthly  temperature  at,  79. 

normal  relative  humidity  at,  209. 

normal  sunshine  at,  230. 

number  of  rainy  days  at,  253. 

precipitation  at,  247,  248. 

river  gauge  at,  447. 

snowfall  at,  251,  252. 

tables  of  data  for,  501,  512. 

temperature  data  at,  91. 

thunder  showers  at,  21,  327. 

variability  of  temperature  at,  90. 

weather  prediction  at,  389. 
Altitude,  barometric  determination  of,  130. 

pressure  variation  with,  132. 
Alto-cumulus,  223. 
Alto-stratus,  223. 
Anemometer,  141. 

deflection,  141. 

Lind's,  142. 

pocket,  144. 

Robinson's,  142,  147. 
Anemoscope,  138. 
Aneroid  barometer,  119. 
Angot,  41. 

Anomalies,  thermal,  98. 
Anticyclones  (see  Chapter  VI,  B). 

definition  of,  L 

description  of,  294. 

distribution  of  elements  about,  294. 

energy  of,  314. 

origin  of,  312. 

structure  above  earth's  surface  of,  279. 

tracks  of,  305. 

velocity  of  motion  of,  306. 


Arago,  69. 

Arctic  winds,  177. 

Argon,  7,  8,  15. 

Aristotle,  3. 

Assmann,  R.,  48,  53,  70. 

Astronomy,  2,  6. 

Atmosphere,  composition  of,  7,  9. 

convection  in,  45. 

definition  of,  6. 

dust  in^ll. 

evolution  of,  15,  16. 

future  of,  16. 

hca*i'lg  *lH  rnn1inp;  nf,    tt 

Iie!ghtof,"l8. 

nocturnal  stability  of,  55._ 

offices  of,  13. 

of  other  heavenly  bodies, Jj}.  M 

pressure  of,  18  (see  Chapter  IV). 

temperature  change  between  day 

night,  42. 

Atmospheric  acoustics,  Chapter  XIII. 
Atmospheric  electricity,  Chapter  XL* 
Atmospheric  optics,  Chapter  XI lf*r 
Atom,  29. 

Aurora  borealis,  19,  479. 
Australia,  176. 
Avalanche  winds,  181. 

B 

Backing  of  wind,  137. 
Bacteria,  10. 
Baguois,  266. 


and 


Barograph, 

Barometer,  accuracy  of  aneroid,  121. 

accuracy  of  mercurial,  119. 

aneroid,  115,  119. 

corrections  to  reading  of  mercurial,  117. 

history  of,  115. 

kinds  Of,  115. 

mercurial,  115. 

mouth,  122. 

Barometric  gradient,  159. 
Battles  and  rain, 
Beaufort  wind  scale,  139. 
Bench  mark,  446: 
Bentley,  241. 
Bibliography  (see  end  of  each  chapt- 

Appendix  IX). 
Bigelow,  302,  311,  312. 


543 


544 


INDEX 


[The  numbers  refer  to  pages.] 


Black  bulb  thermometer,  75. 

Blizzard,  344. 

Blue  Hill,  145,  225,  291,  297,  375. 

Bora,  348. 

Bowie,  E.  H.,  385. 

Brandes,  H.  W.,  361. 

Breezes,  land  and  sea,  177. 

mountain  and  valley,  179. 
Bruckner,  E.,  411. 
Buran,  347. 
Buys  Ballot's  law,  163. 


Calm  belts,  165. 

Campbell-Stokes  sunshine  recorder,  227. 

Canadian  weather  service,  374. 

Capacity  of  air  for  water  vapor,  194. 

Carbon  dioxide,  7,  14. 

Celsius,  63. 

Centigrade,  61,  63. 

Chalk  plate  process,  368. 

Chemical  hygrometer,  198. 

Chemistry,  2,  6. 

Chinook,  345. 

Cirro-cumulus,  222. 

Cirro-stratus,  222. 

Cirrus,  220. 

City  fogs,  217. 

Classification  of  clouds,  218. 

Climate  (see  Chapter  IX). 

constancy  of,  437. 

definition  of,  20. 
Climatic  data,  427. 
Climatic  factors,  429. 
Climatic  subdivisions,  430. 
Cloud  colors,  493.V 
Cloudiness,  226. 
Clouds  (see  Chapter  V,  C). 

classification  of,  219. 

formation  of,  233-239. 

height  of,  225. 

history  of  classification  of,  218. 

sequence  of  forms,  224. 

thirteen  forms  of,  220. 

velocity  of,  226. 
Cloud  shadows,  490. 
Code  of  U.  S.  Weather  Bureau,  363. 
Cold  wave,  344,  400. 

cause  of,  344. 
Color,  of  clouds,  493. 

of  sky  ^490^   ^^-^ 

sunrise  and  sunset,  492. 
Commercial  weather  map,  366. 
Composition  of  atmosphere,  7,  9. 
Condensation,  definition,  191,  210. 

forms  of,  210. 

latent  heat  of,  191,  212. 

methods  of,  233. 

nuclei  of,  231. 


Conduction,  44. 

Conductivity  of  atmosphere,  460. 
Congresses,  meteorological,  374. 
Constancy  of  climate,  437. 
Continental  winds,  171. 
Convection,  44,  156,  194,  235. 

condition  of,  56. 

evidences  of,  46. 
Cooperative  stations,  76. 
Corona,  487. 

Corrections  to  barometer  readings,  gravity, 
119. 

meniscus,  118. 

temperature,  118. 
Correlation  of  the  meteorological  elements, 

319. 
Counter  current  ^theory,  of  highs,  312. 

of  lows,  311. 
Credulity,  popular,  413. 
Cumulo-nimbus,  224. 
Cumulus,  47,  221. 

Cyclones    (see    tropical     or    extratropical 
cyclones) . 

D 

Davis,  W.  M.,  35,  165. 

De  Bort,  53,  412. 

Deductive  method,  23,  164,  278. 

Deflective  force  of  earth's  rotation,    160, 

280. 

Deserts,  255. 
Dew,  211. 

amount  of,  212. 

conditions  for,  213. 
Dew  point,  196. 
Diffusion,  JQ4  y 
Digest  of  literature,  539. 
Doldrums,  168,  279. 
Dove,  H.  W.,  165. 
Draper  thermograph,  71. 
Dust,  11,  12,  231,  491. 

amount  of,  12. ' 

effect  of,  12. 

source  of,  12. 
Dust  counter,  11. 


Earth  currents,  465. 

Earth,  orbit  of,  32. 

Echo,  497. 

Eclipse  winds,  180. 

Eiffel  Tower,  52,  145,,  146,  150. 

Electricity,  atmospheric  (see  Chapter  XI). 

Electrometer,  457. 

Elements,  meteorological,  20. 

Elevation  from  barometer,  130. 

English  weather  service,  373. 

Equinoctial  storms,  416. 


INDEX 


545 


[The  numbers  refer  to  pages.] 


Eqviipotential  surfaces,  459. 

Ether,  29. 

Evaporation,  definition,  191. 

amount  of,  192. 
Extratropical  cyclones  (see  Chapter  VI,  B). 

definition,  283. 

description,  283. 

distribution  of  elements  about,  283. 

energy  of,  313. 

locating  center  24  hours  ahead,  383. 

origin  of,  308. 

structure  above  earth's  surface,  289. 

tracks  of,  299,  385. 

velocity  of  motion  of,  304. 


Fahrenheit,  61,  62. 
Ferrel,  W.,  161,  280. 
Flags,  signal,  364. 
Floods  (see  Chapter  X). 

kinds,  449. 

prediction  of,  451. 
Foehn,  345. 
Fog  (see  Chapter  V,  B),  #16. 

city,  217. 

Forecast  districts,  355,  3&1. 
Forecasting  (see  weather  ^prediction). 
Forecasts,  value  of,  4. 
Forests  and  rainfall,  258.  " 
Fracto-cumulus,  223. 
Fracto-nimbus,  224. 
Fracto-stratus,  223. 
Franklin,  B.,  353,  454, 
Frost  (see  Chapter  V,  B),  213. 

causes  of,  214. 

prediction  of,  214. 

protection  from,  215. 
Funnel  cloud  of  tornado,  336. 


Galileo,  62,  115. 
Galveston  hurricane,  270. 
Gas,  properties  of,  7. 
Geosphere,  17. 
Glaciers,  259. 

Government  publications,  530. 
Gradient,  average    value    of  .  tempest ure, 
50. 

barometric,  159. 

definition,  48. 

poleward  temperature,  98. 

vertical  temperature,  48. 
Graphical  representation,  22. 
Gravity,  17. 
Gregale,  348. 
Guilbert,  386. 
(iuilbert'a  rules,  386. 
Gulf  Stream,  95. 

2N 


Hail,  241,  333. 
Hailstones,  241. 
Hair  hygrometer,  198. 
Halos,  488. 

Hann,  J.,  40,  43,  427,  459. 
Harmattan,  348. 
Harrington,  M.  W.,  4,  354. 
Haze,  232,  493. 
Heat,  nature  of,  29. 

sources  of  atmospheric,  31. 
Heat  equator,  94. 
Height  of  atmosphere,  18? 

of  clouds,  225. 
Helium,  8,  9. 
Hergesell,  H.,  53. 
Highs  (see  anticyclones). 
Horse  latitudes,  168. 
Hot  wave,  344. 
Howard,  L.,  218. 
Humidity,  195. 

absolute,  196,  198,  204. 

observations  of,  203. 

relative,  196,  198,  207. 
Humphreys,  W.  J.,  9,  53,^173. 
Hurricanes,  266  (see  tropical  cyclones). 
Hydrogen,  7,  9,  15. 
Hydrosphere,  17. 
Hygrometer,  chemical,  198. 

definition,  197. 

dew  point,  200. 

hair,  198. 

recording,  202. 

I 

Ice  storms,  242. 
India  monsoons,  174. 
Indian  summer,  416. 
Inductive  method,  23,  164. 
Insolation,  31. 

absorption  of,  40. 

distribution  over  earth  of,  35. 

value  of,  40. 

International  storm  warnings,  374. 
Interrelation  between  thermometric  scales, 

61. 
Inversion  of  temperature,  55. 

Ions,  231,  233,  461.  ' 

Isobaric  surfaces,  134.     . 
Isobars.  132. 

cnaracterisxi      of  annual,  133. 

.Ifinuary,  13o. 

July,  135. 

Isothermal  layer,  52. 
Isotherms,  characteristics  of,  94-98. 

construction  of,  94. 


Jenner,  418. 

Jordan  sunshine  recorder,  228. 


546 


INDEX 


[The  numbers  refer  to  pages.] 


Khamsin,  348. 
Kites,  48. 
Krakatoa,  12. 
Krypton,  8. 


Lake  temperatures,  105. 
Land  breeze,  177. 
Latent  heat,  38,  191. 
Leste,  347. 
Leveche,  347. 
Lightning,  beaded,  470. 

cause  of,  466. 

color  of,  467. 

damage  by,  473. 

danger  from,  470. 

heat,  469. 

kinds  of,  466. 

protection  from,  442. 

rods,  472,  475. 

sheet,  469. 

spectrum  of,  467. 

zigzag,  466. 

Lind's  pressure  anemometer,  142. 
Linnaeus,  62. 

Literature  of  meteorology,  518. 
London  fogs,  217. 
Looming,  487. 
Lows  (see  extratropical  cyclones). 

M 

Mars,  15. 
Marvin,  49. 
Meniscus,  118. 
Meteorology,  definition,  2. 

history,  3. 

utility,  3-5. 
Meteors,  12,  17,  19. 
Metric  system,  503. 
Migration  annual  of  the  winds,  170. 
Migration  of  isotherms,  97. 
Mirage,  46,  486. 
Mistral,  348. 

Moisture  (see  Chapter  V). 
Molecule,  29. 
Monsoon,  174. 
Moon  and  weather,  414. 
Moore,  Willis  L.,  355,  440. 
Mountain  and  valley  breeze,  179>* 
Mountain  observations,  146. 
Mouth-barometer,  122. 


N 


Natural  sciences,  definition,  1. 
enumeration  of,  2. 


Neon,  8. 

Nephoscope,  226. 
Nimbus,  222. 
Nitrogen,  7,  14. 
Normal  values,  21,  76. 

in  connection  with  precipitation,  246- 
253. 

of  cloudiness,  229. 

of  fog,  218. 

of  frost,  216. 

of  moisture,  203-209. 

of  number  of  thunder  showers,  327. 

of  pressure,  124-129. 

of  temperature,  76-91. 

of  wind,  147-155. 
Northern  lights,  19>-470.K 
Nuclei  of  condensation,  231. 


N 


Ocean  currents,  95. 
Ocean  temperature,  105. 

temperature  change  between  day  and 

night,  41. 

Optics,  atmospheric  (see  Chapter  XII). 
Oxygen,  7. 
Ozone,  12,  55. 


Pampero,  347. 

Parallax  in  reading  thermometer,  65. 

Particles,  231,  491. 

inorganic,  11. 

organic,  10. 
Pericyclonic  ring,  268. 
Periodicals,  540. 
Physics,  2,  6. 
Piche  evaporimeter,  193. 
Planetary  winds,  165. 
Planets,  wind  system  of,  183. 
Polar  temperatures,  101. 
Precipitation    (see    Chapter    V,    D ;     also 

rainfall  and  sn9wfall). 
Prediction,  accuracy  of,  407. 

by  similarity,  398. 

cold  wave,  400. 

flood,  404. 

frost,  399. 

general  method  of,  379. 

long  range,  407. 

storm,  403. 

system  of  verifying,  405. 

terms  used  in,  404. 

tornado,  339,  403. 

when  high  dominates,  398. 

when  low  dominates, 
Pressure  anemometer, 
Pressure  change  with 


nmates,  398. 
unates,  383.   ,j 
eter,  141. 
vith  altitude.  \\  .J9. 


INDEX 


547 


[The  numbers  refer  to  pages.] 


Pressure  of  atmosphere,  114.     (See  Chap- 
ter IV.) 

observations  of,  123. 
Pressure  of  wind,  137. 
Prevailing  westerly  winds,  169. 
Prevailing  winds,  147. 

of  the  world,  155. 
Prognostics,  419. 
Proverbs,  weather,  418. 
Psychrometer,  200. 


R 


Radiant  energy,  30. 
Rain,  239. 

cooling  caused  by,  243. 

on  July  4,  243. 
Rainbow,  489. 
Raindrops,  formation  of,  239. 

size  of,  240. 
Rainfall,  distribution  of,  254. 

measurement  of,  244. 

relation  of,  to  agriculture,  257. 

relation  of,  to  forests,  258. 
Rain  gauge,  244. 
Rain  making,  243. 
Range,  annual,  of  temperature,  100. 
Range  of  temperature,  52,  87. 
Reaumur,  61,  63. 
Reflection,  36.- 

of  insolation,t;87. 
Refraction,  484. 
Regimen,  443. 

Richard  Freres  barograph,  121. 
Richard  Freres  thermograph,  72,  76. 
River  stage,  444,  446. 
River  temperature,  106. 
Robinson's  cup  anemometer,  142. 
Rods,  lightning,  472,  475. 
Rotch,  A.  L.,  53,  111,  146,  375. 
Run  off,  443. 


St.  Elmo's  fire,  479. 

Sanctorius,  62. 

Saturation,  194. 

Sea  breeze,  177. 

Sea-level  correction,  508. 

Secondary  lows,  397. 

Seiche,  452. 

Shelter,  thermometer,  67. 

Siberia,  101. 

Sirocco,  344. 

Six  thermometer,  73. 

Sky  colors,  490. 

Sleet,  :>4L'. 

Sling  thermometer,  69. 

Snow,  240. 


Snowfall,  chart  of,  for  the  United  States 
256. 

effects  of,  258. 

measurement  of,  245. 

normal  values  of,  252. 

observations  of,  246. 
Snowflakes,  240. 
Snow  line,  438. 
Solano,  347. 
Solar  constant,  39. 
Sound,  496. 
Squall,  321. 
Stations  of  the  U.  S.  Weather  Bureau,  76, 

356,  513. 

Storms  (see  Chapter  VI). 
Strato-cumulus,  224. 
Stratus,  221. 

Subequatorial  wind  belt,  170. 
Subtropical  wind  belt,  170. 
Sun,  31. 
Sun  dogs,  488. 

Sunrise  and  sunset  colors,  492. 
Sunshine,  amount  of,  230. 
Sunshine  recorders,  227. 
Symbols  on  weather  map,  365. 


Teaching  meteorology,  517. 
Teisserenc  de  Bort,  53,  412. 
Telethermometer,  75. 
Temperature  (see  Chapter  III). 

annual  range  of,  100. 

annual  variation  in,  85. 

anomalies,  98. 

below  surface  of  the  land,  106. 

daily  variation  in,  84. 

differences  over  a  limited  area,  92. 

differences  with  altitude^  91. 

distribution  over  the  earth,  93-105. 

highest  ever,  101. 

inaccuracies  in  determining,  65. 

lake,  105. 

lowest  ever,  101. 

normal,  76. 

normal  values  of,  76-91. 
*ocean,  105. 
/polar,  101. 

range  of,  87. 

real  air,  67. 
Terrestrial  winds,  169. 
Thalweg,  443. 
Thermographs,  71. 
Thermo-isopleths,  81. 
Thermometer,  black  bulb,  75. 

Centigrade,  63. 

i  ruction  of,  63. 

cost  of,  66. 

errors  in  construction  and  use,  66. 


548 


INDEX 


[The  numbers  refer  to  pages.] 


Thermometer,  Fahrenheit,  62. 

invention  of,  62. 

location  of,  67. 

maximum  and  minimum,  73. 

parallax  in  reading,  65. 

Reaumur,  63. 

shelter,  67. 

Six,  73. 

sling,  69.  *. 

ventilated,  70.  / 

Thermometry,  60. 
Thunder,  497. 
Thundershowers  (see  Chapter  VI,  C). 

at  Albany,  21.  / 

cross  section  of,  324. 

cyclonic,  334. 

definition,  320. 

direction  of  motion  of,  329. 

distribution  of  elements  about,  322. 

geographical  distribution,  326. 

kinds  of,  330. 

observations  of,  325. 

origin  of,  330. 

path  of,  328. 

periodicity  of,  330. 

relation      to      extratropical      cyclones, 
326. 

relation      to      V-shaped      depressions, 
328. 

time  of  occurrence,  329. 

velocity  of  motion,  329. 
Tidal  winds,  181. 
Tornadoes  (see  Chapter  VI,  D). 

definition,  335. 

destruction  by,  336. 

distribution  of  elements  about,  337. 

geographical  distribution  of,  338. 

insurance,  342. 

number  of,  338. 

observations  of,  338. 

origin  of,  340. 

path  of,  340. 

protection  from,  341. 

relation      to      extratropical      cyclones, 
339. 

relation  to  thundershowers,  337. 

time  of  occurrence,  340.  4, 

Torricelli,  115.  % 

Tracks  of  lows,  299.  *. 

Trade  winds,  168. 
Transmission,  37. 

of  insolation,  37. 

Transparency  of  atmosphere,  493. 
Tropical  cyclone  (see  Chapter  VI,  A). 

dangerous  half,  273. 

definition  of,  266. 

description  of,  267. 

Galveston,  270. 

origin  of,  277. 


regions  of  occurrence,  273. 

tracks  of,  274. 
Twilight,  18,  493. 
Twinkling  of  stars,  485. 
Typhoons,  266. 


U 


United  States  Weather  Bureau  (see  Chap- 
ter VII). 
history  of,  353. 
present  organization,  355. 
publications  of,  372,  530. 
stations  of,  76,  356,  513. 
thermometer  shelter  of,  67. 
Washington  station,  358. 


Valley  breeze,  179. 
Van  Cleef,  301. 
Vane,  wind,  138. 
Varenius,  266. 

Variability  of  temperature,  88,  101. 
Variation,    annual,  in    absolute    humidity, 
204. 

annual,  in  amount  of  precipitation,  250. 

annual,  in  conductivity,  462. 

annual,  in  potential  difference,  460. 

annual,  in  pressure,  128. 

annual,  in  relative  humidity,  208. 

annual,  in  temperature,  85. 

annual,  in  wind  direction,  153. 

annual,  in  wind  velocity,  151. 

daily,  in  absolute  humidity,  204. 

daily,  in  amount  of  precipitation,  246. 

daily,  in  conductivity,  462. 

daily,  in  potential  difference,  459. 

daily,  in  pressure,  124. 

daily,  in  relative  humidity,  207. 

daily,  in  temperature,  84. 

daily,  in  wind  direction,  150. 

daily,  in  wind  velocity,  149. 

geographical,  in  absolute  humidity,  204. 

geographical,  in  relative  humidity,  208. 

irregular,  in  pressure,  128. 

irregular,  in  temperature,  86. 

irregular,  in  wind,  153. 

of  insolation  with  latitude,  34. 

of  insolation  with  time,  35. 

of  pressure  with  altitude,  132. 

periodic,  20. 
Veering  of  wind,  137. 
Velocity  of  flow  of  rivers,  444. 
Venus,  15. 

wind  system  of,  183. 
Verification  of  prediction,  405. 
Volcanic  winds,  181. 
V-shaped  depressions,  328,  397. 


INDEX 


549 


[The  numbers  refer  to  pages.] 


w 


Ward,  R.  de  CM  376,  440,  517. 
Waterspout,  342. 
Water  vapor,  191. 

distribution  of,  194. 
Waves,  cold,  400. 
Weather,  definition,  20. 
Weather     Bureau       (see     United     States 

Weather  Bureau). 
Weather  cycles,  410. 
Weather  house,  199. 

Weather   map,    charting   observations   on, 
364. 

commercial,  366. 

construction  of,  367. 

distribution  of,  368. 

English,  373. 

history  of,  361. 

symbols  on,  365, 


Weather  prediction  (see  prediction). 

Weather  proverbs,  416. 

Werchojansk,  82,  86,  100. 

Whirlwinds,  47,  343. 

Williwaus,  348. 

Wind  (see  Chapter  IV,  B,  C,  D). 

definition,  136. 

observation^  of ,  146. 

pressurejjf,  1^77 

roses,  147. 

scales,  139. 

vane,  138. 
Wind  shift  line,  287. 


Xenon,  8. 

Zones,  climatic,  430. 


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the  strength. 

(Based  on  the  Segelhandbuch  der  Deutschen  Seewarte;  similar  to  the  Charts  in  ANQOT'S  Traite  elementaire 

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CHART  XIV.  —  Air  Circulation  of  the  Atlantic  Ocean  for  July  and  August.  The  length 
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strength. 

(Based  on  the  SepeWumd&ucfc  der  Deutachen  Seewarte;  similar  to  the  Chart  in  ANGOT'S  Traite  Uementaire 

de  meteor ologie.} 


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